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MG MGA - Fuel Gauge issue

I have the last job to figure out on the B that I have rewired. The fuel tank has fuel, but the gauge shows empty. The gauge systemn is teh same as used on the MGA

I have checked the Green/Black wire to the tank sender, it shows continuity for the whole length.

I have taken the Green/Black wire off the back of the gauge, and it then instantly reads full.

The gauge worked before the rewire, so did the sender, which has not been disturbed apart from tightening the screw holding the tab for the earth cable (that it has an earth tab on the sender is the only difference to the MGA system). Earth also shows no resistance.
dominic clancy

Disconnect wire from sending unit and check sender resistance. MGA sender should be 0-ohms when empty, about 70 ohms when full, and roughly linear in between. Gauge reading empty at all times implies a short to ground in the sender unit. Disconnected sender wire will peg the gauge past the "F" mark.
Barney Gaylord

Sender tests out just fine.
dominic clancy

Either no gas in the tank, or the sender wire is shorted to ground.
Barney Gaylord

If you can maybe try running a temporary wire outside of the car to either confirm or eliminate the car's wire.
L Poupard

is the B sender unit of the same design as the MGA?
If yes, it might be that the little brass bolt where the wire is connected to is touching the sender housing giving ground that way?

I understood that the A fuel gauge operated opposite the manner of the B gauge. If you have an A gauge, maybe this is the problem?
Mike Parker

The early B is very very similar in all electrical respects to a late A, and bears no resemblance to the later MGBs except in cosmetics. Same wiper motor, same lighting layout, same heater motor, same Jaeger instruments (although speedo is calibrated differently) ,

I'll test the suggestions made so far and report back
dominic clancy

I am not very technical but if I remember correctly I have been advised to make sure to fit the sender unit the right way up. In fact my fuel gauge had the same symptoms and it resulted that the sender unit was upside down. If I remember well there is also a mark on the unit to indicate correct position.
c vassallo

This thread was discussed between 16/12/2011 and 17/12/2011

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