2004 Chevrolet Impala 3.4
Cooling fans do not kick in when engine starts to overheat.
Fans do run when air conditioner is turned on.
Checked the three relays in fuse block. OK
Replaced the thermo switch in Intake manifold by the thermostat.
Still not working.
Unplug thermo switch and fans run when I start engine.
The heat gauge works.
Last edited by thomasoshkosh (10/5/2015 4:09 PM)
When you unplug the temperature sensor you are telling the PCM there is no temperature signal and the system goes into a safe mode by turning the fans on at high speed.
This is a difficult electrical circuit; it is a series/parallel circuit that allows the PCM to operate both fans at high or low speeds.
It is hard to diagnosis without seeing the problem my self but this is what I came up with according to the information you supplied us.
To diagnosis the problem you need a bi-directional scan tool which can help the mechanic by giving him the ability to turn the fans on and off remotely from the scanner. This can help the mechanic to eliminate any problems with the PCM. The BCM is also in the circuit also but it tells the PCM what is going on in the A/C system which also operates the cooling fans through the inputs of pressure sensors in the A/C system.
It seems to appear the problem might be related to a ground problem in the low speed circuit. The ground is supplied by the PCM and also ground 101. The problem could be poor connection, broken wire, bad relay and the worst would be a bad PCM not supplying the ground for the cooling fans. There are also three fan relays that are in the circuit to control both fans for high and low speeds.
Just make sure you check fuse (FAN CONT#1 25 AMP) and (FAN CONT #2 25 AMP).
Check relays FAN CONT#1, FAN CONT #2 and FAN CONT #3 for good connections with the junction block connections. Clean if necessary.
This is a complicated problem and electrical circuit where a trained mechanic and scan tool is needed to help simplify the diagnostics.
If you can use a wire schematic for the fans circuit I supplied it below.
Alldata Automotive website.