Cooling Fans Running Constantly! Sounds like booster Rocket

Hi all, just bought a used Minivan, getting rid of my old 2000 (Still working well but rotten underneath!)and I have discovered the following issue a week after the sale

I live in Canada, it is currently -20 Deg C. (Cold)
When I start the vehicle, after about 10 seconds of running, the fans (both) start.
They don't shut-off until I turn the vehicle off.

I have changed all of the 3 relays that are labelled FAN1, FAN2 and FAN3. They all shutoff the fan if they are unplugged. (And restart if they are re-inserted), so I think that all of the relays are good.

I connected my OBDII reader to it, and the ECT is reading 65-90DegC, (the temperature gauge on the dash is in the middle). (From what I understand the trigger is over that temperature to start the fan/s.) The sensor does change as the vehicle warms up as well, I have not experienced any over-temperature conditions to date.

I am fairly technical, so I am at my wits end to know why these things keep running. I am unsure if there is a larger issue with this van or computer that could be causing this issue.

I have heard that the AC system may be overcharged as well, but I have not the tools to check that, nor do I know if it has anything to do with it.

Any help would be appreciated!

Sorry, it is a 2008 Pontiac SV6!

Have you check for codes?

Being that cold, I doubt your compressor is running with defrost.....

at 90C, it's not overheating.....

Thanks for the reply!

There are no codes. That is the puzzling part.
I have searched high and low on the internet, seems like some people had this issue, but there are no clear results about what was the cause. Some say the temp sensor, but I am confident that this one is working in the range that it should be. I disconnected it and started the vehicle, and a message about the AC Cutting Out was indicated. After reconnecting the sensor again and clearing the fault, the same problem occurred (2 fans running)

I understand that the ECU reads the temperature at the Engine Coolant Sensor and then provides a 0-reference out to the relay's in order to start the FAN1, FAN2 and then High Speed.

I think that the next step is to take it to a GM dealer to have it scanned.

I wish I had some friends that had this wonderful scan tool.

Well, you said it had no codes......some codes do not turn on the CE light right away, they have to fail on two consecutive drive cycles......

But if certain PCM codes are set, it will cause the fans to run by default......I don't think your vehicle has a separate coolant sensor for gauge and PCM, so if the gauge is not reading that it's overheating, then I doubt it's the sensor...

Is there any way to find out what PCM codes will turn on the fans by default?

I have a service manual that I purchased online and it doesn't mention anything about other PCMs, other than use a scan tool as the very first step.

Simdonald6, are you using a brush or your tongue?



There is only once PCM......so if your scan tool shows no current/history codes, that is not the problem....

On initial start up, how does the ECT compare to the IAT....they should read the same.....if it does, a control circuit to one of the fan relays could be grounded(rub through condition), or there is a PCM problem....

Is the IAT the Incoming Air Temp?
I will do a screen shot of the variables sou you can see them.

On the morning that I took screenshots, (but I can't find them for some reason) it was -20 deg C outside, and the coolant temp from the ECT was -5deg, and started climbing when started. I didn't think that the outside air had anything to do with it.

Knowing that the ECM is located in the air intake, (I located the colours of the wires and referenced them to my schematic for the fans and relays) maybe it is the issue?

For now, I have simply unplugged the connector that feeds the fans at the RAD. With the temperature being cold I should not need to worry about requiring the fans. I do want to fix it though, I am just wondering what tools GM has that could possibly find out more information without a $500 bill. Is there scanner able to see things in the deeper part of the computer?

From what I see, the relays are OK, the wiring is OK from the relays to the FAN/s, and that the wiring is good from the ECM to the Relays, because there is a 10s delay before the fans are commanded to start. I would also think that the coolant sensor is OK, because it is reading some good range and not outside of the expected range.

I really like the van, so I am hoping that this is a relatively small problem, I don't have the cash to spend tons fixing this issue. I am lucky that it is the winter here now, if this was the summer time I would be really annoyed with the jet-airplane 3.9l. roaring away.

Ok, I have taken some screen captures of the engine stats while it is cold, off and started. (Due to the temperature, the fans should not be told to run)



What else tells the fans to run? I have seen that the AC pressure being too high will turn them on as well. I have a recharge kit with a gauge, should I bother checking?

Once again, I appreciate the help!

Scan tools never tell what is wrong with a car......only a good tech who can interpret the data, can do that......

Yes, that 10 second delay, does make me think, the PCM is COMMANDING the fans on.......

