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Cooling Fans not turning on unless AC is on, so overheats when AC isn't on


Eddie
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I just recently purchased a 2005 Buick Century and love the car! However, just recently the car started hotter than normal when idling or driving slowly, definition of hotter than normal is a bit over halfway up, that is unless the AC is turned on.

Turns out my cooling fan isn't kicking on. Mechanic replaced both the coolant temperature sensor and one of the relays for the fan. I am at a completely loss. The only other option I found via googling this issue is the PCM/ Computer acting funny. Which replacing that part on the Flagship One website would cost $115, and they program the part for you.

Also, would a PCM Malfunction cause the rear dome lights not to turn on, both of them are deader than dead even when you turn them on manually.

Of note, the car starts shifting rough between 2nd and 3rd when hot, like someone is tapping your car. Also, the power steering gets a little on the loud side when this happens. Fluid in both the tranny and P.S pump is fine, both systems also don't do this when at normal operating tempature.

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25 minutes ago, digitaloutsider said:

What temperature is it actually hitting? The fan turn-on temps are pretty high in the factory calibration, and the gauges are only vague indications of what is actually occurring.

About 240 degrees using a scantool

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You have to diagnose the circuit. I don't have a wiring diagram in front of me but I believe the command for the fan relays is the same for AC vs. engine temperature, so the circuit post pcm should be okay. If you confirm the pcm is reading the engine temp correctly and still not commanding fans, it might be shot. I'd double check all circuit continuity and all the relays. There should be three of them for the fans. The pcm has two outputs for the fans, a high and a low. Both would have to be dead for you to lose all cooling. I'm finding that hard to believe.

Edited by vipmiller803
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My first thought is seeing as you've been into a shop to have this rectified what got in the way of that shop NOT solving the problem?

If you've paid out money for their time & diagnoses why is the car not trouble free?

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The fuses and relays were checked. Will follow vipmiller803's advise and check the circuits, as well as as PCM Temperature. Thanks everyone! Will have that checked within the next couple of weeks.

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I'll post your schematics tonight, if your cannot figure this out quickly  it would be better to cut the green wire going to the primary relay and ground the relay side temporarily to force the fan with ignition power. 

Yeah, that's ghetto but weighed against driving around 30* above boiling it's not a bad option. 

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That looks like the typical of that era gm setup for low and high fans. No primary or secondary fans there. Still, though, it is the same post ecm circuit for either engine temp or AC. I'd be looking at the cts again and its wiring. If you confirm the ecm is reading that correctly, it could be an internal ecm problem. Also, confirm the scan tool is reading reliably by measuring engine temp manually and by starting cold and watching the scan tool read ambient temp first. 

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  • 2 weeks later...

Could be the cooling fan motor? We had that issue when the shop thought it was the coolant temp sensor. Chances are it was the fan motor itself not getting any power.

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Well the cooling fans are getting power, as they do turn on when the AC is on.

On another note, the mechanic replaced the PCM with a new one off of Flagship One, that was supposedly reprogrammed to my VIN (Hence Plug-and-Play). However, it doesn't crank at all, only the dash lights come on. Tried calling and e-mailing the company, have yet to hear back. Just hoping that the company didn't program the PCM to the wrong Vin, or give me a PCM for another engine, in which case my car is out of commission for a week or two while waiting for a new PCM.  

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1 hour ago, Eddie said:

 

On another note, the mechanic replaced the PCM with a new one off of Flagship One, that was supposedly reprogrammed to my VIN (Hence Plug-and-Play).

The plot thickens.......

 

what became of the original PCM for the car?

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This has crossed the line from diagnostics to throwing money at it in an expensive and wasteful game of Pin-the-tail-on-the-Donkey.

Did you check any of the points of reference on the schematic?

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I think this was only slightly jumping the gun.

Of course a few continuity checks with a test light are warranted here. Then, I'd want to confirm the tcs reading with another scan tool first, making sure it does start out reading cold. It wouldn't be the first time scan tool data is read incorrectly. But really, what else will you do if the pcm won't command fans from engine temp but will for AC? It's the same fan circuit.

Edited by vipmiller803
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Agreed, PCM replacement should come into play IF it can be proven that it sees the ECTS properly and the rest of the cooling fan circuit is verified.

Testing the circuits is as simple as disconnecting the battery, unplugging the PCM, hooking the battery back up and jumping the 2 fan relay pins on the PCM connectors to ground.

The ECTS is easy enough to test without a scan tool from the PCM connectors with an ohmmeter and non-contact thermometer, the truth table is out there.

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Imp558, I printed out the schematics a few weeks ago and gave them off to my mechanic. He said everything checked off as far when if came to getting proper voltage through the wiring system, he also was able to run the fan via a portable battery. Fuses and relays were checked during this time, with one of them even being replaced to no avail. The Coolant Temperature Sensor, a common point of failure with W-Body's was also replaced. Unless an important step was skipped without my knowledge, my mechanic has been attempting to properly diagnose the system.  

Edited by Eddie
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If you can see the proper temp on a scan tool and cycle the fans with either the scan tool or by grounding the wires at the PCM plug then you're on the right track. 

Did you have any success with VATS relearn?

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No luck with the VATS relearn yet, I have a suspicion that the company may have shipped me a PCM for a different model.  For instance, the car won't even crank after the relearn. For what I understand, a car that is started with a blank or key not programmed for a particular car would actually start the engine, but will turn off after 15 seconds or so. My GF's 2001 Grand Prix strangely enough would crank with no key in it, that was until the ignition on it was replaced. 

I would also liked to add that this car had the VATS completely bypassed a few years back, the previous owner had to literally retrain the key everyday before he could drive it. So, instead of fixing the ignition, he bought a resistor pack online to bypass the system. 

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That sounds funny, shouldn't that be a transponder key? After the re-learn the security light should go out, some cars respond to the 3 X 15 minute re-learn and not the 3 X 10 min.

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