GM probs '05 G6 3500 - bizarre behaviour

[PeterPolyol]

Couple days back had some strange idling problem, a random instability.. yesterday I heard the cooling fans rev up on hi and not stop. Today, checked the ECT sensor and it's fine. Noticed some old rat damage on the sheath, wire wasnt broken but insulation compromised. Snipped and fixed it up hoping that's what it was. No luck.

The sensor seems fine, tested with another one also with same reult. Temp gauge works fine, Air/Fuel seems ok so ECU seems to be getting a correct engine temp reading, only problem is cooling fans coming on shortly after the basic warm up, temp gauge rightly shows a slightly low operating temp. It's below freezing outside.

Here's the bizarre behavior. The fans go on high, as they would during a very hot day with the AC going but just after the basic warm up (and at subzero temps). I think it has something to do with the AC system because when I toggle the AC switch, compressor kicks on and RPMs flare a lot higher than they normally would, briefly. Then when I toggle AC switch off, compressor disengages fine but the idle RPMs drop like a rock and become very unstable-- stalling half the time if not settling down after a few seconds.

I'm at a loss right now, is there a temperature switch somewhere in the AC system besides the thermister in the evap coil? What is the logic behind running the fan on high if there is a proper ECT signal? What really gets me is the near stalling when turning off the AC ... any ideas? Starts/runs just fine otherwise..

 

[eljefino]

I would get a phone with torque and obd-ii dongle (elm327) and watch live data. Maybe idle the car and tug on the wiring harness while doing this. Get every temp PID that's in the stream including ATF if possible. Torque logs the high points on the little gauges even if it's for 1/2 second.

You can also disconnect the AC pressure switches for diagnostics. If they can't communicate they have pressure, the compressor won't kick on, and the car won't demand fans for AC reasons.

 

[PeterPolyol]

Got it solved! Thanks eljefino, good call on that pressure switch. Just pulled the combo hi/lo AC pressure switch went for a drive and problem is gone. Looks like the high pressure switch is stuck on making the ECU run the fans to try and bring pressure down (logic probably assumes it's over-pressurized from heat). The RPM flare up must be from the ECU expecting a higher compressor load than exists, and the near stall must be the opposite side of the coin- ECU thinks it's dropping a huge load (that isn't there). Clever GM programming logic lol