Petrol guage

[jbeemer09]

Hi the petrol guage is not working on my 325i any ideas where to start with this problem?? thanks

[Brianmoooore]

Earth the gauge wire at the plug and socket under the rear seat cushion, and see if the gauge goes to full.

[Grrrmachine]

Can you give a more elaborate test brian? My fuel guage starts around the 40-50l mark when you turn on the circuits, then wobbles up to the 60l mark with the engine running. It never drops from that position, although the red warning light has shown up to tell me how cripplingly low on motion lotion I am.

I'll check the resistance values of each sender, but does this sound like the wire to the guage is grounding somewhere? I've already tightened the brass nut on the back of the clocks.

[Brianmoooore]

The fuel gauge wire is brown/green at the plug/socket under the rear seat cushion. Pull it apart, turn the ignition on, and the gauge should read less than zero.
Short the brown/green (in the half that comes across the top of the seat pan) to earth, and the gauge should read full.
Connect a 56 ohm resistor from the wire to earth, and the gauge should read zero if you have a small tank, or half if you have a large tank.
Connect two 56 ohm resistors in parallel, and a small tank should read half.
Connect two 56 ohm resistors in series and a large tank should read zero.
If your car passes all these tests, the fault is with the tank end of things, and vice versa.

[Grrrmachine]

Thanks for the comprehensive check Brian, very useful. My problem appears to be a short across the LH sender (not the one that's part of the fuel pump.) Disconnecting that plug send the fuel guage to empty, and putting a 56-ohm resistor on that wire brings the reading to 1/2, so looks like a knackered sender.

I thought these units broke rarely?

EDIT: Confirmed - resistive wire snapped in the small sender (the one under the round inspection hatch), causing the guage to read full. The search begins for a replacement.