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MG MGB Technical - Alternator problem
|Well, I think I have a problem. I don't think my alternator, a normal Lucas 16 ACR, is up to snuff. I am finding that even on shortish trips of a few miles the battery voltage keeps getting lower. Down to 10 volts or less. One one particularily nasty night it was reading 9 or so when everything was on the the blinkers turning on and off made a noticable movement on my volt meter.|
I have upgraded all the wiring (and Advance Autowire loom which has MUCH fatter conductors than stock) and everything is relayed. I also upgraded my headlights to halogens and I tend to drive with them on all the time for visibility reasons. I also have a aftermarket electric radiator fan.
The battery is new and should be good (it's an Endurant Crank Master) and when the car isn't used it is always connected to a battery conditioner to keep it charged up.
Is there any way I can test the alternator? I guess I could find a decent ammeter and connect that into circuit to see exactly what sort of current I am pulling.
The alt itself does seem to charge as I can see the voltage jump to just under 14 when the revs increase. The ignition light behaves as normal. On when you start the car then it goes off once you increase revs and the alt kicks in. It doesn't come on when driving.
I know I probably want a beefier alt but would like to test this one before I go that route. There's lots of info in the archives about replacements I know but unfortunately the numbers aren't internationalised and the cars people suggest to get alts off aren't easily available here. The local equavilent Bosch is about $450NZ new here. More than twice a new Lucas!
Things I've been caught out with in the past are a loose fan belt and worn field brushes. The fan belt is easy to check / fix, but the brushes are much harder. I think the alternator needs to be disassembled to get at them.
|Hi Herb, fan belt is good. It was the first thing I checked. Brushes is a possibility. I am sure I saw the voltage flickering once even when the engine was above idle revs.|
The alternator was rebuilt during the 6 years I spent restoring the car so theoretically it should be as good as new!
I guess I can take it back to the auto electricians who did it? They are usually very good so I wouldn't normally question their work. Would they able to test it for me?
Yes you could take it back to the auto electrician for testing. I don't know how they would test the brushes, though, other than a test that the alternator doesn't produce full power. According to the manual the test of the brushes is to measure how far they stick out of the carrier (0.2" min.), which unfortunately requires disassembly.
Years ago I had a Golf, with a Bosch alt. and it had a plastic "lump" on the rear, which was the regulator and brush assembly all in one. Very easy to remove and service. Every alt. I've worked on since has the brush assembly buried in the casing and requires hours of work to do a relative simple job.
|Simon. First, as I remember it, you have a rubber bumper, North American specification car. This should have an 18ACR, not the lower powered 16 ACR alternator. |
Second, there is a rather simple test to see if the alternator is charging at the correct voltage. Charge your battery. Take a voltage reading at the battery terminals, then on the clamps. The voltage should be the same on each reading and the volt meter should show 12.5 to 13.0 volts. Then, start the car and take a voltage reading on the battery terminal clamps and you should show 13.5 to 14.5 volts. If not, there is a problem with the alternator.
The last test is the alternator output measured in amps. I have always had my local mechanic make that reading because I do not have the correct measuring devices. But, on the 16AC alternator on my 68 BGT, the output was maxed at 17 amps (rather than the nominal 35 it should have been putting out--especially after having been professionally rebuilt). Like your indications, the alternator was not capable of keeping the battery fully charged and I replaced the 16AC alternator with an 18ACR.
You might want to do some simple tests/have them done and see what you can find out. The fellow who rebuilt the alternator is, frequently, the last one who will tell you that the root cause of your problem is a bad rebuild.
|Is the ignition warning light working? It needs this to start charging, without it you may have to rev it to over 3000rpm to get it to start, although once started it should then charge normally down to about 600rpm or so before it drops out. Other than the 14v you get occasionally the alternator clearly isn't charging, so unless it is a loose connection on the charge wire between the alternator and the solenoid the alternator is faulty. Unless you have significant extra loads either the 16, 17 or 18ACR will usually be fine except for the later cars with electric cooling fans which need the 18. |
According to Clausager all cars irrespective of market got the 16ACR in 1969, then the 17ACR in Feb 73, and the 18ACR in June 76.
