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MG MGA - fuel guage
| Hi all, here's a strange one my fuel guage is reading very oddly, when I first switch on the ignition but dont start the engine the guage reads correctly then after a few mins the needle starts falling until it gets just below half full, I have tried two different guages and they both do the same thing, also operating the wiper switch can send the needle either full or empty, I have had all the wiring apart to the guage and switches even swapped switches over all with no effect, anyone got any ideas regards gordon |
| g c pugh |
| Gordon Do you have a sender with a plastic float? The wire on mine wore through the float to make it a partial "sink" but it only behaved that way when I had the tank full. On half or less of a tank it behaved correctly. |
| dominic clancy |
| Hi Dominic I should have also mentioned that I have a new fuel tanl and sender fitted, the fault was there with old unit as well, so I think that I have elimated the sender and the guage and the switches must be something to do with voltages somewhere gordon |
| g c pugh |
| You have a poor connection somewhere in the circuit. Get out a wiring diagram identify and clean every connection in the circuit. Especially the ones you are absolutly sure are clean. Starting at the battery, cable, starter switch, wire, regulator, wire, ignition switch, wire, fuse box terminal, fuse, other fuse box terminal, wire, now we finally get to the fuel gauge, wire ,bullet terminal, wire, sending unit, fuel tank, ground to frame ,back to ground cable, battery. As you see there are almost 20 connection points in that circuit that can cause trouble. The trouble can be the accumulated resistance in all connection points and not just one bad connection. If you run a jumper wire from the body of the sender to the ground terminal of the battery and the gauge works then you have a poor connection between the tank and a ground. good hunting |
| R J Brown |
| Gordon Sounds like a classic case of escaping smoke from the wiring. I will look out for you on Ebay for a smoke recharging kit. They do come up occasionally. Steve |
| Steve Gyles |
| Clean the fuse contacts. Increasing resistance in the sender circuit would make the MG fuel gauge read higher, not lower. This must be a dirty fuse contact with a high resistance supply circuit. The fuel gauge requires only a small current to work, which can be drawn through a high resistance supply curcuit with a moderate drop in voltage. The gauge works on a balanced magnetic field which is not much affected by change of input voltage, until the power lever goes so low that the weak magnetic field cannot counter the force of the counterweight on the needle, which is when it starts dropping. When you switch on the wipers the heavier current draw immediately drops the voltage in the high resistance supply circuit, and the gauge reading dies with the failing power. |
| Barney Gaylord |
| Check that the gauge case is properly earthed. One of the two coils, I think the "tank full" ones is connected to earth through the gauge case. The wiper earth is in the same wiring run. |
| Malcolm Asquith |
| A suggestion: Try powering the gauge independently of the car circuit with an external 12V power supply. (A 12V battery charger will work fine.) Disconnect the wiring harness lead(s) from the fuel gauge B terminal and connect the neg. terminal of the external PS to the gauge terminal (assuming the car is still positive ground). Then clip the positive lead of the PS to the gauge case. (No need to disconnect the black ground wire on the gauge bracket thumbscrew.) If you still have the problem, it is with the gauge, sending unit and/or sending unit to gauge wiring. With the remote power supply, you can also remove the gauge from the dash and connect it directly to the tank sender unit with a short jumper to eliminate the wiring harness connection between the gauge and sending unit. Alternatively, you can run connections directly from the battery to the connections indicated, but in that case, it would be a good idea to have a voltmeter to confirm that the battery voltage does not drop when you perform the tests. Steve |
| Steve Brandt |
| Hi many thanks to everyone for their suggestions, it will obviously be a long proccess to find culprit, Barney I tried your idea of the fuses first as being the easy solution so I cleaned all points of contact but unfortunately didn't help, so back to square one. regards Gordon |
| g c pugh |
| Steve it's so comforting to know you are there for me Gordon |
| g c pugh |
| Gordon Am sure many others are reading this with interest too, but like me cannot help here. I reckon Barney's on the right sort of track. At least we now know it's not the fuses. Have you checked nobody has put an in-line fuse somewhere and hidden it for originality. I had that on my fuel pump circuit. That corroded fuse took a bit of finding I can tell you, but now I know just where it is, its clean and covered in petroleum jelly too! Good luck Pete |
| Pete Tipping |
| Gordon Just reread your opener. You say this all happens with ignition on but not starting the engine. Asking the obvious question, what happens when you do eventually get round to firing her up? Steve |
| Steve Gyles |
| Hi Steve, it is the same when running, I think I must let my local experts have a look for me it is, as you say, obviously in the wiring/fuses somewhere gordon |
| g c pugh |
