1999 cooling fan will not operate

I have a 1999 Monte Carlo 3.1 motor with 180,000 miles. I changed both fan relays, the temperature sensor (and the connector), and the main cooling fan motor. The temp gauge works but fan will still not work..I swapped the relay between the ignition relay and the fan relay to make sure they worked (they all do )..I checked continuity between the sensor and the relay and continuity between the relay and the fan motor everything seems to be getting through the wiring. only thing is when I disconnect the sensor the car wont start. what could be keeping the fans from working?? Is there a separate sensor that operates the fans??

The fans come on with the a/c or at temps above 222, is this not happening?

The fans come on with the a/c or at temps above 222, is this not happening?

not coming on with the a/c or high temp....

You can try direct wiring the motor from the battery to see if the motor turns......or get the car warm, turn the A/C on, and give the motor housing a love tap with a hammer...

You can try direct wiring the motor from the battery to see if the motor turns......or get the car warm, turn the A/C on, and give the motor housing a love tap with a hammer...

its a new motor would i need to do that?? I will try it tho...

no deal on tapping the fan motor....

Still have not answered the question, under what condition does the fan not work, over 222, when the a/c is on on a warm day?

check the fuses. with a meter check for voltage at the relay control contacts. check for voltage at the fan.

get the old fan connect 12 volts to it see if it runs. if it does then you may have a control/ground/connector /fuse issue.

with the ac on the fan should run.

Still have not answered the question, under what condition does the fan not work, over 222, when the a/c is on on a warm day?

I only had the car 4 weeks....so I never new what will happen if its above 53 degrees outside.....fan wont operate at all..even if the A/C on or when the temperature gauge is just below the red (on the right of the gauge).

However I did flush the radiator and put in fresh dexcoolant....

check the fuses. with a meter check for voltage at the relay control contacts. check for voltage at the fan.

get the old fan connect 12 volts to it see if it runs. if it does then you may have a control/ground/connector /fuse issue.

with the ac on the fan should run.

I checked the fuses- even cleaned the terminals.... there is continuity between the sensor and the relays and from the relay to the fan...no leakages (or shorts) on the wires...

I was wondering about a ground fault...where is the ground set at on that car....

I did some more checking. The Old motor DID work when I hooked it to the battery.I pulled the relay and check for continuity. when i looked at the pins this is what I found:

Pin 85 ( when I go to ground I get Power coming from it)
Pin 87 ( when I go to ground I get Power coming from it)
Pin 86 ( when I go to Power I get a Ground from it)
Pin 30 (goes to the sensor and I checked continuity and its good coming from the sensor- thats the Dark Green wire- the one by itself on the sensor)

Pin 86 is a Light Green wire and it goes to the fan motor and there is continuity.

thats what I found so far....im thinking that the problem is in the PCM. any thoughts on this??

You can command the fans on with a pro scanner to see if the pcm can operate them, also if you unplug the coolant temp sensor they should come on.

You can command the fans on with a pro scanner to see if the pcm can operate them, also if you unplug the coolant temp sensor they should come on.


I will try that...I know before when I unplugged the sensor and tried to start the car, it wouldnt start.....also what if they dont start up??

Get the car started, warmed up, then shut it off momentarily, disconnect the coolant sensor, then start it up and see if the fans come on.

I did some more checking. The Old motor DID work when I hooked it to the battery.I pulled the relay and check for continuity. when i looked at the pins this is what I found:

Pin 85 ( when I go to ground I get Power coming from it)
Pin 87 ( when I go to ground I get Power coming from it)
Pin 86 ( when I go to Power I get a Ground from it)
Pin 30 (goes to the sensor and I checked continuity and its good coming from the sensor- thats the Dark Green wire- the one by itself on the sensor)

Pin 86 is a Light Green wire and it goes to the fan motor and there is continuity.

thats what I found so far....im thinking that the problem is in the PCM. any thoughts on this??

85-86 is the coil control. 86 is the ground.

87 should go to the fan , 30 is the 12 volt feed. short out 87 to 30 does fan work ? is there 12 volts across the fan power input. do not use body ground use the return wires/ground wire.

the fat wires go to the fan motor , the small wires are the contol coil relay wires.

your finds appear wrong .

Coolant fans won't come on with a/c UNLESS there is 175 psi a/c pressure generated (On GM W-cars as is yours) So does your a/c work? Is It capable of 175 psi head oressure?

Coolant fans won't come on with a/c UNLESS there is 175 psi a/c pressure generated (On GM W-cars as is yours) So does your a/c work? Is It capable of 175 psi head oressure?




compressor dosent engage....

You can command the fans on with a pro scanner to see if the pcm can operate them, also if you unplug the coolant temp sensor they should come on.

When I pull the sensor connector both fans WORK....when i reconnect, they stop....

85-86 is the coil control. 86 is the ground.

87 should go to the fan , 30 is the 12 volt feed. short out 87 to 30 does fan work ? is there 12 volts across the fan power input. do not use body ground use the return wires/ground wire.

the fat wires go to the fan motor , the small wires are the contol coil relay wires.

your finds appear wrong .


If you are facing the fuse block.
The fat green wire is left front. the thin dark green one is to the right front

do you have a diagram??

If the fans work sensor disconnected the circuits are all good, the pcm does this as a default to prevent overheating when it cannot read engine temp. So now get a scanner on it and watch the temps, as they approach 222, the fans should come on, reconnect sensor of course.

If the fans work sensor disconnected the circuits are all good, the pcm does this as a default to prevent overheating when it cannot read engine temp. So now get a scanner on it and watch the temps, as they approach 222, the fans should come on, reconnect sensor of course.

Will try that next......