Faulty fuel gauge in instrument cluster

sogama · 10-01-2007, 07:09 AM

I have a 1991 928 S4 (Euro) with digital dash. The analogue fuel gauge is misbehaving - it shows full when I fill the petrol tank, but once it gets to 3/4 full, the dial suddenly moves to above full and stays there until the tank is refilled. When I turn off the engine, the dial stays at 3/4 full for some time - all I know is that if I drive the car the following morning, the dial has returned to its proper resting position (zero). If I turn the ignition key without starting the engine, the dial also goes to above full.

I have taken the pod off, cleaned all the contacts but to no avail. I know the problem is unlikely to be with the fuel tank unit gauge unit as the digital readout for how many more miles the car can go seems to be accurate. It seems to me that there is a short somewhere in the instrument cluster which comes in to effect when the petrol tank is 3/4 full, and stops the petrol gauge dial from functioning normally. One thing I found when I took of the clear plastic cover off the cluster is that the fuel gauge needle can be turned all the way round (i.e. 360 degrees) unlike the other dials. I am not sure if this is normal or not, or if this is in some way related to the problem. I have not dared to disassemble the instrument cluster any further for fear of damaging something.

As on a previous occasion, I would be grateful for any suggestions?

jstobo · 10-10-2007, 11:23 AM

The analog fuel gauge needle moves based on the voltage that is sent to it. The sending unit in the tank changes the resistance as its float moves up and down. Higher resistance the lower the float so it will lower the voltage to the gauge as the tank goes to empty. This is how I tested mine. I removed the fuel gauge from the instrument panel(probably can do this without removing the gauge using the contacts on the back. Then I sent different voltages to the gauge. using differnt batteries. 6volt, 2-1.5 volts. At 6 volts my gauge read almost full at 1.5v empty and combing the two 1.5 volt batteries 3 volts was about half a tank. Since the gauge changed with different voltages I assumed the sending unit was bad. Replaced the sending unit and the problem was fixed. It was funny, the sending unit still changed resistance as the float moved but the resistance was too high so my gauge always read empty. Hope this helps.

sogama · 10-10-2007, 11:29 AM

Thank you for the suggestion. What I don't understand is why the digital readout with respect to how many miles the car can do with the available petrol appears to be accurate? I take this to mean that the signal from the sending unit must be OK. What do you think?

ROG100 · 10-12-2007, 05:40 PM

My educated guess is that you have bad soldered joint on the back of the fuel gauge.
This means you have to take the cluster out of the pod and look at the soldered joints.
It is possible to apply heat to the joints to reconnect the joint, however care is needed and if you are not able to do this take it to someone who knows what they are doing.