Hi,

My 2005 A6 diesel fuel gauge is playing up. It was incorrectly showing empty but is now permanently showing full, with a half empty tank. I know this is a problem with the fuel sender (part of the fuel pump) or possibly the wiring to the fuel sender (a common issue I understand).

So far as I understand, the fuel sender is based on a variable resistor and the fuel level is measured by measuring the resistance to ground, i.e. 0 ohms should be an empty tank and , say, 100 ohms (or whatever is the correct resistance for my sender ???) should be a full tank. For example, if the gauge always shows empty then there is no resistance to ground and perhaps the sender wire and a ground are touching. However, for an always full reading there is 100% resistance, indicating a fault such as a break in the wire.

So, how do I diagnose this with my multimeter before opening up fuel tank and/or replacing the fuel tank and sender unit? I can see the top of the plug has five wires - a chunky earth and power wire at each of the plug, presumably to power the pump, and then three thinner wires in the middle - purple, brown and white. I'm a bit stuck about what to do next. I think there might be two senders and the third wire is to the instrument cluster. Does anyone have a wiring diagram, the pin outs for the plug or better information on this. I'm not sure how to proceed.

Thanks, Keith