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MG MGB Technical - VDO Gauge Wiring Problem
| Hi I have installed my VDO gauges. And I am having some difficulties When I installed the VDO tach I used the existing tach green wire in my 74.5 MGB. Now just turning the ignition on causes the tach to jump. If the wipers are on when I turn the ignition on, the tach goes up to @1500 (and stays there) with just the ignition on. Looking at the information provided at Paul’s’ site (The Pages Of Bee & Vee) “Wire Colours Fuses Terminal Numbering” I can see why. Almost all the gauges run via the green switched wire. I forgot to install power to the speedometer and was going to use this same green switched circuit to power the speedometer. Is this just going to create more problems? Basically how can I isolate these two components and still have a fused switched wire? I do have an upgraded fuse box with several more slots available Can I tie into the turn signal system and get my switched fused power from there without creating the 'surge? Or, is it anything to even worry about if the gauge stabilities after the engine starts? I did try running a different ground and that didn’t make a difference. And the gauges are still wired through the voltage stabilizer Thanks Bruce |
| Bruce Mills |
| Install a new relay that is controlled by the white wire and will supply power to the guages from the brown circuit and isolate it from the green circuit. Wiring guide for the Bosch relay to the guages pin 85 - white wire, switched pin 86 - black wire, ground pin 30 - brown wire, unswitched power from the starter pin 87 - new wire, supplies power to the guages. Install a fuse in the new circuit you have created after pin 87. |
| Kimberly |
| G'day. Not strictly part of this thread, but does anyone know why the relays are numbered as such. Why not 1,2,3,4? Tony |
| Tony Oliver |
| Tony - Don't know why, but it does seem to be an industry standard for most makes of vehicles. My guess on the 5 pin Bosch relays is that 87a stands for 87 auxiliary. Lucas relays are marked W1, W2, C1, and C2. I forgot what the C stands for, but the W stands for winding. |
| Kimberly |
| C= contacts Clifton |
| Clifton Gordon |
| Thanks Kimberly Any idea what is causing the tach to react like that? I wouldn't want to leave a potential future problem unsolved Bruce |
| Bruce Mills |
| Bruce- You stated, "the guages are wired through the voltage stabilizer." Is this the stock voltage stabilizer? With a stock setup, only the temp and fuel guages are wired through the voltage stabilizer. The tach gets direct 12 volts. The stock voltage stabilizer fluctuates between 0 and 12 volts, averaging about 10 volts. What are the requirements for the VDO guages? What's the model of the guages? |
| Kimberly |
| Kimberly Yes it is the stock voltage stabilizer. And I guess I just assumed all the gauges went through the voltage stabilizer. There are 2 green wires going to the unit and one LGG. I have heard that the VDO gauges do not require a voltage stabilizer. The Gauges are the Cockpit Royal. Here are the results of my tests I am using the Fig 10:31 Typical Wiring Diagram 1973-74 N American Models. At the instrument voltage stabilizer I have 2 joined green wires and one LGG 1) With the ignition on and the wire disconnected: I turn the heater on the tach jumps up and then back to zero. I turn the wiper on (high) and the tach jumps up to 1400 and bounces between that and @1200 corresponding with the sweep of the wiper. I have no gas gauge or temperature gauge. 2) With ignition on, G wire disconnected LGG connected: Same as #1 3) With ignition on LGG and G connected together: Same as #1 but I have gas gauge and temperature gauge. 4) With the car running and the wire connected to the instrument voltage stabilizer: Turning on the wipers or heat produces no fluctuation with the tach needle. 5) With the car running and the G wire disconnected from the voltage stabilizer: The fuel gauge and temperature gauge don’t work but there is no fluctuation in the tach needle when the wiper or the heater is turned on. 6) With the car running and the G wire and LGG wire joined together: There is no fluctuation in the tach needle when the wiper or the heater is turned on. Installing the VDO gauges wasn't that hard. All the wiring is there for the temperature gauge (I used the required sender) as well as the tach and gas gauge. The only additional wiring is adding extra RW & B wires for the dual light bulbs. Simple enough. I didn't have a problem with the Smith gauges I wonder if the additional bulbs are drawing too much current and causing this? Bruce |
| Bruce Mills |
| Bruce I looked up the wiring diagrams for the VDO Cockpit Royal tach and speedometer. The tach does not use a voltage stabilizer. Use one of the switch controlled fuses in your fuse box to supply power to pin #2 on the tach. Make sure pin #3 is connected to a good ground. The white/black wire from the negative side of the coil connects to pin #4 on the tach. The speedometer does not use a voltage stabilizer. You can use the same switch controlled fuse that you are using for the tach. Connect the power supply to pin #4 on the speedometer. Make sure pin #3 is connected to a good ground. Follow the diagram for the speedometer to connect it to its sensor. http://www.advanceautowire.com has two wiring diagrams for your model year. Click on stock schematics and find the one that best fits your car (diagram 16 or 17). |
| Kimberly |
| do you have electronic ignition as this causes problems with some electric tachos and as yours are new its a poss if you do ste |
| Ste Brown |
| Hi Kimberly I followed your advise and ran a new fused switch wire over to the tach and speedometer and all worked very well. I really appreciate the time you took to help me. Thanks very much. Bruce |
| Bruce Mills |
| You're welcome. |
| Kimberly |
| Hi As I mentioned above I ran a new white fused switch wire over to power the tach and speedometer and this appeared to work well. (When I had it running through the switched green wires, turning on the wipers would cause the tach to jump) That is until I took it for a test run and stepped on the brake. The tach goes up and the speedometer drops as soon as I step on the brakes or turn the fan on. I had all the gauge wires grounded to the same spot under the dash so I ran new grounds (grounded to the body) for the tach and speedometer. But it made no difference. My pump is on the same white circuit and I can hear it drop when the fan or brakes are applied. This is the switched white wire which I have a 65 amp alternator so it should be supplying enough power. I am also running 100/80W halogen bulbs and 2 50W driving lights. But this has never been a problem before. I have no problems putting in a relay and thanks Kimberly for the instructions above, but will this fixed the problem? Do I need a new (bigger) alternator. Thanks Bruce |
| Bruce Mills |
| Are you having the same problems when the lights are off? The lights, fuel pump, and brake lights are all on different circuits, so I don't think a relay will help. All a relay would have done is isolate the main source of power for your new guages from your ignition switch. Do you have enough power? The headlights and driving lights alone use two to three times more power than the stock setup. I don't know what other accessories you have, car stereo, phone charger, etc. Does the speedometer drop when you turn on the fan while you are driving on the freeway? |
