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John Miller
John Miller, ASE Master Technician
Category: Car
Satisfied Customers: 199
Experience:  37 yrs auto repair experience, host The Car Counselor Show on Talk Radio 950 KPRC Houston TX
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I have a 2001 Oldsmobile Alero and the cooling fans run ...

Customer Question

I have a 2001 Oldsmobile Alero and the cooling fans run continuously,even when the car is cold,just started. Why is this?
Submitted: 7 years ago.
Category: Car
Expert:  John Miller replied 7 years ago.

The most common cause of this is that the coolant temp sesnor is either bad of disconnected. the system will default to high fan operation if it detects a bad coolant sensor. It will also turn the fans on if it detect high freon pressure or a diconnected a/c pressure switch.

Here is the operational theroy of the system for you.

Notes

COOLING SYSTEM DESCRIPTION AND OPERATION

Cooling Fan Control
The engine cooling fan system consists of two electrical cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the Powertrain Control Module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood junction block. The ground path is provided at G103.

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the cool fan 1 fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan S/P relay and the right cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay control circuit. After a 3-second delay, the PCM supplies a ground path for the cooling fan 2 relay and the cooling fan S/P relay through the high speed cooling fan relay control circuit. This energizes the cooling fan S/P relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time the cooling fan 2 relay coil is energized closing the relay contacts and provides battery positive voltage from the cool fan 2 fuse on the cooling fan motor supply voltage circuit to the right cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

The PCM commands Low Speed Fans ON under the following conditions:

  • The engine coolant temperature exceeds approximately 106°C (223°F).
  • The A/C refrigerant pressure exceeds 1310 kPa (190 psi).
  • The vehicle is shut off when the engine coolant temperature is greater than 140°C (284°F) and system voltage is more than 12 volts. The fans will stay ON for approximately 3 minutes.

The PCM commands High Speed Fans on under the following conditions:

  • The engine coolant temperature reaches 110°C (230°F).
  • The A/C refrigerant pressure exceeds 1655 kPa (240 psi).
Customer: replied 7 years ago.
Reply to XXXXX XXXXX's Post: JOHN, A LITTLE BACK GROUND ON THE ALERO JUST SO YOU KNOW THOSE WERE GOOD SUGGESTIONS BUT THONGS I HAVE ALREADY CHECKED I.E. THE FANS RUN WHEN THE COOLANT TEMP IS LOW EVEN ON INITIAL START UP. THE A.C.PRESSURE IS 59 P.S.I.   I CAN COMMAND THE COOLING FANS OFF WITH THE MODIS I KNOW THAT MOST OF THE PEOPLE YOU DEAL WITH ARE AMAUTER BUT I DO HAVE A LITTLE EXPERIENCE AND WOULDNT BE BOTHERING YOU IF IT WAS SOMETHING THAT SIMPLE I HAVE ALSO RULED OUT THE POSSIBILITY OF P.C.M FAILURE DO TO THE FACT THAT THERE ARE 2 DIFFRENT SOURCES FROM THE P.C.M. FOR THE COOLING FANS ...ALBEIT RIGHT NEXT TO EACH OTHER ALSO THE FACT THAT I CAN COMMAND THE COOLING FANS OFF W/SCANNER
Expert:  John Miller replied 7 years ago.

Looking at the diagram and reading the operational detail the only thnig I can see if the pcm is good is that realy #1 is stuck. This would give you continous fan operation any time there is voltage to the fans. Control is all ground side on this for the left fan and power side for the right fan, but it uses the right fan in the ground for the left fan for low speed operation of both fans.

Check at the relays to see if the ground is being commanded by the pcm, if not it has to be a relay.

The Modis should show fan command status in the pcm and it should be off.

Do you use I-atn?

 

Customer: replied 7 years ago.
MODIS SHOWS PCM FANS BEING COMMANDED ON IF I UNPLUG ANY OF THE THREE RELAYS THE FANS JUST SWITCH FROM LEFT TO RIGHT I HAVE ALSO DONE THE SWITCH AROO W/ ALL 3 RELAYS SAME THING FANS JUST RUN BUT TODAY THEY ARE ONLY RUNNING ON LOW [YESTERDAY IT WAS HIGH AND ALL THE TIME] AND THEY ONLY COME ON AT 225[NORMAL] BUT THEN THEY WONT SHUT OFF EVEN WHEN THE TEMP DROPS TO 185 BUT IF I TURN THE KEY OFF THEN I CAN GO THRU THE WHOLE SCENARIO AGAIN W/ THE SAME RESULTS I ALSO TOLD MIKE[THE BOSS]THAT WE SHOULD BE ON IATN
Expert:  John Miller replied 7 years ago.

