Renix Temperature gauge

I have a 89 4.0 Cherokee. I am having issues with the dash temp gauge. With the key off the needle points to the left of straight up. When the key is cranked it bounces around the high end of registering. Once it is running the needle goes back to the low end of registering and stays there. I have driven it and let it idle in the driveway for 20 minutes and still doesn't register. The radiator and hoses are building heat. Any ideas? Does anyone have a part number for a new sensor?

Sounds like what mine used to do before I cleaned all the oil off the connector and bent the ears a little to tighten it up. May be your issue, may not, but something to look at before buying a new one. You might also want to take a close look at the wire.

The connector at the instrument cluster is prone to oxidation, it usually show sup as a flaky fuel gauge but may be something else to look at. A little scrubbing with a pencil eraser and a wipe down with contact cleaner is something I do every time I change out an instrument bulb. There is a TSB abut this issue, so it is fairly common.

Pull the connector off the sensor on the left rear of your head, right next to the intake.

Ground it with your key on.

Does your gauge peg out full hot now?

If yes = replace sending unit.

If no = check wiring / cluster.

The behavior of the needle pegging hot when cranking the engine is normal. It's a built in "bulb check" type of thing.

Floating and stopping at a random spot with the key off is also normal.

I have been having similar issues with my 96 4.0 ever since I pulled the transmission. Not sure if they they are related but thats some solid advice before purchasing new parts. Thanks!