Cooling fans not turning on

RSEVILLA86X

MEMBER
1998 OLDSMOBILE CUTLASS
3.1L
V6
2WD
AUTOMATIC
175,000 MILES

A few days ago, I swapped out my nineteen year old condenser. When I reinstalled the condenser/radiator/fan assembly, and confirmed no leaks, I was excited that I would soon be able to vacuum and recharge the A/C, until I noticed the cooling fans were not turning on even though the engine had reached normal temperature. I tested the fan motors with direct current and they are fine. Relays and the relay panel tested out okay. I jumped the relays (cooling fan number one and number two and fan control mode) and the fans turned on. My dash is reading the engine temperature perfectly fine so it seems the coolant temperature sensor is okay. Thinking about splicing the sensor's wire to a grounding switch in the cabin if I cannot get some help figuring it out. Thanks

3 Replies

CARADIODOC

MECHANIC
33,871 POSTS

Before you get too concerned, be aware that on a lot of GM vehicles, the engine has to get as high as 226 degrees before the radiator fan turns on. Some people find that can be even higher on some models. Sounds like you have not observed any symptoms related to overheating, so you may not have a problem. I suspect you will find that once the AC compressor cycles on, the fan will cycle on and off at the same time.

On a 1998 model, it is likely you have two coolant temperature sensors, a two-wire sensor for the Engine Computer, which runs the radiator fan relays, and a single-wire sensor that is for the dash gauge. On newer models they just use the two-wire sensor, then the Engine Computer sends data to the instrument cluster, which is another computer.

A quick test is to unplug the two-wire coolant temperature sensor while the engine is running. That will set a diagnostic fault code, but that should force the computer to turn the radiator fan on as a safeguard in case the engine is overheating. The computer will not know actual engine temperature, so they do that as a default just in case. If you do that, look at the dash gauge too. If it is still reading normal temperature, there is a separate sending unit for that function.

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Tuesday, August 21st, 2018 AT 8:40 PM

RSEVILLA86X

MEMBER
12 POSTS

Thank you so much for your reply. I unplugged the sensor and sure enough the fans engaged right away. First on a low setting for a few moments and then on high. Turns out there is only one sensor on my particular model because the dash had no reading while it was unplugged. Anways, recharged the system now and everything is running smoothly, well as smooth as an Oldsmobile with high mileage can run. The A/C is nice and cold, sometimes even a little too cold, haha. I plugged in my OB2 and seems coolant temperature stays right around 195 degrees F. Strange that I never noticed that the fans don't engage until a higher temperature is reached. Thanks again @CARADIODOC.

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Tuesday, August 28th, 2018 AT 5:18 PM

CARADIODOC