| Kimberly |
| Yes I am having the same problem with the lights on or off. Perhaps a little more pronounced when they are on The brake pressure failure switch (16) comes to a junction of grounds (black wires). As I follow it around it seems to tie into other grounds as well. I thought that perhaps being tied into the same ground might be causing the problem. But when I ran the grounds from the tach and the speedometer to a different grounding point it didn't solve anything. "Do I have enough power" Well now I am starting to wonder. This set up worked with the Smiths gauges, perhaps the Smiths aren't as sensitive, or didn't draw the power the VDO requires, or perhaps this is just enough to put me over the top? I have a stereo and an amplifier but that is it, no phone charger etc. And the drop happens with or without the stereo on. Am I looking at a new, bigger alternator? The fan does cause the speedometer to drop when turned on at highway speed. But strangely enough there is hardly and drop in the speedo or increase in the tach at lower RPM (idle) Is there anything else I can look for. I did replace a couple of the electrical connectors, the blocks where the wire (male & female) join in the harness. But I have checked those several times and all the wires come through the plastic electrical block and are the same color wire as they come out the other side. IS there any other test I can perform before I spend money on a bigger alternator. I guess the best way to figure out what a new alternator must put out to add up the Amp of each item which is on? Remember when I has the VDO gauges hooked up to the green wires, turning the wipers on caused the tach to fluctuate, and that was without anything else on at all?? Thanks Kimberly Bruce |
| Bruce Mills |
| Sounds like high-resistance connections in the feed to your gauges, brake-lights and wipers. The latter should be on the green-circuit, which you say you didn't use because the wipers caused a problem, so it must be somewhere between where your fused white (should be green, the convention is that white is unfused) joins to the output from the ignition switch. Could be in the connections to the switch, the switch itself, or where the browns join the battery and alternator. A decent voltmeter should show a greater volt-drop at the gauges than the battery when the brakes applied, if a bad connection is the problem. If turning on the headlights causes a similar problem then it is more likely to be closer to the solenoid as the headlights don't come through the ignition switch. Unlikely to be a ground as the brake lights don't use a wired ground. The brake balance failure wiring is probably irrelevant as it only lights a low-wattage bulb, and they should be black/white wires, not black ground wires. All this is supposing you have conventional wiring if you have a home-brew or after-market there could be any numer of interactions. |
| Paul Hunt 2 |
| Bruce, I have the complete VDO set up in my MGB. I removed the voltage stabilizer completely. I have the 5 speed Ford Sierra transmission with a hall effect sender. I also have the pertronix ignition and to stop tach jump this is what to do. Will VDO tachs work with points ignition or Pertronix? Yes, with diode in line on the signal lead. Instructions are as follows: Purchase a diode #1N4005 from a local electronics store such as Radio Shack or Circuit City. Cut both ends of the diode so each is approximately 3/4” long. Crimp 1/4” female spade connector on the end of the diode with the silver band. Crimp butt-splice connector on the other end of the diode. Crimp the opposite end of butt-splice connector to the wire connected to the ignition signal source. Connect 1/4” female spade connector used in #3 above to terminal #4 on the back of the tachometer. Connect a ground (-) wire to terminal #3. Connect a switched 12-volt power wire to terminal #2. Set switches for appropriate amount of cylinders. I have not had any problems with my VDO tach now going on two years. Regards, Ray |
| Ray 1977mgb |
| Thanks Paul and Ray I have ordered a diode (it will take up to 10 days to get to me) so in the interim time I will be checking out the wiring, white and green for "high-resistance connections" Bruce |
| Bruce Mills |
| Hi Well I am getting really lost here and more than a little frustrated. It was easier installing new relays and a new fuse box behind the dash than installing VDO gauges. I spent the whole day in the shop trying to solve this problem and I am no further ahead. I re-wired the tach back to the green wire and ran another green wire from the tach over to the speedometer. With the ignition on and turning on the fan the tach goes up to 1000rpm and stays there. With the ignition on and turning on the wipers, the tach fluctuates with the wipers. With the car idling and turning on the fan there is no fluctuation in the tach. With the car idling and turning on the wipers there is a little fluctuation (I have ordered a diode and will install that when it arrives and hope this will solve my tach problems) But here is the real problem. When I took the car for a drive and stepped on the brakes the speedo drops and the tach jumps higher I then unhooked the rear halogen brake lights and with the ignition on there is no fluctuation on the tach when I turn on the wipers or the fan. I went for a drive. Of course there is now no problem. I cleaned up all the rear brake wires(and ground) and installed new connectors with dielectric grease. Turning on the ignition and turning on the fan or the wipers and there is no tach jump. Took the car for a drive and still when applying the brake the speedo drops and the tach goes up. Back to the shop and cleaned all the green wires I could find. But nothing changed I see the tach/speedo share the same fuse as the other green wires (including the brake switch) which gets the power from the white wire via the ignition switch. Could I tie into the white wire, run it to a male connection on my fuse box (I have a couple of spare place's left) then run new green wires from the other side of the fuse over to the tach and speedometer? Would this separate the tach/speedo from the brake switch or would these 3 components still be tied together via the white wire? At this point I don’t know what more can be done I am out of ideas. I am now wondering if I have a bad speedo or hall effect sender? The VDO instructions explain how to test the unit by using a voltmeter and slowly turning the tang which goes into the hall effect sender down by the tranny. It should read @ 4V going up to @ 5V. If there is no difference the unit is bad. I got a reading 4.07V and it didn’t change at all. However the speedo does function and I can read the speed.?????????? Any idea which way I should proceed from here would be greatly appreciated Bruce |
| Bruce Mills |
