dodgy fuel gauge audi 100

[Coxy]

Got a dodgy fuel gauge/sender on my '87 100 sport.
Strange problem. When I turn the ignition, it beeps at me and the petrol warning light comes on and the computer trip display shows L for low fuel (I presume??).
Now, the thing is, I've just put in £25 of Esso's best, and the other thing is when the ignition is off, the fuel gauge shows what it should read instead of going to empty.

My first though is a dodgy earth on the plug that goes in to the top of the fuel tank. Have had the cover off and taken out the plug but nothing untoward, and no corrosion showing on any of the connectors.

Any ideas?

[burny]

Originally Posted by Coxy

Got a dodgy fuel gauge/sender on my '87 100 sport.
Strange problem. When I turn the ignition, it beeps at me and the petrol warning light comes on and the computer trip display shows L for low fuel (I presume??).
Now, the thing is, I've just put in £25 of Esso's best, and the other thing is when the ignition is off, the fuel gauge shows what it should read instead of going to empty.

My first though is a dodgy earth on the plug that goes in to the top of the fuel tank. Have had the cover off and taken out the plug but nothing untoward, and no corrosion showing on any of the connectors.

Any ideas?

the fuel gauge is tne only instrument which fails ?

Maybe (it is just an idea) your panel intrument stays under power.. if you unplug the battery, what happen?

[Woodsman]

Coxy, I think you are probably on the right track, ie dodgy earth.

Does the gauge only fall when the fuel pump is on (engine running) or all the time the ignition is on? Try pulling the fuel pump relay.

I'm sure the earths for the pump and sender are on seperate wires but are grounded on the same tag, this is the first item to check, should be close to where the loom from the tank enters the car, but I can't remember exactly where, under the back seat maybe?

If this is ok then maybe an earth connection to the dash or auto check computer.

A good way of checking earths is to measure for a voltage on the earth wire when the circuit is loaded, tiny voltages <0.5 V are usually no problem, but the lower the better the earth is.
This is usually done with a wire piercing probe (good practice to tape the tiny hole afterwards) or a very fine probe to "backprobe" the plug.

[Coxy]

Not really sure what the problem was as it righted itself the next day and hasn't done it since.
Dodgy earths are normally the answer for a lot of electrical gremlins so will be my first port of call if/when it does happen again!