Strange cooling fan behavior

I have a problem that has me scratching my head. The cooling fans only operate if the A/C is on. If the A/C is off, the engine overheats. The fans never kick in. Turn on the A/C and the fans run and cool down the engine.

The temperature gauge works and there seems to be only one sensor but I am unable to confirm that the gauge is driven by the ECU.

Does anyone have any bright ideas? I am getting a headache. :banghead:

Yes, cooling fans are always on when the ac is on and the car is not moving, this is normal, otherwise ac wouldn't work. condenser needs air flow at all times. As for the overheat, this tells you that the pcm is in control of the cooling fans. Either it is not getting the proper temperature from the temp sensor, which is located by the t-stat, or the coolant is low/air pocket, or you don't have the proper 50/50 antifreeze mix etc. is the check engine light on? does it idle rough after it is warm? check the simple things first.

I have bled the cooling system. No air. No computer codes. Engine idles fine.

I am confused because the computer has control of the fans, the temp gauge reads correctly but the car overheats because the fans don't come on.

What year? What engine? Seeing that you mentioned bleeding the cooling system I assume you have the 3.1L. E-fans are typically set to come on at fairly high temperature, like 210-215°F, so the sending unit might not be seeing temps high enough. If you do have the 3.1L, have you had the intake manifold gaskets replaced? This problem could be caused by leakage and contamination caused by leaky IMG's.

Joel

If your fan isnt coming on then change your temperature sensor, this is what tells the car that the temp of the engine is to hot and turns on the fan, and if that isnt working then your fan is broken.

I have bled the cooling system. No air. No computer codes. Engine idles fine.

I am confused because the computer has control of the fans, the temp gauge reads correctly but the car overheats because the fans don't come on.

What temp do you call overheating, the fans will not come on until 230 deg F.

what mix of cooant to water do you have??? if it's too much water, then it will always overheat. (50-50 mix) did you just drain the cooling system? if so then you might have an air pocket, after the car cools off, try high idling it at 2500 rpm and see if you can purge the air. these cars arent known for trapped air, but you could give it a try. remember to use the bleed screw on top of the water pump as well.

I might start checking relays. The PCM controls two grounds for the cooling fans. There is three relays. According to the diagram I am looking at only one fan should come on with the air condintioning. When the PCM sees AC request it grounds the relay for the right side cooling fan through fan control relay #2. When the PCM sees the right temp, around 230 degrees as stated before, it grounds fan control relay #1 and turns on both fans. My guess is that relay number 1 is bad. It is in the underhood junction box. If you pull the relay and know which posts to jumper the fans should come on but my diagram doesn't show the terminals. Don't guess because you can smoke the computer if done incorrectly. The diagram is for a 99 malibu with a 3.1. Sorry for rambling on but like I said, a bad relay #1 would cause what you described from what I am looking at anyway.

These cars are designed to run with out the fans and not overheat. Overheat is the key word here....

This is how your fans should operate, if its a newr Mailbo since you never realy said.
The PCM commands Low Speed Fans ON under the following conditions:

The engine coolant temperature exceeds approximately 106°C (223°F).
The A/C refrigerant pressure exceeds 1310 kPa (190 psi).
The vehicle is shut off when the engine coolant temperature is greater than 140°C (284°F) and system voltage is more than 12 volts. The fans will stay ON for approximately 3 minutes.

The PCM commands High Speed Fans on under the following conditions:

The engine coolant temperature reaches 110°C (230°F).
The A/C refrigerant pressure exceeds 1655 kPa (240 psi).
When certain DTCs set.