| Bruce, I think you tried to many things mentioned above and sometimes this makes things worse. The VDO Tachometer has to have the Diode I mentioned above. This is a First thing to do and a must. I believe I got a pack of two for $1.25 When I rewired my car and ignition I removed the voltage stabilizer, the resistance wire (which goes all the way to the front of the car and back again), and removed the ignition drive/ballast resistor. Take a look at: http://www.treasureislandjewelers.com/1977mgbSupercharger.htm You will see a picture of the wiring and how I made up a new relay just for the ignition wiring and electric fan. None of my ignition wiring goes to the old fuse box. All wires a seperate and fused. Ray |
| Ray 1977mgb |
| Ray You are probably right. I tried too many things. I was looking at it as 2 problems when it could actually be just one. Add the diode first and go from there. At least I know all my connections are sound! When I rewired the electrical back under the dash I also installed relays for the high and low beam as well as the cooling fan and 2 ford truck horns. (People would laugh at my original horns, now they get out of the way. So I will get the diode in and then go from there. Hopefully it will solve the fluctuating speedometer problem as well. Thanks Bruce |
| Bruce Mills |
| Disconnect the sense wire(s) from the tach, so only the 12v and ground wires are connected, and try again. If it still happens the problem is in the 12v and ground supply, check with a voltmeter connected between the 12v battery post and the 12v connection, and the ground post and ground connection, to see if there is any variation with the other things on and off. If you can't see any variation then I'd power the tach direct off the battery and see if that makes any difference. If that doesn't, then I'd suspect the tach, but how I couldn't say. If it doesn't happen with the sense wires disconnected then there must be a problem with how they, or the ignition, is wired. Do the same voltage tests at the ignition coil and distributor body. Oh, and check again with first 12v then ground disconnected from the tach. And if it still does it, call your car 'Christine'. |
| Paul Hunt 2 |
| Hi Paul The diode has arrived and I will give that a go and see how I get on. If the diode doesn't do the trick I will follow your instructions and report back. My x-wife's name was Christine ;) So it is all starting to make sense now ;) Thanks Bruce |
| Bruce Mills |
| Bruce, Let us know if the diode fixed your problem. Ray |
| Ray 1977mgb |
| This is all to do with bad connections throughout the system, from battery feeds on. Nothing to do witj alternator per se. And possibly you have the tach signal and power reversed. I will send you comprehensive troubleshooting instructions, follow them first. FRM |
| FR Millmore |
| Hi Ray, yes I will post the results of installing FRM, Thanks very much for the information. It appears I have some work to do. I have Oxgard on list of things to try and find tomorrow. Bruce |
| Bruce Mills |
| I managed to spend about 6 hours trying to chase this problem down today. I disconnected the sense wire and with lights on, fan on , applied the brakes and the tach jumps Sense wire disconnected, lights off, fan off, applied brakes and nothing. Went for a drive and applied the brakes and the speedo jumps. With the tach green wire disconnected and speeding on the brakes, the speedo jumps. I installed a diode and it didn’t make any difference. Still applying the brake or turning on the fan cause’s the tach and speedometer to jump. Made worse by having the lights on. I split off the white wires and ran over to the speedo and tach but produced the same results when the brakes were applied. So using Green or white as a power source produces the same result. I hooked one probe of the voltmeter to the battery + and one to the tach green wire and got a reading of .92V. Turning on the fan = 1.69V and the lights 1.78V. I hooked one probe to the battery – and one to the tach – and turning on the fan or the lights nothing changed. Advice/input is much appreciated. |
| Bruce Mills |
| Bruce- ****You have multiple bad connections: the instructions I sent will let you find them - You MUST be methodical - jumping around just wastes time. Start at the Batt, fully charged, with everything ON and engine at 2500. Do the same checks on the ground side. No single drop should be more than 0.1V, and total drop should be max 0.5V From B+ to any hot, or from B- to any ground, with all circuits fully loaded. You can do a measurement every 5 sec, as written. Start with big sections and work to smaller ones, again as written. I disconnected the sense wire and with lights on, fan on , applied the brakes and the tach jumps. Sense wire disconnected, lights off, fan off, applied brakes and nothing. Went for a drive and applied the brakes and the speedo jumps. With the tach green wire disconnected and speeding on the brakes, the speedo jumps. I installed a diode and it didn’t make any difference. Still applying the brake or turning on the fan cause’s the tach and speedometer to jump. Made worse by having the lights on. ****These all indicate bad grounds in the dash/gauges, etc. Run a dedicated ground (Black) from each gauge case and ground terminal if any, AND from the dash itself, to the main ground point under the dash. Check V between this and Batt- should be less than .2V with everything ON and engine at 2500rpm. I split off the white wires and ran over to the speedo and tach but produced the same results when the brakes were applied. So using Green or white as a power source produces the same result. ******** the (major) power side problem is before the browN turns White at the IGN switch. There may be/are additional problems in the W or G after the switch. I hooked one probe of the voltmeter to the battery + and one to the tach green wire and got a reading of .92V. Turning on the fan = 1.69V *** Definitely the N/W/G circuit and the lights 1.78V. ******* Either the N or R feed to the lights AND the dash grounds. I hooked one probe to the battery – and one to the tach – and turning on the fan or the lights nothing changed. ******* Either you wrote this wrong, measured wrong, or I am not reading it right - it is impossible given the last two statements. FRM |
| FR Millmore |
| Assuming the lights are powered from the brown circuit as per factory wiring then for an additional 1v+ drop measured on the green relative to the battery there must be a bad connection between the brown and the battery, and not just in either the white or the green. With such a drop just from the lights, if this were caused by a battery cable connection, there is no way it would crank. So the chances are the bad connection is at the solenoid. For a 74.5 this *should* be where the battery cable and 2 or 3 browns all bolt on to the same solenoid stud, so disconnect the battery ground cable, then remove all the lugs from the solenoid stud, polish them all with emery or wet and dry, then reassemble with copper grease. This is the most likely place, but there is a sealed multi-way connection in the brown behind the dash which could be causing problems, but I have never heard of this happening before. With fan and lights on you could do a series of voltage measurements wrt to the 12 battery *post* - on the 12v battery clamp, the cable where it is bolted into the clamp (non-moulded connectors), the battery cable lug at the solenoid, the solenoid stud it is connected to, and the brown cable lugs on the same stud. With care and a thin probe you should be able to touch each of these lugs individually