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MG MGB Technical - Revcounter and petrol gauge not working

When I bought the gt 6 months ago the rev counter did ot work and now the petrol gauge is not working.I've changed the fuses and cleaned the fusebox and cleaned the wire on the revcounter under the dash.Has anyone any suggestions please?
chris hebditch

What year? Both tach and fuel gauge are fed off the green circuit - the 2nd fuse up in the fusebox, but so is a lot of other stuff as well like indicators, brake lights, reversing lights, wipers, heater fan, electric washers etc. Depending on the year and what other stuff is and isn't working it is usually possible to isolate it to one or two of the many connectors in the green circuit. However with a car that is new to you, the tach wasn't working to begin with, and now the fuel gauge, it could just as easily be two or more faults and so take longer to resolve.
Paul Hunt 2

hi paul it's a 1978 model and everything else is working.I have changed the fuses and cleaned the fusebox terminals.
chris hebditch

There is a voltage stabilizer on the green circuit that feeds the tach, fuel guage, and (I thought) the temp gauge. It's a small rectangular metal box-like affair, mounted above the footwell on the inside of the bulkhead. ' Can't remember if it's on driver's side or passenger side. Check the connections on it, and if that makes no difference, try replacing it. Maybe you can barrow one from a friend's MGB to try before you go to the expense of buying one.

Paul, please correct me if I'm wrong. You're usually right about these things.

Allen Bachelder


go here for a really good wiring diagram, you are probably looking for page 25...

The voltage stabilizer Allen mentions feeds the Temperature gauge and the Fuel gauge, not the tachometer. The power for the Tach (green wire) goes from the fuse, to the wiper washer switch to the Tach and then to the heater fan switch. There are a couple of bullet connectors on the way which could be causing trouble. Sensing for the tachometer is via the ignition coil with a white black wire. So you should see two wires on the coil that are white and black one to the distributer and one to the tach.

If your temperature gauge works, and your fuel gauge doesn't, check the wiring between the two gauges, and from the fuel gauge to the fuel sender.

good luck,
Dave Braun

As Dave says the voltage stabiliser doesn't *feed* the tach, only the fuel and electric temp gauge, and a UK 78 should have the latter. If that's working then it isn't the stabiliser, or the 12v feed to it i.e. the fusebox, that's affecting the fuel gauge. In that case locate the fual gauge sender on the side of the tank by the right rear wheel. It should have a green/black wire on it. Connect a good earth to this wire with the ignition on and watch the gauge. As soon as it starts to move remove the earth as that shows the wiring and the gauge are OK. Next is to scratch a clean bit on the body of the sender and connect the earth to that. If the gauge rises now then it is the tank earthing that is the problem. Until the 78 year there was an earthing wire, but after that it relied on the mechanical fixings of sender to tank and tank to body, so corrosion could be the problem. If there was movement of the gauge on the green/black wire but not the sender body then the sender itself is at fault. If earthing the green/black at the sender did nothing check the green/black wires at the 2-way bullet connector in the mass by the fusebox where the rear harness joins the main harness, connecting the earth to the bullet on the wire coming from the main harness and checking the gauge for movement. If still nothing connect an earth to the green/black on the back of the fuel gauge *NOT* the light-green/green or this may blow the fuse. If that works then there is only one other connector and that is part of a multi-way plug and socket behid the dash. If still no go check for 12v on the light-green/green at the gauge with a voltmeter or test-lamp and the ignition on. Normally this is 12v switching on and off, and if present the gauge is faulty. The stabilised feed to the temp gauge comes via the fuel gauge, so as long as the wiring connector is making good contact with the fuel gauge spade that should be OK.

The tach needs a 12v supply on the green, an earth on the black, and signals from the coil on the white/black. Check with a voltmeter for 12v between the green and black, and if that is OK, with the engine running connect an earth to the black/white. That should stop the engine, if it does then the tach itself is faulty, if not the white/black is open circuit, and the only connector is another multi-way plug and socket behind the dash.
Paul Hunt 2

This thread was discussed between 18/01/2008 and 20/01/2008

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