V8S petrol sender

[morph2006]

The problem ...

Petrol gauge on dash shows the following ...

Ignition off - Empty
Ignition on but engine not running - Full
Engine running for a short time - Empty

There is petrol in the tank and seems to make no difference how much fuel is in the tank!
I've cleaned the two contacts on the fuel sender unit.

Help please, do I need a new sender unit or should I check the connections / wires again ???

[morph2006]

On closer inspection and unlike as described in the manual I have three wires on my fuel sender ???

Bottom wire - green and black
Middle wire - black

(These two could be the other way round as they have the same plug)

Top - brown wire

[greenv8s]

Disconnect the wires at the tank and measure the resistance between the two contacts with a full, half full and empty tank. If you get a nice linear change in resistance then the sender is probably OK and you have a gauge/wiring fault. Have you had any problems with the alarm? Sometimes the alarm is wired into the fuel gauge circuit (to make the gauge read 'empty' when the alarm is armed, I guess it is one more thing to discourage a would-be thief) and this gives one more point of failure.

[morph2006]

Well I think it is electrics, measured the resistance with not much petrol in tank and now have just filled tank and off to measure it again but on way back put on lights and petrol guage went from full to empty. It seems that the guage is not showing how much petrol but if the dipped headlights or on or off (full = off) (empty = on) !!!