The commanded on means that the PCM is for some reason keeping your fans on. I would look at the data stream and see if anything is in the parameters that are listed above to keep fans on.

If you have good accurate inputs and the fans should be off but the pcm is still commanding on then the "logic" says that you have bad pcm.

I am going to paste the replies to a question just like this one on I-ATN, but be sure to look at the last reply. I never did ask if you had any trouble codes, they will keep the fans in default.

GM-Engine: 01 Oldsmobile Silhouette, Cooling, COOLING FANS
From:Merle Pfeifer
Status:Closed w/Fix (Reply to this Fix)
Post Date:Jan 8, 2004 12:00 PM
Rating:(Rate this Fix)

When van first came in we tested several circuits and found the fan relay, a/c relay, coolant temp sensor and thermostat were all working properly.Yet the computer was commanding the fans to stay on. It doesn't matter if the A/C is off, hea...

Hide Quoted Text
Reply 1 of 6
From: Randy from Pennsylvania
Date: Jan 8, 2004 2:13 PM

A very common failure mode in GM PCMs, is uncommanded
cooling fan operation. If all the PIDs show that the fan
operation is not requested, and you have proved out the
associated control devices, it certainly seems like the
computer is toast.

Reply 2 of 6
From: William from Florida
Date: Jan 8, 2004 4:14 PM

computer+flash again

Reply 3 of 6
From: Epifanio from Georgia
Date: Jan 8, 2004 6:11 PM

disconect the coolant sensor and see what happend you may
have bad sensor else replace the relay from the dealer and
the coolant sensor..good luck

Reply 4 of 6
From: John from Ohio
Date: Jan 8, 2004 8:29 PM

Check for a bad/shorted a/c high pressure switch. If the
pcm senses high pressure it turns the fans on, regardless
of compressor operation.

Reply 5 of 6
From: Troy from Minnesota
Date: Jan 10, 2004 6:35 AM

I have a 2001 chev Monte with the same complaint right now.
I have had it for a week and have done everything you have
done plus sensors and relays. Can you give me more info on
yours? today i am attacking the ac system.

This help request was closed with the following on Jan 15, 2004 7:53 AM:

First thanks to all of you that sent us an answer. A couple of you said GM is having trouble with ther computers controlling the fans in this year. We went through the systems and did not see any reason for the computer failing to control the fan operation. But sure enough the new computer had failed. We installed a second computer, flashed burned it and now the fans operate correctly again. We are a little concerned as this fixed this car once before. Only time will tell if it will last. Thanks again for all your help and suggestions.

The following replies were received after this help request was closed:

Reply 6 of 6
From: Troy from Minnesota
Date: Jan 15, 2004 9:42 AM

It might be back. I found the real problem on mine to be a
tstat. What happens is once the code is set it has a
default to turn the fans on. So once that code returns the
fans will stay on. I would call the customer and replace
the tstat. Sounds stupid but it fixed mine. The code set
because the colling system does not get to operating temp.
Good luck Email me and let me know

 

Customer: replied 7 years ago.
Reply to XXXXX XXXXX's Post: wow here we go again!!!! ha ha aha lol it did have a t stat code...i cleared it yesterday and the problem was still there i also logically figured that i had an engine not to temp code because.....the fan is always running seems logical??? well at this point unfortunately it looks like it might be going to the dealer i personally feel that it probably has a bad pcm but at this point it is a very expensive non returnable needs to be flashed guess and i [the mechanic] have already lost my a** on this car so i am cutting my losses at this point i do appreciate all your time but this will have to fall in the you just have to let some go catergory
Expert:  John Miller replied 7 years ago.

Been there doing that for the last 40 years!

You can get a tech subscription to I-ATN for $10 a month. Considering the time you lost here it would probably be worth it even if the boss wouldn't pay for it.

A shop subscription cost $20 a month. We have both here and use it ALL the time. It has saved me tons of money.

 

Customer: replied 7 years ago.
JOHN THANX AGAIN SORRY YOU LOST TOME ON ME BUT I DO APPRECIATE ITI ACTUALLY HAVE ABOUT 12 YEARS OF EXPERINCE IN DRIVEABILITY BUT WAS STUMPED W/ THIS ONE AND IT IS A GOOD CUSTOMER I REALLY WANTED TO HELP....I APPRECIATE ALL YOUR TIME AND IF I CAN EVER HELP YOU FEEL FREE TO E-MAIL ME ATXXX@XXXXXX.XXX YOU NEVER KNOW
                    THANX AGAIN
                          JOHN EASA

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