You say the fans come on, 10 seconds after you start the car...... does the same thing happen, if you just turn the key to on(without starting the car)?

When the car is started, do your have your Defroster on? If yes, shut the car off, turn the HVAC control head to off.....wait one minute.....now restart the car......if the cooling fan does not come on, then you might be onto something with the A/C pressure being too high.......it doesn't mean that your pressure is actually high, it could be a bad sensor sending a high pressure signal, when it shouldn't be(if temp is around 4C, pressure, even in a fully charged system, should be very low)......scan through the data to see if you can find A/C pressure......I believe low, would be below 1.0 volt, average running pressure would be around 2.5 volts, and high would be 4.0-5.0 volts....if you can't find this data, follow your A/C lines and look for a 3 wire connector to the sensor in the line.....disconnect it......now start the car.....turn on defrost and see if the fans run.......If they don't, then you need a new A/C pressure sensor....

Thanks Tech II for your help.

I was very optimistic about your grounding theory. If the harness at some-point on the two negatives feeding the Relay Panel were shorted, they would indeed be turning on the fans uncommanded. (They would both need to be grounded)

After thinking about it, it doesn't make sense, due to the time delay being consistent at 10sec for the fans starting after the engine is running. If they started right away with the engine in RUN, that would make perfect sense. I tested the circuit to the relay solenoid, and the ground is removed when the key is out of the RUN mode, so the PCM is definitely telling the fans to run.

If the key is turned forward, the fans do not start, only after the engine has been running for 10 seconds.

Do you think that GM would have the magical tool that would reveal why this is turning on, or do you think that I would have to pay for several hours of troubleshooting to get to where I am already?

There is no magical tool.....it's possible, there is a TSB out there that mentions your symptoms.......

It's possible, that a good tech when looking at scan data, may notice something out of the norm.....

Did you try the other tests I mentioned?

I can't thank you enough for the replies Tech II,

Yes, I tried your suggestions with the keyswitch in the ON position, with the engine not running. They don't turn on. (the fans)

According to the service manual, they are supposed to run it would appear.
How do you interperet the manual? (attached)

I have not looked into the AC thing at this point.
(The defrost is OFF, the fan knob is OFF) but after the engine is started the fans start up again, same old sequence.

That thumbnail does not open.......just for ha-ha's, disconnect the pressure sensor in the A/C line, and then start the car....see if the fans come on......

Sadly the pressure switch being disconnected had no effect.

I am completely out of thoughts. It has to be the PCM for some reason or another.
I will get it to the dealer once I get enough time......

One of the front wheel bearings just started to fail now.....this is starting to remind me of my last Montana! (Once I replaced the bearings they lasted for several years though)

I am re-posting the picture to see if it works here. It is funny, the pic is displayed, but it won't launch. edit: wont let me re-attach the file, it says that it is already displayed!

Sadly the pressure switch being disconnected had no effect.

I am completely out of thoughts. It has to be the PCM for some reason or another.
I will get it to the dealer once I get enough time......

One of the front wheel bearings just started to fail now.....this is starting to remind me of my last Montana! (Once I replaced the bearings they lasted for several years though)

I am re-posting the picture to see if it works here. It is funny, the pic is displayed, but it won't launch. edit: wont let me re-attach the file, it says that it is already displayed!
Perhaps the wires that go to the pressure switch are shorted together.

Well, I finally grew the courage to take the van to the dealer (GM)
I took it in and accepted a $129 diagnostic fee, in order to find out the root cause of the cooling fans running.
After a whole day of not hearing from the dealership, I was starting to get worried. Finally near the end of the day I heard from the service advisor indicating that the techs had been working on the vehicle for the better part of the day. It had numerous codes set, (I could not read them due to the fact that you need the GM scan-tool).
They reported to me that the reason it seemed was from the AC pressure on the High Side. It required an AC service, to relieve the pressure, and bring the codes back to normal.
(I did remove the sensor and verify that this was not the issue....although I could not clear the codes that were set).
So, after close to $500 dollars spent on servicing the AC system (Evac/test/recharge) and a sensor connector replacement, the vehicle is working again.

I want to thank everyone whom offered advice. I have always leaned on these forums for help, and in most cases the problems were solved with working with experienced individuals who provide advice. I feel a little bad, that I couldn't resolve the situation, having a process control background. (I guess the fact that the GM PCM holds and retains codes, it will set outputs accordingly until cleared by a factory GM Scan tool)

Once again, thanks for the help! Now onto the next issue! The darn heat thing, (no heat at idle)