I have a 97 Deville that overheats. I looked to see if the cooling fans were running the other day and they were. I then borrowed scan tool from a friend and tried to check and see if the high and low speeds come on. In the tool it had option for fan #1 which i assume is low speed. Both fans operated when acrivated. It then had an option for fans #1 #2 #3 to be activated all at once. Both fans again came on but didn't seem to be spinning any faster. Then it had an option for fans #2 #3 and when this one was turned on nothing happened.

My question is where should I start looking to fix this problem? I swapped the relays around with each other down below the radiator and the same symptoms existed.

When it first started afterheating I saw the coolant was low and pressure tested the system and found that just about every hose and even the water pump housing the the engine was leaking. So I started replacing all the hoses, seals, overflow tank/cap(for low coolant light), water pump and thermostat while I was in there. Also, since the AC compressor was meaking horrible noises I replaced that. When I replaced the compressor I had to repair the wires for the clutch since they were rubbed through.

The issue with the fans existed before any repairs were made but I thought the other things needed addressed before the cooling fans. So now that all that's fixed can anyone help with the fans?

Someone posted a pre-2000 fan control circuit in Northstar Performance or Seville a couple of weeks ago. If you can find that it will help troubleshoot. If the fans run (slow in series, fast in parallel) when commanded, perhaps you have lost a PCM function or have a bad signal from the coolant temperature sensor. The fans should run in slow any time an A/C function is commanded. Otherwise fans go to slow at 224 degrees and fast at 236. The car should NOT overheat with fans in slow. The only way I can get my fans to go to fast is to cover the condenser/radiator with a piece of cardboard and let the car idle for a while.

If you don't have a real GM/Helm Service Manual (Chilton's and Haynes are too generic) search eBay or subscribe your car for 3 years to www.alldatadiy.com

If you continue overheating and/or losing coolant, have a good radiator shop test the airspace in the coolant surge tank for the presence of exhaust gas.