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Juan Crespo
Juan Crespo, Tech Trainer
Category: Buick
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have an 2003 rendezvous and the fans will not come on no

Customer Question

i have an 2003 rendezvous and the fans will not come on hi no matter what the temp is. I can hook up my genisus scan tool and command both the low and hi speeds but will not come on otherwise. the truck will not overheat as long as manually operate the fans but will overheat if I don't. I have grounds to my fans but no power on either wires when the temp is 3/4 hot? any suggestions? I think possibly PCM? what are your thoughts
JA: Sometimes things that you think will be really complicated end up being easy to fix with a Buick. The Buick Mechanic I'm going to connect you with knows all the tricks and shortcuts. Are you hoping to fix this yourself?
Customer: Yes..
JA: Great! What have you tried so far?
Customer: thermostat, coolant temp sensor,radiator and have had the heads and gaskets done as well.
JA: Is there anything else important you think the Buick Mechanic should know about your Buick?
Customer: other then I can command them on with my scan tool?
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Buick Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: Buick
Expert:  Juan Crespo replied 1 year ago.

The BCM and PCM command High Speed Fans on under the following conditions:

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

With that in mind, you could choose not to check A/C refrigerant pressure at this time and just focus on using your scan tool to see whether or not there are any current or pending codes, and to monitor engine temperature to see if it gets up to and over the 230 degree enabling point.

Please let me know what you find and we'll take it from there.

Customer: replied 1 year ago.
Not sure of the a/c pressure as the clutch is engaged and I'd blowing some cold air. There are no codes in either the pcm or bcm and temp is well over 230. Goes to 248 before I shut the car down. If I remember correctly while the car is running I can unplug the bcm to see if the fans operate correctly?? True?
Expert:  Juan Crespo replied 1 year ago.

On your opening post you said you could make the fans come on high and low speeds with the scan tool. If that is true, then I don't see how unplugging the BCM would help us in our diagnosis, so let's leave it alone for now.

Now, we know that in this system, the BCM is "the brain" as it determines how long, when and what speed should turn on. The PCM is "the muscle" that engages the cooling fan relays, plus it will control the fans independently if the BCM fails.

What we need to find out is why the fans are not coming on when the coolant temp sensor is telling the BCM that temp is at 248 degrees. In order to do that, we need to confirm the BCM is indeed getting that data, and also whether or not it is commanding the PCM to activate the relays - we could only do that with the help of a Tech-II or equivalent scan tool loaded with the proper software for this MY Rendezvous. Without that tool, all we could do would be guess at the problem - something I don't recommend doing because it can get very expensive very fast.

So, is your current scan tool capable of reading BCM input and ouput PID? If not, can you get one that does?

Expert:  Juan Crespo replied 1 year ago.

Hi again. Just checking to see if the information I provided helped you solve your issue. If it did, then kindly accept and rate my answer now. If it didn't, please reply.

Best Regards.