The 17ACR is a bit different to the others in that it used battery sensing to control the voltage regulator and not machine sensing as before and after. This uses an extra, standard gauge brown wire (making three wires at the alternator in all) which goes down to the solenoid as well as the main heavy gauge brown output wire. If your new harness doesn't include this wire you will almost certainly have charging issues, unless you link the two spades at the alternator together.
|I owned a '75 MB 300D, several years ago, that had similar problems. The electrical shop, that I deal with, rebuilt the alternator, only to have it fail once again. This happened 3 times. On the 4th go around, I watched them run the alternator up to 110% of power for 30 minutes. This time it passed. It turned out that they had received a bad batch of components which they used to rebuild the alternators. RAY|
|Hi chaps, thanks for the advice. Les, you're right about the spec of my car. It's been so messed about before though nothing is standard on it so having the wrong alt doesn't surprise me. I will try the tests you mention tonight. Currently the car is in bits with no oil and water as I am replacing weeping frost plugs (using your JB Weld tip actually)!|
Paul, the alt light does work. I replaced all the dash bulbs with LEDs except for that one which needs to be a normal bulb to work correctly of course. The light comes on with the ignition on and the engine not running. When you start it I do need to blip the throttle to get it to go out. Then it generally stays out although sometimes it will come back on at idle which I have set to around 650 at the moment. All the connections look good but I will check them again and make sure the bulb is firmly screwed in!
I replaced the two yellow original fans with one, big shrouded fan behind the radiator. I am not sure how many amps it draws but it is quite a few I believe.
When I did a quick mental calculation even a 35 amp 16ACR should be enough for a fairly standard B I think?
The 62-74 Bentley reprint of the WSM has all the tests for up to 16ACR alts. (the 75-on version leaves out a whole bunch of info from earlier cars that you really need to work on an MGB) Not certain of differences for the 17 & 18, but it should put you on track. I think that testing is the same as the earlier 16 ACR except for output. The 16ACR should be adequate if working correctly, but there are several faults that will result in working but low output, like 7 or 10A.
|I finally made some measurements.|
With the battery nicely topped up on the trickle charger I get 13.27V on the battery terminals, on the cable clamps and 13.27 on the starter.
With the ignition on it dropped to 13.08 and the alt light comes on.
Then when I cranked it it dropped to 11.60 before the engine fired (it's fires pretty much right away so no extended cranking).
The choke was at fast idle so the ignition light went out right away and stays out. Around 1000 RPM I was getting 13.80 volts. Revving a little higher and it would go up to around 14.04.
With the lights and the radiator fan on that would drop to about 13.08 at 1000 RPM.
At idle (around 650 RPM) it was 13.26. Turning the lights on dropped that to 12.47 (blast - I didn't try it with the rad fan going too).
I am sure the connections are all good and the ignition light is working. On the second time I tried it out I didn't use the choke to start so the engine fired and the light stayed on until I revved to about 800 RPM.
So I think it is generally working. But for some reason it just can't keep up?
|12.47 just with the headlights on at idle seems a little low to me, so the alternator may be suffering from low output. However the battery should have been left to 'rest' after charging, when you should find it dropping to about 12.8v after a few minutes. If you transferred it straight from charge to test then the 'excess' charge will have been giving you a falsely good picture, at least to begin with. Either that or your meter is reading high, in which case the 'on load' readings are definitely too low. But in any case you need to take each reading in at least two places - the alternator output spades and the fusebox brown, and ideally as the battery 12v *post* as well, as you could still have bad connections and hence voltage drops your readings aren't showing.|
It also depends on where your in-car voltmeter is connected. At the very least this would be an ignition powered circuit i.e. the white, and ideally a fused ignition circuit which could well be the green. These points almost certainly *will* show lower voltage than the alternator and the battery, and it is battery voltage which is important at least to guarantee starting next time. I've also seen digital dashboard voltmeters display something significantly lower to both digital and analogue external instruments, which was puzzling. The analogue instruments with the red and green sections and little by way of actual numbers make by far the most sense for a dashboard instrument, the more numbers the average owner is given the more they will worrit.
|Hi Paul, will have another go at measuring. I use a good Fluke meter so those readings are accurate. The in dash voltage meter is pretty vague as you say. It's connected to the ignition (I have a simple on/off key ignition). It doesn't have much of a scale. The thing that worries me when I am driving isn't so much the actual reading, more that the meter keeps getting lower and lower!|
I'm not the brightest bulb in the bunch when it comes to auto electrics, but I thought a good system was supposed to deliver about 14.5 - 14.7 volts when running. It looks like your highest reading just barely crosses the 14-volt mark without much load. That would make me suspect a weak alternator, regulator, or wiring.
