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Fuel gauge reads full all the time ign on, replaced fuel sender for new item problem the same, wiring on sender ( from gauge to sender Green/black) = 12.5 volts . in the manuals there is a fuel delay unit somewhere can you tell me where it is /what it looks like as I believe the above green/black wire shouldn't read full voltage unless tank is full , my guess is the delay unit is either faulty or the wires have been incorrectly fitted, the orange/green wire on the sender unit comes from this unit which I believe is the true power supply to the sender/gauge.
Can anybody shed any light on the problem please

The delay unit is for the low fuel light and is not part of the gauge circuit. It is located on top of the gauge cluster. It is the small metal box usually attached by on of the screws that hold the clear plastic cover in place. It has one wire to ground (black), one to the sending unit (green/orange), and one to power (green).

The Green/black wire carries full voltage from the gauge to the tank rheostat. That unit is a wire wound rheostat that increases the resistance as the arm is lowered by the fuel level.

You should check for damage to the green/black wire which would allow a ground.

Also if you have the terminals switched on the sending unit and the fuel is low it would cause the gauge to read full as the low fuel circuit is just an on/off switch to ground.

Edward Hamer
Petaluma CA

Thanks for your very detailed reply...my only query is that my 12.5 volts green/black wire is correct but you say it should be checked for a problem with earth surely if that is the case the system would blow a fuse but it isn't that is why I have not considered this as a problem....my brain hurts!!!

Mine was the same when I got it years ago, seems quite common. I checked out the sender and connections (worth a try to make sure of continuity and that the connections are the right way around) but mine was cured by swapping the gauge from a instrument assembly I bought on ebay. As a bonus it also had a clock that worked!

I had exactly the same a year or two ago and like Saabfast mine was a faulty gauge. I also spent weeks trying to figure out what was wrong with the wiring.

The green/black wire completes the circuit to ground through the variable resistance of the fuel gauge sending unit wound resistor coil.

If you look at the wiring diagram you will see that the gauge is feed power from the positive terminal of the battery. This power is feed though the gauge windings and then to the green/black wire to the sending unit.

Edward Hamer
Petaluma CA