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MG MGB Technical - Fuel Guage intermitant problem.

My fuel gauge sometimes works other times no readings. Currently not functioning. Needle all the way to left although tank is almost full. Could this be a short somewhere? Or could it be a faulty guage or a faulty sending unit in the tank? What would be the best steps to diagnosis this problem. The fuse block has been replaced so I don't think this is the problem and all the other guages are operating properly. Thanks for your advice on how I should trace this problem, I just don't know the best direction to proceed.
John Fraioli
John F

What year?
Paul Hanley

Disconnect the wire from the sender (on the side of the tank) and hold it against bare metal whilst somebody turns the ignition on....if the gauge reads full, it's the sender in the tank....mine stuck (77 roadster) over Xmas, but has now decided to behave.

Good luck

M Barnfather

Paul, It is a 1979, worked fine all the way to MG 2005 and back by the way. But it has been doing this Non-reading thing every once in a while over the past 6 years.

Mike, thanks for the starting point. I can check that later today when the wife gets home. I wonder though if it is a stuck sending unit does that mean it is stuck in the down position?

John F.
John F.

John - If the sender was stuck in the down (empty) position, the gauge would read empty all the time, not intermittently. I would suspect a flakey voltage stabilizer or an intermittent open in the wire from the gauge to the sender. A short in that wire would cause the gauge to read full all the time. Since the stabilizers are rather prone to failure, that would be my first suspect. If grounding the wire at the sender does not get a full reading from the gauge, then check to see if you have 10 volts at the output of the stabilizer. Good luck - Dave
David DuBois

Thanks for the input. Here is what I have done so far. I disconnected the wire at the tank sending unit and touched it to ground with the key on and a helper watching the guage. No movement on the gauge needle. I also used an electrical tester between that wire and ground and found no current present.

Next I had a spare fuel gauge so I replaced the gauge. Still not working. I tested the replaced gauge with a 12 volt power source and get the needle to move.

Then I sat down with my haynes wiring diagram. I notice there are two teminals on the gauge. One with a single green wire and the other that has two green wires to it. It looks like the one with the single green (GB) goes directly to the sending unit. The other terminal with the two Greens (LLG) one goes to the stabilizer and the other to the coolant gauge. Since the coolant gauge is working properly. I am thinking I am getting power from the stabilizer.
I will check to see if I have 10 V at the stabilizer and then I will check for voltage at the fuel gauge (at the terminal with the two wires).At this point I am suspecting the wire that goes from the sending unit to the gauge may have a break in continuity(that sometimes has continuity and other times does not)not that the wire is grounding. If I find power at the stabilizer and at the gauge. I will then check the (GB) wire for continuity or I may just run a long (jumper) wire directly from the sending unit to the gauge and see if the needle moves to the correct position.

When I said in my earlier post that the fuel gauge was working intermitantly I ment it would either give the correct reading or NO reading at all, not a false or irradtic readings.

Let me know if you think I am on the right track so far.

John Fraioli '79 Inca yellow MGB
John F.

Yes you are. As you now seem to have access to the back of the gauge check for voltage on the two terminals. With the ignition on for a few seconds you should see 12v switching on and off about once per second on the light-green/green terminal, believe it or not this is the action of the 'stabiliser'. If no voltage there is a disconnection back towards the stabiliser, but as the run goes onto the temp gauge from the fuel gauge and that is OK this is unlikely unless there is no connection between the wiring connector and the gauge spade. If you have the voltage on the light-green/green check on the green/black. No voltage means an open-circuit gauge. 12v switching on and off means the guage is OK but there is an open-ciruit back towrds the sender. Something less than 12v is what you should be seeing with a correctly connected sender, the actual voltage depends on the position of the sender, the greater the fuel the higher the sender and the lower the voltage. A ground on the black-green means that is is shorting to ground somewhere, which should cause the gauge to read above F, which is not good for it and it may have burnt out. If the same 12v on both terminals of the gauge then there is another connection where the main harness joins the rear harness amongst the mass of connectors by the fusebox. If that is OK i.e. you have the same 12v both sides of the bullet connector, and there is still no voltage on the spade at the tank end, then that wire at least is damaged and open-circuit somewhere along the body harness. But a dicky connector by the fusebox is my guess. Be aware that once you start fiddling there, and have dodgy connectors, you could upset a number of other circuits.
Paul Hunt 2

Hi John.

Assuming that the ignition is on but gauge isn't working at the time :

Please check carefully for volts relative to ground on both sides of the gauge. Use a voltmeter or a low power test lamp, not a high power test lamp.

If you have no volts on either side of the gauge there is a break between the voltage reg and the gauge.

If you have volts on both sides of the gauge you have a break between the gauge and the tank sender or the sender is broken.

If you have volts on one side of the gauge but not on the other you have a problem within the gauge (or its tags).

You might like to diconnect one side of the coil while you are exploring this in case the engine is stopped with the points closed.


Hi Paul.

Didn't intend to 'steal your thunder', you just got to 'Submit' before I did.


Don and Paul - You both got to the send button ahead of me. I was slowed down by a doctor's appointment for a follow up exam for my go around with lymphoma. I am happy to report that the doctor says that I am doing great and there is no signs of the lymphoma having come back as of now. It is still in the bone marrow, but as long as it stays there, I am fine.

John - Paul pretty well mirrors what I would have written, so I won't go any further other than to say to check the wires where they are crimpped into the spade lugs as that is the weak point in any wiring.
Cheers - Dave
David DuBois

Thank you all who responded. I am writing to let you know how I made out. My fuel gauge is now working again! Seeing as how I had power to the temp gauge, and I had swapped out the fuel gauge with a (nicer looking) spare I had, I figured I must have power to the guage. Thinking that I must have had a break in the wire that connects the sending unit to the fuel gauge, I made up a long jumper cable and connected it from the spade on the sending unit opened the passanger door and ran it thru and connected the other end to the side of the guage having the single (green-black)wire. Turned on the key and the fuel gauge needle moved up to the correct reading. So now I knew there was a break somewhere in the Green-black wire.
So I unhooked the jumper wire and thought I would check the bullet connection that Paul Hunt had mentioned. The one where the main wiring harness connects to the rear wiring harness. Sure enough that connection had pulled apart enough to make the break. Using two pair of needle nose plyers I re-inserted the wire into the connector. Went back and turned the key to on, and sure enough the fuel gauge is working.

Thank you all for the great advice. It kept me on the correct track. And kept me from getting to frustrated.
John Fraioli
John F

This thread was discussed between 02/01/2006 and 09/01/2006

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