|C R Huff|
|Simon. Your in dash volt meter registers system voltage. When the alternator is insufficient to operate the electrical system, and recharge the battery, on its own, the battery picks up some of the load. When the system is operating fully, or partially, on the battery, the system voltage will drop because the battery is being drained. I agree with Charley, the alternator, with its internal regulator, is not doing its job properly. You can get by with this for some time, especially if you recharge the battery between trips, but it does limit you to shorter, preferably daytime, trips. A properly working alternator, either new or rebuilt, is not that expensive considering the freedom it brings to your motoring.|
|Simon, can you fit a smaller pulley on the alternator ? I've seen this work before. Barrie E|
|It seems to be very common that while alternators may be rebuilt they dont actually work. Yours is in trouble and with 30 amps at 14v should handle the load. Even my car with the twin 6v and a dynamo is capable of keeping its energy budget balanced if you think about what you are doing|
|14.7v is the absolute max for an alternator, can be as low as 14.3v, but this is as measured at the alternator output terminals and on low load. The more load you put on the alternator, and the more wire and connections it has to go though before you measure it the more it will drop. |
It shouldn't need a smaller pulley to spin faster, if it can't maintain battery voltage (if that indeed is what isn't happening) then something is wrong that should be fixed, not worked round. An iffy alt with a smaller pulley probably isn't going to last long-term anyway. However you could have an *over*-sized pulley on it i.e. it is from another vehicle. I kept the Lucas alternator when I scrapped a 1980s BL Metro, the pulley on that is slightly larger than the MGB, but it (the alt) fits so can be used as an emergency spare. That needs more revs to get up to full charging voltage and current, but still has no problem maintaining above 12.8v.
|Well, I took the alternator off the car and toddled down to the local car sparky and he ran it up on his load tester for me. Worked fine up to 35 amps then the output started to drop a little. Seems fine for a 16ACR.|
Next weekend he said go back and he'll check my battery for me and he also suggested I check my engine earth strap, which was fine when I did.
|The engine earth strap is going to have far more immediate effect on cranking than it will on the alternator and charging. You need to measure the voltages along the circuit starting at the alternator spades, with everything turned on, and see what drops in voltage you get.|
|I've found a lot of trouble with earth paths on cars redone by enthusiastic amateurs, since they tend to heavily paint the pieces before bolting it all together. The factory bolted stuff together then spritzed a bit of paint on. Common problem areas are alt to mount to block, engine mount brackets to block, wherever the ground strap(s) is bolted at both ends. Recon alts are commonly lacquered, which stops them earthing unless they have dedicated earth leads, which I strongly recommend. Any connection carrying power must be bare metal at the contact point; this includes all contact points in bolted together parts.|
It all adds up to the fact that an alt that works off the car may not work as installed. I had a Jag here that showed 14.8 at the alt, but only 10V at the battery. There were drops of >1V at the alt mount bracket, and >0.5V at engine mount to block, block to subframe strap both ends, subframe to car strap both ends, Batt cable to car. When I was done the alt was back to 14.5 and the battery at 14.3 under full electrical load = running & everything ON.
Send me an email and ask for "Electrobabble" and I'll send you more than you want to know.
|That reminds me, I did hear of one case where the two halves of the alternator weren't making a good contact, and though both the front and the back are bolted to the block one of those wasn't conducting either, so there was a voltage difference between front and back, and of course it was the lower end that *was* making contact with the block.|
|Yep. Rebuilders frequently bead blast and lacquer the parts then assemble the alt. Then you get no continuity between front and rear cases. Especially a problem when it is an alt (eg. GM) that mounts entirely off the front case, while the regulator is in the rear case.|
|Will get the multimeter out again this weekend and do some measuring.|
|Just an update on this. Things seem to be better now. Perhaps it was due to the removing and reattachment of the alternator or checking that the engine earth was good but now things are behaving. I do see a small voltage drop at idle with the lights on and when the radiator fan kicks in. I have a biggish aftermarket electrical fan and it draws a good deal of current. Will measure it to see exactly how much. When I am actually driving and the revs are up the voltage is fine.|
Went to pick up a loaf of bread Saturday morning (from a place 2 minutes drive away) and it ended up taking me 2.5 hours. I just went on a random drive since the weather was nice, the top as down and the roads were nice and quiet!
This thread was discussed between 17/10/2010 and 07/11/2010
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