while they are all bolted together. A few years ago my roadster had the local garage completely stumped when just such a bad connection happened to develop at that point during the annual test. You need to get rid of what I believe is the main problem in the first paragraph above, check you have got rid of that volt-drop, then see which circuits if any still affect the instruments. I don't believe that bad grounds are the *only* problem (and may not be at all) because with the brake lights at least, these are affecting the instruments but they do not have a wired ground, they pick it up from the physical fixings to the body. There may be additional bad connections in the brown feed to the ignition switch, the switch itself, and the white and green circuit including the fusebox, but other than causing a small drop in voltage at the battery terminals, and hence all circuits, turning the lights on and off won't cause any change in voltage at the instruments. You should also do similar voltage measurements between the battery ground post and ground connection to the instruments, but again any bad connections at the battery or either ground strap would affect cranking. |
| Paul Hunt 2 |
| Thanks Guys It is back to the drawing board. I will post my results. Bruce |
| Bruce Mills |
| All Paul Said is correct, and contained within what I said. It needs a bit more thought though. On the main browN feeds, if this car (not certain when this started) has the flat plastic 4 way brown connector just behind the fusebox, it is a prime source of this problem. "I don't believe that bad grounds are the *only* problem (and may not be at all) because with the brake lights at least, these are affecting the instruments but they do not have a wired ground, they pick it up from the physical fixings to the body." PH It's almost certainly not the "only" problem, and it is precisely things that don't have dedicated grounds that cause this sort of trouble. The principle applies in the boot as well - brake lights grounding back through the park/TS or fuel gauge circuit can get you. The lamps ground through several dissimilar metal junctions in the lamp body, then to the shell. It's a common source of these strange events, especially in the front park/turn lamps, where there's more corrosion. I solder ground leads directly to the bulbholders as a permanent cure. Poor grounds through instrument cases and dash mountings result in applied voltage variations when the dash lights are on, or when any other thing on or grounded through the dash is on. FRM |
| FR Millmore |
I didn't have time to look at the car tonight. But I would like to talk a little about the grounds. When I first wired in the VDO gauges I ran a grounds from each gauge’s ground terminal (plastic gauge case) to the big common ground under the dash (instrument panel) soldered them together and screwed them into a freshly sanded piece of the body. When all the problems started I separated the tach and the speedo from the rest and grounded them to a different spot, (Gas gauge and temp gauge work fine. Although the gas gauge seems much more sensitive than the Smiths) actually where the voltage stabilizer use to be. I am not saying there are not other problems and I am planning on going through the rest of the electrical, following your article and Paul’s advise and replacing the connectors and cleaning up all the bullets and grounds. I wonder if separating the tach and speedo ground wires, drilling another hole, cleaning up the metal and attaching the ground wire from the tach would help? Because the following quote sure describes my problem “Poor grounds through instrument cases and dash mountings result in applied voltage variations when the dash lights are on, or when any other thing on or grounded through the dash is on.” My car does not have the flat plastic 4 way brown connector just behind the fuse box. It does have the sealed multi-way brown wire connection behind the dash, but it appears to be intact. which could be causing problem. Bruce |
| Bruce Mills |
| By "big common ground under dash" do you mean the bolted one above the wiper motor? Every other ground goes through that, and most people never find it. Since the grounds join at various places before that, you can get all sorts of backfeed. The dash light thing is a giveaway for this. Check V from any ground to Batt-, should be very near zero with a fully loaded system. Good idea to run a dedicated ground from the metal dash panel to the main ground point, as a backup for anything grounded through that panel - maybe dash lights. The harness sealed browN junction has never been a problem in my experience - I think the wires are actually welded there. Note that there are no separable connections within the harness covering, so no cutting. The other really big offender is the big wad of W wires at the IGN switch harness takeoff under the steering column. FRM |
| FR Millmore |
| What I meant about the brake light grounds was that problems with them (if they exist) can't be the cause of the *instrument* problem as there is not common circuitry to interact. This is apart from the main battery ground strap of course, but that would be having a far greater effect on cranking, probably to the point where it wouldn't crank at all. They can of course cause problems with other circuits like turn signals, stop lights and parking lights like glowing when they shouldn't or not glowing when they should, possibly varying from side to side. None of the lights in the rear clusters have wired grounds, and so cannot interact with the reversing lights, fuel gauge and fuel pump, which *do* have wired grounds back to a number plate bolt, as well as the number plate lamps on some years. Whilst problems in the brake light 12v supply i.e. the white and green circuits could well be causing problems with the instruments, because the main lights are causing a problem as well i.e. the greater than 1v drop, and these come direct off the brown, there must be a problem between the brown and the battery. Cleaning up the solenoid connections should be the first step, then do the voltage checks again. If the main lights don't then cause problems with the instruments, but the brake lights do, even when the instruments are powered from a white, then again the green circuit can't be causing the problem so it must be the white circuit, ignition switch or brown connections to it. With problems like this you have to deduce the most common factor, eliminate that, then deduce any other less common factors one by one if problems still exist. |
| Paul Hunt 2 |
