Question about 2001 Pontiac Montana

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AC problem with 2001 Montana passenger van

I had an AC problem diagnosed by a professional firm and they determined my compressor is bad. Long story short.... I replaced the compressor, accumulator, and both front and rear expansion valves (2001 Montanna with front and rear air). I pumped vacuum for an hour, then filled with 2 lbs of R134a freon (spec is for 2.2 lbs.) Low side pressure at idle is 50 psi..... at 2000 rpm will drop to 40-45 psi. But high side stays constant around 200-250 psi. And air is only moderately cool coming out of vents.

Is it possible I purchased a bad compressor? It was a reman. I don't know what else it could be. I feel I have replaced everything that is reasonable.

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  • JOHN DAVIDSON Jul 21, 2010

    The pressures seem adequate for cold air flow to be happening...did you put a few ounces of compressor oil in with the charge? If not evacuate a little and add a can of 134a with oil/freon mixture. Check the vents perhaps some of your air is escaping , the compressors are generally made well however it is remotely possible you might have a bad one. be certain the clutch is holding long enough, do your homework and see if this system has an a/c cycle switch that perhaps has gone bad? Hope this works. Happy motoring!!

  • lcwillisjr Jul 21, 2010

    I did put 2 ounces of oil in. Is there a way to tell if you have too much or too little oil?

    The compressor does engage and stays engaged. That indicates to me that the temp sensor doesn't get cold enough to kick the compressor off. I've removed the ducts and can place my hand right on the evaporator and is only mildly cool.

    Also..... when I shut the engine off the low and high side pressures seems to equalize withing a few seconds. I thought this took several minutes as the freon equlized through the expansion valves.

  • lcwillisjr Jul 21, 2010

    It has front and rear air, so yes I replaced both the front and rear expansion valves. No sight glass other than the one where the freon line leads into the AC guages. None on the system.

    I held off adding any more freon as the low pressure side was already reading 50psi even after adding 1.5 lbs of freon. I was hesitant to overcharge although I went ahead and added another .5 lbs bringing the system up to 2 lbs total. So I know I'm little under the 2.2 lbs specified at this point.

    It seems that if the compressor was working properly that the low side would have been lower (20-30 psi) and increase up to 45-50psi as I added the freon. :-(

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Did you replace the expansion valve?
Also try adding just a little more freon and see if that makes a difference....is there a sight glass you can view with the system activated to be sure you have it completely air free?

Posted on Jul 21, 2010

  • JOHN DAVIDSON Jul 21, 2010

    Dont forget the ambient air temp(humidity etc.) has all to do with the variation of readings also...so the more humid the higher pressure and the less cooling effect.

    If the lines are icing uo its probably from the humidity.

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