When I brought my car into the body shop the temp gauge worked. Got it back and of course it didn't. Brand new engine wiring harness and front end harness so the wires work. The temp gauge has been pegged to cold the whole time and has never moved. I took it out and the last single prong on the back is partially broken off. I'm guessing when they took off the old harness they cracked the solder joint on it but it still seems like an ok connection.

They other I had a coolant leak form the water pump, didn't notice it while driving but all of a sudden the temp gauge started to work, even went higher as I drove (that's how I figured something was amiss). So I stopped let the car cool down, filled up the radiator and drove the car home. While I drove home the temp gauge stopped working again until I lost a bit of coolant. Anyone have this happen to them and why would the temp gauge only work when low on coolant - is that a ristance issue or something.

 

Sounds like that terminal needs to be reattached - use a #2 screw and more solder. Busted solder joints are often intermittant like this.

At the temp. goes up, resistance decreases and current increases (not a lot, but that could be why you are seeing the failures when hot.)

Here is how the circuit behaves:
https://classicoldsmobile.com/forums...html#post98959

It is also possible that your sending unit is damaged. If you have some 1/2W resistors in the correct values, you can put them in place of the sender and check it, or just use your ohmmeter.