I have a question about my folks car. It'* a 1995 Chevrolet Lumina (3100 Gen III V6). This vehicle has 99,000 miles and I recently replaced the head gaskets and lower intake manifold gaskets. My problem is, is that the cooling fans will not come on when it gets warm, even when the A/C is running they dont come on. I've tested both fans, and they work. Replaced the Coolant Temp sensor twice, the temperature guage sensor twice, both fan relays twice, the thermostat twice, fuses, and I've bled the coolant system FOUR times....STILL the fans dont work. The only time they work is when I disconnect the coolant temp sensor they both stay on, and when I use jumper wires to jump across where the relays are located. I don't know what to do with this car. It runs awesomely but we can't drive it so it has sat for three months now like this in the shed with ALOT of engine work done on it, its a big disappointment. Could it be an ECM problem, and how much would it cost to have the ECM re-flashed?

If the relays work, and they're not being toggled when required (make sure on this one), it'* almost certainly the PCM. Judging by your troubleshooting thus far, I'd assume the wiring and temp sensor for the fans is good. Disconnecting it sends it to infinite resistance which is translated as very hot by the PCM.

Wait a minute.....

When you disconnect, the PCM commands the fans on? Sounds like the PCM is working right. I'm assuming it works like ours, where the low-speed kicks on at a preset temp, and the High speed kicks on with the AC compressor.

Can you break your troubleshooting down by these two modes so we can figger it out?

When I picked up my 95 the fans wouldn't kick. I found that the relay'* could be jumped, however the contacts didn't look dirty and were. I plugged and unplugged them the 5 magical times with dielectric grease and they began working again. It'* not much, but worth a try.

I've tried everything that I can so far think of, and I'm assuming that it must be the PCM because, I've replaced all the sensors nessisary for the fans to work properly including the fuses, and relays. I guess my next step is to take it to the Chevrolet dealership and pay a fortune to have the PCM re-programmed. I'm most certain that its the PCM because now the A/C compressor doesn't turn on, the car is now doing crazy things like the instrument panel lights, and radio lights flickering at 1500-2500 RPM'* when the car is parked and running. It also RARELLY starts up poorly, and for a car that has had fresh sensors, fluids, parts, ect. It has got to be something electrical.

Are you able to check for voltage at the relays when the car is running with the AC request on that should trip the low voltage and close the relay? I assume you've check the relays for battery voltage. You should be able to see the voltage increase at the correct slot when the computer turns on the relay.

Well your parents took the car to the shop and found out that the guage in the dash is not working correctly. Every thing is fine and the fans do work when the car actually gets to 225-227 degrees. The guage was off by 20 degrees and that is why the fans did not kick on because the car actually was not hot enough.