| Progress report. Some progress has been made. I had an hour to spare the other night thought I might as well start with the basics. I cleaned up the battery terminals and applied dielectric grease. I also cleaned the ground in the trunk and cleaned all the starter wires and the ground located above the wiper motor and replaced the connector. But it didn’t make any difference. I have also been in touch with VDO tech support and sent them a picture of the back of the dash after I had re-wired it. “From the photo, I can see his problem. He has an individual ground wire for each instrument, which is good...but they are all going to a common point on the back of the instrument panel. The problem is that A) the instrument panel is not necessarily a good ground b/c the panel attachment points to the chassis are most likely poor (a few screws, and painted surfaces) B) all of the terminal ends are meeting at the same point and using 1 screw/bolt to make the connection. My recommendation is to have the instrument either a) ground to common point to the battery or engine -using 1 terminal (such as having all the wires crimped to a 4 or 6 gauge eyelet terminal-the kind you would see on a lawnmower battery b) have each instrument ground separately to the engine or battery using 14 gauge wire.” So tonight I continued with my basic stuff and cleaned all the white wire and replaced the connectors. I followed VDO’s advice and ran a #12 wire from the ground wires located under the dash from the instrument panel, and ran it back to the battery. I also ran the tach and speedo wires back to the battery and I made some progress. At an idle the tach will just slightly fluctuate when I turn on the fan. The speedometer still fluctuated drastically when the fan or brakes are applied. Saturday is the next time I will have some time to really get into it and I plan on finishing up the basic cleaning and replacing the connectors on the brown wires under the dash and with my volt meter (if I am still having problems), systematically trying to chase this down. Thanks Bruce |
| Bruce Mills |
| Paul- With all due respect, ANY bad connection anywhere in the car can produce spurious signals that get back to electronic instruments. The typical cracked connector sleeve can for instance cause a high frequency arc that will not show up in the brake lights, since they have thermal lag, but the tach/speedo may well sense the arc. Bad grounds at or within the lamp bodies could do it too. Lack of wired grounds here is a major PITA - I install them if I have any problem with front or rear lights, indicator flashing rates, etc. This implies that any fault in any circuit that causes the problem may be a source of the problem. Everybody would do well to follow the advice of your closing sentence, and the same advice is in the info I sent Bruce. Bruce: The VDO tech people are nuts - 14 gauge is not necessary for instrument grounds, it's a waste of effort and clutters the area with stiff and nasty wires, making everything more difficult. 20 ga would be fine, but it's not necessary, if everything else is correct. They first condemn the multi ground termination at a single point in comment B, para 1, and then tell you to do the same thing at the engine or Battery in comment a, para 2. The only difference is that you are bypassing bad connections at other points which WILL come back in the form of other problems, like slow turn indicators or poor brake lights. I already told you to run a dedicated ground from the panel to a good ground point. The wiper ground point should be perfectly fine, IF the main battery and engine grounds are good. Why do you fixate on the browN wires? Any wire in the instrument circuits - N, W, G, or B (ground) can cause the trouble. And quite possibly any other wire or connection on any of the trouble inducing circuits can do it too. If you quit fooling around hit & miss, and measure things as instructed, you will KNOW where the problem(s) are, and can take appropriate action. START with the meter. You could for instance, measure across the wiper ground point connection and know immediately if it is "good" or not. Or from the dash panel to B-. It takes far less time to learn the procedure and perform it than to write all this out. FRM |
| FR Millmore |
| If I'd picked up from your earlier posts that the gauges were using a ground from the dashboard instead of the body, then I'd have recommended against that. The original instruments use the body ground by the wiper motor, and that should be the one to use, making sure it is sound, precisely because the dashboard 'ground' is not reliable and dependent on paint and physical fixings. This is one of the classic problems on the MGB - where the tach stops operating when the dash lights are turned on. As FRM implies big wires are no use at all if the problem is bad connections, the current for the gauges is negligible - relatively - making the gauge of the wire immaterial. Unless and until you deal with the main brown connections at the solenoid, and take before and after measurements of the volt-drops between the 12v battery post and the 12v supply to the instruments, and the ground supply ditto, you are poking and hoping. |
| Paul Hunt 2 |
| I appreciate the help you guys are giving me and apologies for not using the meter sooner. I can run wires, I can inspect and replace connection and that haphazard way of solving electrical problems has worked for me up to this point. (Eventually I find the problem, but not this time. I can take readings with he multimeter but my problem is I do not understand what the multimeter is telling me. For example tonight I took some readings Battery 13.55 Battery Ground to starter wire 13.26 Alternator to case 13.52 Alternator to body round 13.46 Battery to ground cluster by wiper 13.13 Brown wire cluster 13.39 Brown wire cluster with lights on 12.79 Brown wire cluster with heater fan on 12.70 Brown wire cluster with flasher on 12.67 Brown wire cluster with brakes on 12.58 Note I cleaned the bullets and replaced the connector. As FRM says, (if I understand it correctly) the tests with the brown wire should equal close to the battery 13.55? There is a difference of .94V. Is that within parameters? If I understood what the number mean I am sure the answer is staring me in the face but where do I go from here? Thanks for your patience and help Bruce |
| Bruce Mills |
| Bruce- Keep at it guy, you'll get it. I used to rely on common sense, visuals, and experience. When I got to the Jag mentioned in Electrobabble, there was simply far too much wrong to do that. I found that the systematic approach is much quicker, and you KNOW where the problems are/were. There are fundamentally two tests. First is measuring from any ground to any hot, and comparing it to the reference of Batt or Alt, WITH THE SYSTEM LOADED. That means Batt fully charged, engine at about 2500 so the alt is putting out whatever it does, and everything ON. This gives you a total reading of whatever group of things are "in the loop". Ideally these readings are all the same as the Batt or Alt V. I don't accept anything that is more than 0.5V off the reference. Second is measuring across a connection or a section of circuitry. This gives you the actual drop in that section. You can get down to single connections, and the reading is ideally zero. If more than 0.1V on a heavily loaded circuit like lights, I want to know why. These are typically 0.01V if all is good. Test one is a total voltage available, and test two is the total voltage loss, so any test one reading represents B or A Volts, minus the test two loss for the section under test. ******* I'll run through your readings, with probable trouble points. G refers to ground, H refers to power at that point. Battery 13.55 - Assuming the engine is running above 2000, a little low, likely because the Alt is either not charging enough, or it's getting lost on the way. OK for idle. This should be measured on the actual Batt posts, not on connectors. Battery Ground to starter wire 13.26 - This should be identical to the B voltage, unless the starter is in operation. Unacceptable Alternator to case 13.52 - low if over idle, should be 13.8-14.5 May increase or decrease after the other main connections are fixed - recheck after. Alternator to body ground 13.46 - should be the same as B V, 0.9V = unacceptable; check A case to engine, then engine to body ground point(s) Battery to ground cluster by wiper 13.13 - .42 difference = unacceptable All the above show a problem in the main grounds B>chassis>engine, or power leads from B>Solenoid>Alt Direct measurement from A+ to B+, and from A- (case) to B-, will tell you how much of the loss is in the power and ground sides respectively. Test connections must be clean and solid. Internally Corroded cables are a common source, Check V cable end to end loaded - should be zero. This condition MUST be fixed before you go further. ******** Brown wire cluster 13.39 - to what? Brown wire cluster with lights on 12.79 Brown wire cluster with heater fan on 12.70 Brown wire cluster with flasher on 12.67 Brown wire cluster with brakes on 12.58 All these readings are unacceptable to horrid - and typical. You can see the effect of loading the circuit. These readings will come up after you fix both grounds and power side in the first section. If there is still more than 0.2V difference from the B or A reference, then there is a problem in the N wire circuit itself. I'm not happy if I can't get this drop to 0.1V - but sometimes you have to live with it,IF you know why it's there. Test two from point to point will isolate it. ****** After you get all this fixed, you can start on the White circuit, from the N at the IGN switch. Then follow with the Green circuit, from the W at the fuse box. Do not forget to check the dash ground to Batt-; should be zero, if not fix the ground or run the discrete ground wire suggested. Take two aspirin and call me in the morning! FRM |
| FR Millmore |
| Bruce, When I wired up my dash with all VDO gauges I wired all grounds from one instrument to the next so that my end result is one black ground wire that I attached to the back frame metal panel that is grounded. I can take off my dash by disconnecting the three plugs from the harness, One (1) black ground wire which is attached to the back frame of the car, and two wires from the speedometer. That is it and the complete dash can be removed easily. The ground wire is the same size as any other ground wire.I used the plastic tie wires to make a nice and neat set up of the dash wires. No mess at all and very very clean looking. Ray |
| Ray 1977mgb |
| Hi Ray Thanks for the info on the grounds. I originally took the grounds and ran them to a common point. Now with my new found knowledge I have learned that the point I chose perhaps wasn't the best place. But this is easily rectified. I currently have a couple of # 14 wires running from the back of the dash to the battery ground. This was done to appease VDO tech. If in the future I had tried everything there was to try, and nothing worked then the gauges would have to be suspect. So I wanted to be sure VDO tech was happy that I had followed their instructions. As soon as I solve the problem I will be re-routing the grounds to a more convenient place. Progress report. I spent Sunday after noon checking into my problem.The mini hi-torgue starter was loose at the adapter plate so I pulled the starter and tighten that up. I though a good place to start checking voltage was to check the battery voltage, separate the wires at the stater and check the voltage there. The main wire and the battery had the same voltage 12.55V, I installed the alternator battery and the same voltage was present at the alternator . So far so good. I installed the wire which feed the cockpit accessories and had an immediate drop of @.10 volts which was constant from the main feed through all the accessories (in the cockpit) the brown wire feeds. I was unable to get any power through the system (or the car to start) and had to take a taxi home. I havn't had time to get back to the car but I have had some time to think about it. From the accessory brown wire is another smaller brown wire, this did run to the original alternator. I have installed a new Wilson alternator and can not remember what I did with this brown wire. I suspect I taped it off and left it in the engine compartment harness. So this could be a cause of DPO (with p= presnt). At any rate I could have discouvered the source of a major problem, now I just have to fix it. As FRM stated mentioned, 'it can take longer to fix the problem than to find it'. Once I can get this problem sorted out, I am going to run through the rest of the systems with my voltmeter and FRM's instructions and check over all my 32 year old elctrical connections. Thanks again. And I will post my results. Bruce |
| Bruce Mills |
| AS FRM says you must measure with loads connected. Also you list several loads but do not indicate whether these were one at a time or progressively added. The greater the load the bigger the indication and the easier to read on a meter. With as much load on as you can, you need to measure the voltages: 1. Between the battery posts (battery voltage, your starting point). 2. Between the battery connectors (battery voltage, different to 1 shows losses in the connectors). 3. Between the 12v battery post and shiny metal somewhere on the body (battery voltage, difference to 2 shows any loss in the battery ground strap). 4. Between the 12v battery post and the starter solenoid stud (losses in the battery cable and its connection to the stud). 5. Between the 12v battery post and the alternator (losses in the battery cable, its connection to the stud, and the alternator brown to the stud). 6. Between the 12v battery post and the brown at the fusebox, and ditto the white and the green (losses in the battery cable, its connection to the solenoid stud, connections of the browns to the stud. Additionally in the white and green at the fusebox compared to the brown at the fusebox shows losses in the ignition switch and fusebox). 7. Between battery ground post and your instrument grounds (compared to 3 shows losses between the two ground points). BTW, it is normal for the tach to jump when turning on the ignition, it does respond to pulses, and this is one such pulse. And of course, you could have a faulty speedo - oversensitive to voltage variations. I'd have thought they should be capable of ignoring supply variations of a couple of volts at least. |
| Paul Hunt 2 |
