Cooling fans not working

GENERICNJ

MEMBER
2000 OLDSMOBILE ALERO
3.4L
V6
2WD
AUTOMATIC
37,000 MILES

I recently purchased a 2000 Olds Alero which was very lightly used over the last 14 years. I replaced the upper and lower intake manifold gaskets for good measure, but before that noticed that the cooling fans do not automatically turn on. I replaced the coolant temp sensor also and the fans still do not come on.
- they come on both full when coolant temp sensor is unplugged
- they come on both low when AC is turned on and engine warm
- neither comes on when stopped with engine running and needle going above the middle temp and near 3/4ths.

What else could I check? Thank you!

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

If the fans are not coming on when the system is intact then more then likely you have a coolant temperature sensor issue. If you have replaced that then we need to find out what the PCM is seeing as the temp. You use a scan tool for this. If the PCM is seeing the correct temperature then we need to check the relays and find out if the PCM is grounding the relays.

This is what is called a low side driver or ground side control. Basically the PCM applies ground to the relays and this will energize the relay and close the contact which then turns the fan on.

The fact that they only turn on when the sensor is unplugged points to a sensor issue but again, if you replace it then I suspect you have a PCM issue.

I am attaching the wiring diagram below and here are a couple guides that will help with testing relays.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Please see the wiring diagram below and let us know what you find with this.

Image (Click to make bigger)