This one has me scratching my head.

A repairer had my car and ran it out of fuel. The fuel pump died so they fitted a new pump supplied by me. When I got the car back all fine but when I filled it up the gauge only went to just over half full.

I dragged out the book and checked the sender which was showing 110 Ohms, about right for the gauge reading. So I checked wiring etc found nothing. I pulled the sender and pump unit and checked the sender all good but the resistance was intermittent above that 110-120 Ohms mark. I thought maybe the sender was somehow damaged.

Taking the easy option I bought another sender unit complete from the wreckers. I transferred the sender itself from one body to mine (saving the pump transfer) and refitted it. Now the strange bit - when I turn on the car the gauge goes to 'full' immediately and then seems to switch off and drop straight back to the bottom stop. Disconnecting the wiring at the sender makes no difference, i.e. the gauge still goes straight to 'full'. When I test at the sender end plug the car side wiring shows circa 17-18 Volts (I take this to mean power on) and no short across the wires in terms of resistance.

So, irrespective of the sender being connected in the circuit, what has changed now????

Any ideas on this one?

I think I solved this one. The factory manual says that with the plug at sender disconnected the reading should be 'Empty'. I checked the schematics and the sender is the ground to instrument bus ground. This appears to be a balanced ground so if the plug is disconnected the gauge has no balance and should go to 'Full' since resistance is highest. This makes sense if you check the table for sender resistances which show full is 180 Ohms and Empty is 2 Ohms.

I tested by placing a test light across the plug and sure enough the gauge sat near empty.

This suggests the factory workshop manual is opposite to the reality! I then tested on a second sender unit outside the tank and traced the fault to a bad connection at the sender body terminal on the inside of the tank flange. Seems fixed now but will see....