Fans don't start - PCM?

I have a 1999 Pontiac Grand Am SE 4dr 2.4L engine. I noticed for a while my engine was running a little hot but it wasn’t over heating. Recently I was in a drive thru waiting and it got a little too hot so I pulled over. I checked the fans and they weren’t running even though the car was overheating. The fuses looked good. I verified the fans are able to run by putting a jumper in their relays and they both started. I didn’t check the relays per/say however I moved them around and the fans still didn’t turn on. Although I didn’t check the relays I don’t think they would all be bad. Then I pulled off the connector to the temperature sensor. I jumped it and the fans didn’t start. I measured the voltage on it and there was none. I believe this should show 5 volts. Just for verification this sensor is on the right side of the engine (from the front of the car) in a plastic multi-hose connector. I also turned the air conditioner on which should make the fans run but nothing.

Another thing that has occurred is the air conditioner stopped running. I’m not sure why. It worked fine before winter but this summer nothing. I checked the low pressure with a self fill kit and it showed it has the needed pressure.

I’m also having an issue that recently started where the oil pressure light turns on sporadically even when I’m not low on oil. It seems to come on usually when I’m slowing down or stopped then turns off when I’m driving. It is sporadic though and doesn’t do this every time.

The final thing that happened a year or so ago is my gauges all stopped working. This was on a trip and I was driving several hundred miles coming back. I drove for a while without the gauges and stopped in a small town. When I started the car the gauges started working again. This has never happened since.

My suspicion is my PCM might be having issues. Please let me know your thoughts and what, if anything, I should try?

I would be curious about trouble codes. Have you checked for codes? Just because the light’s not on doesn’t mean there isn’t a code stored that might help get you in the right direction.

When I take my car to Advance or Auto Zone they tell me if the check engine light isn’t on they can’t get any codes. I had a conversation with them about this and they keep telling me the check engine light needs to. I don’t own a code reader. Are there different types like ones that can get more information than others?

inactive codes may not make the check engine light stay on, and some codes won’t make it come on at all- but your computer can still have them stored, and can help get you in the right direction.

Yes, there are better code readers than others. Some much, much better than others.

I would get the codes professional scanned (not an auto parts store,) and go from there.

For the fan problem, it depends on what event exactly causes the fan to turn on, I mean by the manufacturer’s design. I had this problem on my early 90’s Corolla a three times, and the first time it was a bad wire splice in the wire going to power the fan. The other two times were caused by the coolant temp switch for the fan going bad. On my Corolla when that switch turns off, a relay turns on and the radiator fan runs. That switch sits in hot coolant all the time and I guess it gets stressed out and fails. I’ve had to replace it twice. Sometimes it fails stuck on, and sometimes stuck off. The way I tell it has failed is by removing the connector to the switch when the coolant is hot enough to turn the fan on, but the fan isn’t coming on. That makes the relay think the switch is in the off position, and will cause the fan to run, proving the problem is the switch.

On later models, perhaps like yours, the engine computer operates the radiator fan based on the engine coolant temperature sensor (a thermistor). There’s no radiator fan switch in the coolant.

For the oil light problem, suggest to measure the oil pressure with a shop gauge. You need some kind of independent measurement of the oil pressure. You might get lucky and just need a new oil pressure sensor.

For my fans I know that both of them run because I pulled the two relays and jumped the circuit that the relays sit in and both of the fans started. This verifies the fuses and the fans work. I’ll have to verify the relays work however there are three of them and I moved them around so I would suspect they work. I also pulled the connector to the temperature sensor. I checked for voltage on the connector and there was none, and I connected the contacts which should start the fans (is this correct) and they didn’t start. I believe this would mean either there’s a broken wire or the PCM is having issues (is this correct?). I also tried to start the radiator fans by turning on the air conditioning and the fans don’t start.

Please let me know any thoughts why the fans don’t start?

The problem is most likely with the PCM.

The PCM is what grounds the three relays for the fans.

There’s a left fan relay, a right fan relay, and a mode relay.

The coolant temp sensor should have a 5 volt reference signal from the PCM.

When the coolant reaches a certain temperature, the signal from the coolant temp sensor commands the PCM to ground the primary side of the right side fan relay which closes the contacts in the relay causing the fan to come on.

When the AC is turned on, the PCM is commanded to ground the primary side of the mode relay, which then causes the contacts to close which then grounds both the left and right side fan relays causing both fans to run.


I checked for voltage on the [temp sensor] connector and there was none, and I connected the contacts which should start the fans (is this correct) and they didn't start.

The temp sensor is probably a thermistor, a variable resistor that changes its resistance vs. temperature. You could remove it and heat it on the bench with a hair dryer while you measured its resistance as an experiment to determine the resistance vs temperature curve. For the engine computer to measure the sensor’s resistance to calculate the coolant temperature, you are right, it has to output a voltage and measure the current. So there should be a voltage at that connector, like you say probably 5 volts. That’s the voltage my Corolla computer uses anyway. The voltage may not be output all the time though. It may only output that voltage when it is time to measure the sensor. Depends on the computer’s design. A factory service manual would probably have this info.

