Problem oil temp read out

The car is a 512TR: when at idle the gauge functions normally. As soon as the engine revs past say 1200, the reading drops to the left peg, or effectively no reading. I have checked the sender unit which shows a 100 ohm resistance at ambient, same as a new one.

I think I have a ground problem - does anyone know how to find the part# on the wiring diagram?

Any suggestions?