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I've moved on to troubleshooting the cooling system and can't seem to get it right. Neither of my cooling fans will run, not even when I turn on the A/C. Before I replaced the coolant temp sensor and the coolant temp switch/sender, my gauge was accurate. Now it just shoots over to 280 degrees as soon as I turn the ignition switch on. I know the fans and motors are good because when I plug a GM scan tool in under the dash, the A/C compressor clutch snaps shut and the fans run full speed. So I believe the relay to be good, but I can't explain the screwy gauge reading and still no cooling fans. If you guys have any ideas, please let me know!

Your gauge shouldn't be spiking especially if you replaced the sending unit. That would indicate it's grounding out. Recheck your work and make sure all connections are tight and clean.

There are 2 sensors. Which one did you replace?

http://www.pontiacbonnevilleclub.com/fo ... %20sensors

Well, I replaced both the temp sender and coolant temp sensor. I know the sender is responsible for the gauge, and it worked fine before I replaced it. I probably should have left well enough alone there, but I figured I would just go on and replace it since it was original. I did notice that there is only one wire running into the harness for the sender; it looks like there is a hole in it for another wire but there isn't one there. I thought this could be the issue but how would that explain my old sender working when I reinstall it? I'm leaning towards a possible bad sensor, but I hate to blame one right out of the box! Plus, the cooling fans still don't come on when the A/C is on or at all, for that matter. Should I suspect a bad relay there?

Did you use any sealant on it? I always put them in dry because it uses the sensor for the ground to complete the circuit. If you did, try removing them and clean the threads and reinstall.

It only has one wire because all your gauges run off of variable resistance. I'll admit it is very strange that the new one isn't working properly. Did you make sure it's specifically made for the gauge? It'll be different from the ECT or Engine Coolant Temp Sensor. That one has two wires compared to the coolant temp sending unit.

I think you guys hit the nail on the head. I did use sealant because I tried installing it dry and it was leaking when the engine reached operating temperature. That makes sense that it cannot find a ground and the gauge registers incorrect temperatures. I think I'll try cleaning it up when I get home from work and I'll let you guys know how it goes!

I've replaced mine and used thread tape. No issues there for me. I'd look carefully at the body of the sensor though. They are easy to crack. I've broken 3 of them.

Thread tape is actually what I used, not sealant. I had some Teflon tape laying around from a plumbing job I just finished and figured that it would work well on that sensor. I'm running to grab a biscuit and I'll let you know here in a couple hours how the uninstall/reinstall goes

Alright... So I used the thread tape with the old sensor (which wasn't bad to begin with, just old) and my gauge is working properly again. I still can't explain the cooling fans not coming on. The coolant temp sensor is now also brand new, but still no change. I tried cutting the A/C on too, still no cooling fans. What gets me is that I can plug the GM/Saturn diagnostic tool into the port below my steering column, turn the ignition switch to "on", and the A/C compressor clutch will engage along with the cooling fans. So I know I have power to them and the motors are good, and wouldn't this also mean the relays are good? The 40A fuse in the maxifuse panel is also good. Is there any other fuse to troubleshoot here or am I looking at either a defective PCM or a defective coolant temp sensor?

Finally figured this one out! My low cooling fan relay was defective, but the high was good. I swapped out the bad relay and now my fans are cooling when the car reaches about 200 degrees. I'm satisfied now, I'm gonna take this thing out for a spin since I've been hesitant to drive it in the summer without fans!

Thank you guys!