| Well this is getting more interesting every time I work on the car. As you remember last week I had to leave the car in the shop and take a taxi home because the car would not start so I was unable to load up the system to look for my problem. Last week I pulled and re-tighten the bolts which hold, the mini hi-torque starter to the adapter plate. After re-installing the starter I had the same voltage readings at the starter with the main battery cable connected and the main brown alternator wire connected but as soon as I installed the brown wire which feeds power to the cockpit electrical system and the alternator plug my power dropped drastically. Enough so that the car would not turn over and the alternator light would not light. I have spent a most of my free time in the last few weeks trying to sort this problem out and after last weekend I was pretty discouraged and had to take some time to catch up on other chores. Today I got back to the car. Last week I reported I might have terminate a brown wire in the harness when I changed over to the Wilson alternator. Well it is actually a GW wire which used to run to the original coil. I replace the coil with an internal ballasted coil so I capped the wire off at both ends in case I to replace the coil and only had access to a non ballasted coil. (One of those emergence road side repair ideas) I put all the wiring back together and ran some tests. (Unloaded and car not running) Battery + post to battery – post 12.61 V Battery + to body ground 12.61 V Starter wire to body ground strap 12.61 V Alternator to ground 12.60V Alternator to case 12.61 V So I turned the key, the alternator light came on, the fuel pump came on (they didn’t last week) and it tried to turn over and then alternator light went out, car would not turn over. Nothing. I checked every thing again and had a repeat of last week. As soon as I put the wire on which feed the interior electrical system and the alternator plug the power just drops. Last week when this happened I pulled every brown wire in the cockpit in an attempt to isolate the problem but nothing changed. (That is when I left) The is week I pulled the plug wire to the alternator and checked the voltage again. Now I registered the same voltage (with .01V). I checked the alternator plug and it too was the same voltage. This leads me to believe I have a dead short in the alternator. Could this have been causing all the trouble I have been experiencing? Since it looks like I have to replace the alternator, I would like to replace it with a smaller more power full one which is easily obtainable in North America. Chrome would be nice Could any one make any suggestions. |
| Bruce Mills |
| The Bosch alternator for a 1980 Ford Fiesta is a popular option because it doesn't require any modifications to fit and plug in. May have to buy a pulley if the replacement doesn't come with one. For more power, many have used the Delco alternator. The modifications are fairly easy. http://www.onefastmgb.com/alternatorupgrade.html Parts stores should be able to tell you what alternator is available in chrome. |
| Kimberly |
| Bruce- You MUST understand that unloaded voltage readings mean almost nothing. A single strand of wire will give perfectly good readings without load, but will drop to near zero under load. A bad connection will do the same. It does sound as though the alternator may be DOA, unplug it and try to start the car so you can get loaded readings on the rest of the system. If that fails, then try for loaded using battery only with it not running. If you can get good readings at the solenoid terminal, then disconnect all browN wires there and run a jumper to the coil+ and fuel pump W power for testing. Then reconnect brown wires one by one and test each circuit in turn. If there is a rectifier fault in the Alt, then it may be feeding AC through the system, which would certainly drive the Speedo & Tach nutso, since it is pulses. Take the alt to a shop and have it tested. Note that a bad connection between the Batt & Alt could (have) kill(ed) the Alt - "Do not run engine with battery disconnected". If any wire is connected to the Alt, all of them must be, and the browN circuit must be good all the way to the Battery. FRM |
| FR Millmore |
| Everything dying when you turn the key to crank, which magically repairs itself a while later, is almost certainly a bad connection at the battery or solenoid (something I have been banging on about since this first cropped up). It can supply ignition current, but as soon at this becomes starter current the tiny bit of metal that is all that is left of the connection 'burns away' like a fuse and everything dies. Also banged on about is the futility of looking for bad connections with no load on the circuits, all that will show you is a complete disconnection. What does "Now I registered the same voltage (with .01V)" mean? Should it actually read "within 0.1v"? Or did you measure 0.1v? The former is as meaningless as all the other off-load measurements, the latter indicates a problem at the battery or solenoid (bang, bang). In either case I don't see that indicates a faulty alternator, particularly with a stopped engine. If there were a short in the alternator the wires would be burning and the battery boiling. The only way a dead-short alternator *wouldn't* fry the wires, is if there is a bad connection like at the solenoid or battery as well! (bang, bang). Note that you say "As soon as I put the wire on which feed the interior electrical system and the alternator plug the power just drops." i.e. as well as connecting the alternator when the power dropped, you connected the interior electrical system. So any load coming from the interior electrical system which could be as little as the interior light, is enough to drop all the battery voltage across the bad connection, i.e. at the battery or solenoid (bang, bang). |
| Paul Hunt 2 |
| Hi I guess I am not explaining myself clearly Paul. Lack of proper terminology I guess. Basically I started the car as normal last weekend, drove to the shop to fix the loose starter adapter. While the starter was out I again cleaned the starter terminal's and the ends of the wires which attach to the starter. I also cleaned up the battery terminals again and insured the negative lead to the body was attached to bright metal I then decided (right or wrong) to check the voltage at the battery. My thinking is that the readings through out a circuit should be close. If the reading are not close without a load, it would be worse under load. Battery read 12.61 V Battery cable to starter (at starter end read 12.61V) Hooked up wire from battery to starter and it read 12.61V Hooked up big brown wire which goes to the alternator + and it registered 12.60 to ground and 12.61 to the alternator case. Now this is where it gets interesting. I hooked up the connection at the starter which has 2 wires on it, a brown wire which goes to the brown wire welded together under the passengers side dash. The other brown wire goes up to plug on the side of the alternator and a brown/yellow wire joins the plug I checked the voltage at the starter again and the voltage had dropped drastically. Like down to 5V at the starter terminal. I unhooked the big wire/smaller brown wire connection at the starter checked the voltage at the starter again. It was back up around 12.58V. I pulled the plug (brown wire and a brown/yellow wire) at the alternator, hooked the big brown wire and smaller brown wire back up and checked the plug and got a reading of 12.58V. Same as at the starter terminal. From this I concluded that there was a problem at the alternator. As I mentioned I have thoroughly cleaned the starter wires and terminal and since I get an identical reading at the battery and starter terminals without the suspected big brown wire and small brown wire hooked up, I am assuming that the starter connections are ok At this point I am unable to test the system with a load because the car won’t start. Your right it actually have read ‘within 0.1V As the reading at the battery was 12.61V and the reading