Is there a Radiator fan switch screwed into the cooling system somewhere or does the ecm control the fans?


Asked by Feb 23, 2014 at 11:22 PM about the 2004 Buick Rendezvous CXL

Question type: Maintenance & Repair

I am usually pretty good at diagnosing my cars as I used to work as a tech for a GM dealer in the mid 90's... however I am quickly seeing that things have obviously changed...

I have a 2004 Buick Rendezvous with the 3.4 and it keeps running hot (221* on my scanner)  when my wife gets stuck in traffic... I noticed the fans were not coming on until 221* which I believe/feel is too late.  I also had a code P0128 which indicated my Coolant Temperature Below Thermostat Regulating Temperature so I replaced the Coolant temp sensor... It didn't change anything in regards to when the fans turn on.
I then replaced the serp belt due to cracks in it and while doing that I noticed some play in the water pump bearing so I replaced the water pump. It too didn't change anything in regards to it running hot in heavy traffic.
I have double checked the system is completely purged of air and everything else looks fine...  I remember all the GM's having one coolant temp sensor (for the ecm and gauge) and one radiator fan switch (turned on the fans) on all the electric fan vehicles back in the 90's but I can't find a Radiator fan switch on this Buick so I believe they now have the fans controlled by the ecm/pcm...   

I am also a bit concerned with how slow the fans run when they do turn on... Fans are coming on at approx 221* on my scanner and shutting off at 212* they only run for maybe 60 second and shut back off...  Of course I can leave the AC on max and they run continuously but again, they seem to be running at a slow speed and not moving much air.

If somebody could help me out here I sure would appreciate it... Thanks

8 Answers


Did you install a Delco coolant temperature sensor? I work at NAPA and we have trouble with GMs and aftermarket coolant temperature sensors. You are correct, the cooling fans are commanded on by the PCM. If your '04 Buick is like my '00 Grand Prix there's 3 fan relays. Two 4 pin relays and one 5 pin relay. The 4 pin relays are for each fan for low speed and the one 5 pin is for both fans high speed. With the 3.4 I'd check for a blown head gasket. These engines are notorious for that. HTH. -Jim

2 out of 2 people think this is helpful.

"code P0128 which indicated my Coolant Temperature Below Thermostat Regulating Temperature" Have you changed the thermostat or checked yours in hot water. It might be passing causing the engine to heat up. 221 is the trip point, they are operating normally. One sensor is the ECT that feeds the PCM, engine temp for fuel calculations and operates the fans. The other sensor is for the gauge or indicator light which ever the vehicle has.

4 out of 4 people think this is helpful.

JS08016.....I installed a Wells part which in my experience has always been a good product... After replacing the coolant temp sensor I no longer have the PO128... It doesn't have a blown head gasket. YET lol... It keeps running hot it may end up with one. Yetilikesbeer.... The code PO128 basically states that the vehicle is running "Below" the thermostat regulating temperature and is a strong indication the coolant temp sensor has failed. It is not sending a signal to the PCM setting the code... I replaced it and no longer have the code, however It continues to run hot in bumper to bumper traffic. Thanks guys, I appreciate the input.

2 out of 2 people think this is helpful.

I will also add that I have now replaced the Coolant temp sensor, Water Pump, Serp bent and Radiator cap... The Radiator looks very good and the car has always had Dexcool in it since it was new... I am going out to replace the thermostat now.

1 out of 1 people think this is helpful.

For warm up and highway traffic the coolant can be cooled by the rad as it passes through the faulty thermostat. As you hit heavy loads, high ambient temps or stop and go traffic the constant flow of passing fluid slowly heats up the entire coolant system. The rad is most efficent when the thermostat opens/closes allowing the stationary fluid to cool in the core, not just blast through. Trust me, I've had enough passing thermostats to know what happens, I'm to the point were I will not install the POS's untill I cycle them in a pot of water and confirm they open and close properly. For the code, there are multiple triggers in the ECU to trigger the code, there is most likely a runtime condition based on temp and running conditions to trigger it. I'm not bored enough to scroll through the code but I'm sure it's there. BTW If the fans triggered at the same time with the old ECT and the new one they are both good.

1 out of 1 people think this is helpful.

Test it in a pot of water. I pop them a few times then set them upside down and put water on the beast. If it drips toss it (a drip at 0 pressure is a spray at 12 ish). If it holds water it should be good.

1 out of 1 people think this is helpful.

You're welcome. Glad to help. -Jim


I bought a 2005 Buick Rendezvous, the mechanic has changed the radiator, the water pump, the thermostat, the temp switch and it's still running hot...Help!!!!

1 out of 1 people think this is helpful.

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