ok, i purchased this car a couple of months ago, did the whole tuneup thing 97 3.8l w/ 5spd manual i did manage to get 3 plugs from the top though. my problem is when i started driving the car i noticed the temp rarely moved past the 160 degree mark, me being in tennessee in the fall i figure thermostat stuck open. i purchased a 180 degree thrmo instead on the 195 degree. installed it and found i was correct when i removed the old thermostat. my temp now runs about 180 degrees during my cummute, but when i get into traffic i noticed it climbing to 210 or better(Oh SH*T my first reaction) as i traveled a little it went back down. i found the fans are not kicking in, was going to turn the a/c on and see if they would come on as they are suppose to but the a/c is inoperable(i will have to fix it later but winter is here and im not worried about it right know.) i replaced all three relays and checked the fuse at the junction box under the hood, still nothing. please tell me its not a commin thing for the pcm to malfunction and ground out the relays for the fans. i havnt replaced the temp sensor as the gauge is working fine and all the schematics i have seen do not show the temp sender to work the gauge and the fans, although it is a three wire sensor. from what i can tell from the schematics the pcm reads temp from the sensor and grounds the relays. if there is another sensor that controls the fans please let me know. or if you have any other ideas im game.

 

The Engine Coolant Sensor is what the PCM looks to for the coolant temp. The sensor can go bad without setting a code. You can verify if the PCM is getting a reasonable signal, by using a scan tool & reading what temp the PCM is seeing. Comapre that reading with what the temp gauge is showing. If the temps are more than 15 deg apart, put in a new sensor. If the temps match, you will have to find out why the fans do not run. Check the fuse powering the relay winding, and the fusible link providing power to the fans.

 

just run a direct hot to the fans and tie into it with a toggle switch.

 

That is what people do if they can't find the problem. But the other problem is that if the temp sensor is bad, it gives the PCM the wrong info, and the PCM pays you back with the wrong A/F mixture and possible loss of power.
Chad, not to get off the subject, but I like the color of your car, is that one of those paint jobs that change color when viewed from different angles?

 

yea it changes from like green to blue and a couple of other colors depending on how you look at it.

 

Yeah, i work for Detroit Diesel -allison as a diesel mechanic & Transmission tech. so the only scanner i had was a snap-on and it was close to the gauge temp. i didnt pay too much attention to how close it was from the temp gauge but my scanner said the temp was up to 215 degree at one point and the fan still didnt kick on.
As for running a toggle switch i would think about that for situations where i wanted to control it manuallly, but i don't like to jerry rig stuff. I want it to work as designed and not just go to the simple but tacky way of doing things. i figure since the scanner temp is pretty close to gauge temp the sensor is fine, i know some cars had a two sided temp switch one side to control fans and the other side feeds pcm and gauge but wasnt sure if this one worked that way. if its not the sensor,relays (which i have replaced), fuses it has to be a corroded terminal at the pcm or the pcm itself, unless anyone knows of a alternate sensor for the fans only. i guess i will check the terminals at the pcm next and if thats not it maybe find another used pcm and have it reflashed at local dealer. we do favors for them all the time since the new trucks run the allison transmission and thats what i work on, maybe i can talk them into doing it for free if i find one. worst case i have gotten new parts at real low prices from them and maybe the pcm wont be too bad if that turns out to be the culpret.

 

Might sound like a stupid question, but have you checked the ground for the fan motors? And the blades spin free?

I learned the hard way, check the simple stuff 1st, it's cheaper.

 

the fans work when i jumped the relays also i jumped the a/c pessure switch and they came on. it seems like the pcm is not going to ground when the temp comes up. i pretty sure that the pcm grounds the coil in the fan relays to turn them on so in my mind it seems that it has to be the wire from pcm to relays or pcm unless its coolant sensor, i dont think its the pcm because when i jump the high side pressure switch the fans came on which means the pcm is controlling the relays. the only thing that baffles me is its not requesting them on except when it sees the a/c pressure switch closed. they are supose to come on when the engine is off and engine temp is above 140 degrees and they are not coming on. i think im just going to purchase a temp sensor and give it a shot, although i dont like to throw parts at it until i have verified that its the problem.

 

Before you through a pcm in your scanner should tell when there is a fan request. I have the Modis and have done this before.

 

I think you found your problem. No AC charge. ("the fans work when i jumped the relays also i jumped the a/c pessure switch and they came on") GM has the AC engage when the heater is in the defroster mode to remove excess moisture from the incoming air. Below 28 degrees (outside air temp) there isn't enough humidity in the air to need this feature. A co-worker had the same problem with a '97 Fbird. Charged the AC and the fans worked.

You can jump the low side switch and pull the AC relay (to prevent compressor damage) until a proper fix can be done.