My '04 Bonnie SLE has a possesed water temp gauge. Occasionaly it trys to convince me that the temp is R (The R for reverse on the gear indicator in the center of the instrument panel)! I replace the sending unit or sensor, disconnected the negative terminal on the battery, all that got me two days of normal gauge redings. Since then it is hit or miss. The needle on the gauge sometimes (most times) wont return to 0 when the key is off, or bonce from 0 to max, then to actual temp as the other gauges do. Any suggestions?

Sounds like a ground problem. Have you tried searching cleaning ground here?

This thread was about cleaning them.
https://www.gmforum.com/t279127/

No, I haven't tried the ground connections yet. The thing is wierd. The needle moves the correct distance, just from and to the wrong points. I mean it will move the same distance as it should from 0 to 200, but the start and stop point are in the wrong place. Sometimes it starts at 260, some times at 180... The funny thing is the car is supposed to shut down AC and some other things once the needle gets in the red "idiot" end of the gauge if I'm not mistaken. I know my dads '02 Bonnie has had the gas gauge problems. I haven't heard of anyone else having the temp gauge problems. Has anyone?

The bouncing to each end and back when the key is turned on is a Normal function. They all do it. A few newer ones have reported bad gauges in the cluster, mostly voltage though. I dont remember a newer one having a fuel gauge problem here.

He'* having problems with the coolant temp. Seems like the gauge might be shot if it'* not finding it'* zero point.

I'm still trying to understand this. From the sounds of this it makes me thing something is shorting out. You really need to look over the ground block under the dash.

This is from Billboost.

Right. All the gauges do bounce at startup, the temp gauge does too. Just not from 0 to max. It picks different points all on it'* own. The car is home in VA, I'm in MD right now. I will look at the ground connections this weekend. If anyone has any other ideas, I'd like to hear them, Where is the "ground buss" located, is it under the pannels near the parking brake, under the dash?

The ground bus on the drivers side that if you pull back the plastic trim that covers things just to the left of the hood release. Same thing on the passenger side.

Can the plug onto the temp sensor be put on backwards?

I dont know what may get turned off by the PCM when the engine gets hot, But it reads from the sensor itself, not the gauge, so one is not reliant on the other.

OK, that explains why the only thing effected is the reading on the gauge itsself. I am going to be doing alot of neglected work to the ol' girl this weekend, brakes, tranny, radiator, struts & shocks. No football or NASCAR on other than the shoot-out, so I should be able to track down some gremlins in the gauge. (It'* my wifes car) Do all wives drive like they are pissed off, or am I imagining things? Mat the peddle on the right, then mash the peddle on the left? Oh yeah, I have to tear the drivers side door apart too, we had one of those "free" oil changes done at the dealer. The dumas there lowered the car on the lift perfectly down onto a floor jack handle running it into the door. The body shop never got the trim clips on right (used the original ones again I guess) so it squeeks and rattles like crazy.

How were you able to solve this problem? I am having the same problem since I replaced the coolant sensor on my 2005 Bonneville.