Fuel Gauge Not Working

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Neil Evans
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Fuel Gauge Not Working

Post by Neil Evans »

Fuel Gauge

Remove the wire from the tank unit and short it to earth on the top of the tank unit. With the ignition on it should read 'full'.

If it doesn't, short the wire to the car body. If it now goes to 'full' it's that the tank is too well painted/undersealed and is not making contact with the body. Sometimes it's necessary to run a short earth lead from the body to the sender unit.

If it now reads full the sender unit is faulty.

If if the above doesn't get the gauge to read 'full' you'll need to pull the speedo out, then...

First check that the gauge is wired up correctly. There should be three or four green wires going to the terminal marked 'B' (battery) and a single green/purple to the 'T' terminal.

The multiple green wires should light a test lamp when the ignition is on. If the heater, wipers etc work it's almost certain this will be live as the feeds to these other components are the additional wires here.

Short the 'T' terminal to earth and the gauge should show 'full'. If it does, the fault is somewhere in the wiring to the rear. If it doesn't you have a faulty gauge...
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esmeandgiles
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Re: Fuel Gauge Not Working

Post by esmeandgiles »

Just wondered if there are different versions of fuel gauge and/or sender unit as Neil's instructions didn't agree with what I found when troubleshooting a fuel gauge which always read "Full" when the ignition was turned on, despite the position of the float arm on the sender.

After resolving my gauge problem I found that shorting the wire from the tank unit to earth and switching on the ignition gave an "Empty" reading on the gauge, not "Full" as in the instructions.

When the 'T' terminal on the gauge is earthed (which is the same test as above but isolates the wire from the sender unit) the gauge again read "Empty" and not "Full".

My gauge constantly read "Full" because the the sender unit wasn't fitted to the tank and the earth I'd connected to it wasn't earthing because it went via a high level brake light that I haven't yet wired up as the rear screen is awaiting the headlining installation...
Reconnecting the sender unit, providing a real earth and moving the float up and down to simulate varying amounts of petrol in the tank now produces a reasonably correct reading on the gauge.

Giles
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Re: Fuel Gauge Not Working

Post by gazza82 »

Pretty sure that they are all the same. Not converted to negative earth is it?
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Re: Fuel Gauge Not Working

Post by esmeandgiles »

Many thanks. It is negative earth but I'm not aware that that would make a difference. As the car is a work in very slow progress I might try the same tests with positive earth, just to be sure.
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Re: Fuel Gauge Not Working

Post by gazza82 »

I know you DON'T reverse the wiring on the fuel gauge when you switch .. so it isn't that

https://www.ahexp.com/phorum/read.php?14,105969
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Re: Fuel Gauge Not Working

Post by esmeandgiles »

Thanks for the link. Interesting read.
Having checked the workshop manual (and my tests) it's clear that when the 'T' terminal is earthed the gauge should read "Empty" and not "Full" as Neil suggests, so he may have made a typo or two.
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Re: Fuel Gauge Not Working

Post by gazza82 »

It would be unlike him .. but he carries all this knowledge in his head! :-)
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