If all this is correct, and it is a thermistor as confirmed by Tester’s informative post above, shorting the connector contacts might not work to turn the fan on. That would correspond to zero ohms, and that is probably out of range for the thermistor they use. I’d expect a shorted connector would generate a diagnostic code more likely than turn the fan on. Zero ohms could conceivably damage the PCM, but that’s unlikely. The designers anticipate that happening and put in fail safes to protect the PCM.

Looking at some info for your model it shows that there is a yellow wire tied to the temperature sensor that should supply power from the ECU to that sensor. It doesn’t show how it wired and most likely isn’t directly tied to the 5 volt supply. Since you saw no voltage between that lead and the return lead it may mean that the yellow wire has a break in it. The yellow wire should tie to pin 26 of PCM. The return side of the sensor should tie to pin 12.

Constant power should tie to the fan relays so you should be able to ground the dark blue wire, pin D11, and turn on fan 2. If that works okay then the fan circuit operation should be okay and the PCM isn’t turning on the fans as it should. That may be due to a problem with the temperature sensor wiring or the PCM itself. All power inputs to the PCM should be verified before calling the PCM bad. I suggest you purchase a good code reader or scanner so you can look at live data like the temperature sensor to see what the PCM is looking at. Something like the Innova 3040 would do the job. Ebay is a good place to find code readers for a good price.

I am pretty sure the a/c does not work because the fan does not come on. I don’t know if you have one or two fans, but assume it is one only. The fan cools the water as well as the freon inside the condenser coils turning the freon from a gas to a liquid again… check the high pressure a/c line sensor.

I have two fans in front of the radiator. I have pulled out the relay and jumped the connection to start the fans (individually, both relays separately) and both fans start and work.

It seems weird that neither fan will start when turning on the Air or when the car is heated up. For it to be a wire that’s broken wouldn’t it have to actually be two wires that are broken (one for each fan coming from the PCM)? Or wouldn’t it have to be two relay’s that are broken? Etc…

I know I should have checked more but a bought a programmed PCM and put it in. There was no change, the fans still don’t start.

On another site a person listed 6 reasons to why the fan don’t turn on:

  • blew 12V fuse (Unsure, but fans work when jumped)
  • blew ground fuse [not all model have this one](Unsure, but fans work when jumped)
  • failed relay (Could be, but I swapped relays including one that’s for the horn that worked and the swapped one worked on the horn)
  • fault in wire from ECU to relay (Could be, but wouldn’t it have to be two bad wires?)
  • fault in wire from relay to fan (Not - can jump and start)
  • faulty ECU (Not - Replaced PCM no change. Is the ECU the PCM?)

I checked many fuses and all were good. Can someone list all the fuses that could be related to the radiator fans not running?

Many questions, please help.

In case you don’t already have it, here’s the fan wiring schematic to help with your troubleshooting (click for full size):

Note that the mode relay is a different type than the other two, being double throw vs. single throw. Also note that both fans always run together, in either a low or high speed mode. Low speed runs the fans in series, while high speed separates them. The 4 cyl. doesn’t have the ground fuse.

The coolant temp sensor receives a 5v reference on the yellow wire from PCM connector 1, pin 54. It is grounded on the blk/wht wire at PCM connector 2, pin 27. The pin numbers given by @Cougar above are for the V6 wiring. Jumping the wires at the sensor connector would make the PCM think that the sensor has zero resistance (i.e. the coolant is extremely hot), so that should trigger the fans on high, though it may also set a DTC

This could also be a BCM (body control module) issue. According to the written description of the fan system in the GM manual, the PCM and BCM work together to determine when the fans are triggered. A BCM problem could potentially explain the A/C and gauge issues also, though the A/C issue could be any number of problems.

I’m learning a lot about fans and their circuitry. Thank you Bugmenot for the illustration. Until I figure out what the issue is I put a switch in the car to turn the fans on manually (I ran a wire into the car from pin 85 on the cooling fan #1 relay and put it on a switch to ground). The biggest fear with the fan switch is that I leave it on and burn out the battery. I don’t want to keep it this way.

When I have more time I’m going to test the Temperature Sensor wires going to the PCM and also the Fan Relay wires. My suspicion is either a wire is broken from the temperature sensor or the Body Control Module is bad.

The questions I have are:

Does a replacement Body Control Module need to be programmed or can I just buy the correct model from a salvage yard and put it in?

Where can I get the complete PCM (16228016) pin diagram?

On a side note, I’ve been getting the best mileage I ever had with the car. I’ve been getting 30+ miles per gallon. I assume it’s because of not having the radiator fans running or the air running.

From the GM service manual:
The BCM must be programmed with the proper RPO (regular production option) configurations. The BCM stores the information regarding the vehicle options and if the BCM is not properly configured with the correct RPO codes the BCM will not control the features properly.

Here are the PCM connector pin charts (click for full size, & please forgive the poor resolution):

As you may already know, the ECU controls the fan relays by making a ground connection to the relay coils to turn on the relays. You have proved that the relays and fans work using your bypass switch. Check for 5 volt power on pin 54 that ties to the yellow wire going to the coolant sensor. If you have no voltage on that pin then make sure that all the 12 volt inputs to the ECM are good. If that is okay then the ECM is suspect.

Another thing you might check if power isn’t getting to the OBD-2 connector, find out where that power comes from and why it isn’t getting to the connector.