| g c, - Don't give up so easy. This is a pretty simple circuit. http://mgaguru.com/mgtech/electric/circ_f1b.htm You have only three connections on the gauge. Install a jumper wire from battery hot terminal to gauge "B". If the problem goes away it's in the supply circuit. Install a jumper wire from battery ground terminal to the gauge case. If the problem goes away the gauge is not properly grounded. Disconnect the signal wire from the "T" terminal on the gauge. Install a fixed resistor between "T" and ground, any value between 30-70 ohms for the higher range of the gauge. If the problem goes away it's in the sender circuit. If none of these things cure the problem, it's in the gauge. Once you have isolated the problem to power, ground, signal, or gauge, life gets a lot simpler. Let us know what you find. |
| Barney Gaylord |
| Sorry Barney you lost me after "don't give up so easy" lol Gordon |
| g c pugh |
| LUCAS SMOKE PRESERVER http://www.mez.co.uk/lucas.html |
| Sandy Sanders |
| Sandy that is great, now where can I get one , can't find it on ebay gordon |
| g c pugh |
| Hi again, does anyone think that it might be the battery with perhaps a cell gone, as the guage gives up after a couple of mins, then if I switch off and wait 5 mins then switch on again it is fine for a few mins gordon |
| g c pugh |
| Gordon: According to Barney's lower resistance value for the sender unit, the gauge draws about 400 mA (neglecting the internal resistance of the gauge). If the battery is so weak that the gauge drops out after a few minutes, it would be surprising if the car cranks over. The starter and other subsystems, including the lights and wind screen wiper, draw much more current than the gauge. For example, together the headlights draw about 6 amps, or 15 times the current of the gauge. As a simple empirical test, try turning on the lights without starting the car. If the battery is causing the gauge to drop, the lights should go very dim in less than a minute. As a tougher battery test, try starting the car with the lights on. Steve |
| Steve Brandt |
| Gordon If the car starts OK, the battery is firing on all cells. You have loads of power if the starter turns the engine and it starts. This falling gauge reading has to be a change in resistance occuring somewhere. That change must be a poor connection or a partially broken wire with a high resistance which we can only assume is getting higher as it gets hotter. Forget the switches, there are none in the fuel gauge circuit. Presumably turning anything on causes the gauge to change or only the wipers? Is the new tank and sender unit earthed? To check fix a wire under one of the sender flange fixing screws and connect to earth. Don't give up yet. Pete |
| Pete Tipping |
| Hi Pete yes car starts fine, tank and sender earthed properly and the guage readings can alter with almost all switches gordon |
| g c pugh |
| So the gauge is OK as two gauges performed similarly. Is the gauge earthed OK as Barney suggests. (This 'earthing' isn't shown in my 1600 wiring diagram) The sender and tank new. New senders are very suspect, but usually of too low resistance, giving wrong readings and gaps in the coil giving the worst kind of twitchiness possible to the gauge reading. I assume you have a manual where you will see the wiring could not be simpler. One wire goes from gauge at "T" to sender, but there should be a 'snap connector' somewhere along that wire. Have you found that? If you have a 1500, there appears to be a lot more wires connected to the gauge. Power comes from the fuse to the fuel gauge which in turn should provide power to the heater fan switch and the indicator switch. Assume they are OK. Now then the black earth wires seem to be common with the panel lights and the WINDSCREEN WIPER switch! The actual earthing wire should first of all connect to the temp/oil gauge and then jump off to earth all the gauges' lights, the windscreen wiper switch earth wire should come from the fuel gauge at "8" too..! What's the reckoning it's in that earth wire somewhere, it could easily be a dodgey connection in one of the gauges earthing terminals. If the panel lights dim when you switch on the wipers, it could even be the actual earth connection itself. While laying under the dashboard with your feet in the air (we must all do it), just watch you don't short anything under there. Disconnect the battery or the battery switch if you have one when you think it prudent. Those sparks can make you jump! Over to you Gordon mate. Pete |
| Pete Tipping |
| Gordon Most people are hinting at your earth connections and they are possibly right. Thinking outside of the box for a minute, the fuel gauge is the most likely instrument to show you that you have a problem because it has obvious visual clues - a moving pointer. All the other electrical items will mainfest problems in a different way that you may not notice: Slightly dimmer panel lighting - would you notice? Horn not so loud - anything to compare the sound to? probably not. Windscreen wipers a bit slow - nothing to compare the speed to. Heater fan not spinning so fast - again, nothing to compare to. So, what I am saying is that you may be zeroing-in on the wrong problem. Look more generally and not specifically at the fuel gauge. Steve |
| Steve Gyles |
| Well still no further forward the poor chap who runs the local MG center is now threatining to retire if I take the car out there again,only joking , they spent a long time trying to locate problem with no success, tonight I have spent upside down under the dash board checking every connection I can get at, changed both fuses etc and still no different I think this is going to remain a mystery gordon (with headache too much blood to brain) |
| g c pugh |
| Gordon Good to see you burning the midnight oil on the problem. Getting down to the specifics on your earth leads. Can you tell us what you are using as your prime earth between all the instruments/switches/lights and the dashboard. How is the dashboard itself earthed? Steve |
| Steve Gyles |
This thread was discussed between 06/12/2006 and 13/12/2006
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