between the alternator and ground was 12.61V >“In either case I don't see that indicates a faulty alternator, particularly with a stopped engine. If there were a short in the alternator the wires would be burning and the battery boiling.” The was no smell of burnt wiring > The only way a dead-short alternator *wouldn't* fry the wires, is if there is a bad connection like at the solenoid or battery as well! (bang, bang).” As I mentioned I have thoroughly cleaned this area twice. Am I missing something here? >Note that you say "As soon as I put the wire on which feed the interior electrical system and the alternator plug the power just drops." i.e. as well as connecting the alternator when the power dropped, you connected the interior electrical system. So any load coming from the interior electrical system which could be as little as the interior light, is enough to drop all the battery voltage across the bad connection, i.e. at the battery or solenoid (bang, bang).” When this originally happened last weekend (not such a drastic drop) I unhooked every brown wire in the passenger compartment trying to isolate the problem Including the starter relay The four relays’ I have installed, hi & low bean, cooling fan and horn relay. The ignition switch The headlight switch The hazard flasher And pulled the brown wire of the fuse block as well. Nothing made any difference. I have pulled the alternator and taken it into the only place in my small community to have it checked out. Unfortunately their test has broken down and they are not sure when they will have it working. So I have no way to test the alternator. All I can say Paul is I again will check the solenoid/starter wires. FRM I do understand that testing the system under load is the optimum was to test it but under load or not, should the reading not be fairly close? >It does sound as though the alternator may be DOA, unplug it and try to start the car so you can get loaded readings on the rest of the system. If that fails, then try for loaded using battery only with it not running. I will give this a try tomorrow after work >If you can get good readings at the solenoid terminal, then disconnect all browN wires there and run a jumper to the coil+ and fuel pump W power for testing. I am not sure I totally understand this “and run a jumper to the coil+ and fuel pump W power” Do you mean run a wire from the coil + to the fuel pump white wire? Thanks guys. I’ll keep plugging away until I can get this sorted out. Thanks again Bruce |
| Bruce Mills |
| Bruce- "I checked the voltage at the starter again and the voltage had dropped drastically. Like down to 5V at the starter terminal. " This means that there is a bad connection before the solenoid (batt+ cable), or on the ground side - main engine and battery grounds. It is not uncommon to have corrosion inside a battery cable, several inches from the battery terminals. This problem MUST be fixed first! The alternator should not be pulling ANY current with the key OFF, and a very small amount through the NY indicator wire with it ON. This is why I think the alt may be bad. A dead short would burn wires, as Paul said; but a bad rectifier might pull a significant amount without cooking stuff. "I do understand that testing the system under load is the optimum was to test it but under load or not, should the reading not be fairly close?" NO, it could be equal with no load, to the limits of a normal meter; and near zero under even a small load if there is very bad connection or wire in the circuit. >If you can get good readings at the solenoid terminal, then disconnect all browN wires there and run a jumper to the coil+ and fuel pump W power for testing. "I am not sure I totally understand this “and run a jumper to the coil+ and fuel pump W power” Do you mean run a wire from the coil + to the fuel pump white wire" Jumper from solenoid Batt cable to both Coil+ and to W to pump. You'll also need a temporary jumper from the Batt cable to the White/browN terminal of the starter solenoid to make the starter operate. The engine will run like this, but nothing else will work. FRM |
| FR Millmore |
| OK Bruce, in these very long posts I missed the cleaning of the solenoid connections. But to reiterate, you are likely to read the same voltage along a length of wet string with no load, which will be very different to when it is carrying a load. It's Ohm's Law. A bad connection is a resistance that shouldn't be there. Pass no current through it, or the microscopic current that a voltmeter draws, and the volt-drop will be nothing to negliglible, hence you will measure the same voltage both sides of the fault, even with a digital voltmeter. Try and power a load through the bad connection, and take a noticeable current, and you will get a noticeable volt-drop at the bad connection, hence less voltage at the load compared to the source. the greater the load i.e. the more things you switch on, the greater the current drawn, the greater the volt-drop at the bad connection, and the lower the voltage left to power the circuits past the bad connection. So cleaning isn't enough, after the cleaning and reconnection, with as much load as you can reasonably manage, you have to measure again. To test the battery and starter connections I disconnect the coil, and while cranking measure the voltage between the 12v battery post and the solenoid stud, and again between the battery ground post and the starter body. You should be able to get these down to 2 or 3 tenths of a volt each, but I have seen as much as 3v lost in one 'leg' and it still allowed cranking albeit slowly, and greatly dropped the voltage available for the ignition. This is with starter current, which can be 100 amps or more. Compare this to the nigh-on 2v lost between the battery and the green just with things like lights and fan, and you have a very bad connection indeed. This is highly unlikely to be in the main battery and ground cable connections or it would affect cranking so much it wouldn't turn over at all. If by connecting the brown wires the voltage at the starter dropped to 5v, then, if it still cranks OK, then the only place the fault can be *is* at the starter connections. However whether it is still cranking or not I don't know, neither do I know whether the battery is still any good or not. Any fault which drags the battery voltage down to 5v, without there being bad connections, is very definitely going to be drawing enough current to burn wiring, even 100 amps plus only drags a good battery down to 10v, and if you used the brown wires for that would definitely burn them. Even headlights should only drop the battery voltage a couple of tenths, and cause virtually no volt-drop anywhere in the wiring or connections. If you can still get the voltage to drop at the starter to 5v (with respect to the body I suppose), and there is no smoke or flames, then while it is in that condition measure the voltage between the battery posts, the battery connectors, then between the battery 12v post and the body, and the battery ground post and the solenoid stud. From what you say it really can only be in one (or more) of half a dozen places around the battery and starter. |
| Paul Hunt 2 |
This thread was discussed between 06/05/2007 and 12/06/2007
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