Automotive Air Conditioning Information Forum (Archives)

Bosterfeld on Fri May 13, 2011 3:44 PM

Year: 1997
Make: Pontiac
Model: Grand Prix
Engine Size: 3.8l
Refrigerant Type: R134
Ambient Temp: 80F
Pressure Low: 70psi
Pressure High: 150psi
Country of Origin: United States

Hi guys,

I purchased this car a couple of years ago and the ac has never worked very good if at all. I decided to start fresh and evacuate it and charge it to spec. After evacuating for 45 minutes and recharging to spec the air is barely cooling if at all. My readings are 70 on the low side and 150 on the high side. The compressor runs all the time. Both cooling fans are running. I wanted to consult the forum before going any further. Do you guys have any suggestions? Know of any common problems on these that give off similar symptoms? Let me know what you think. Any help would be greatly appreciated!

GM Tech on Fri May 13, 2011 6:58 PM

70 low side is bad--Your V-5 compressor is designed to run all the time - but I'd venture to say you have a broken suction or discharge reed in compressor. Is compressor noisy- if so, that pretty much confirms a discharge reed issue.

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

GM Tech on Fri May 13, 2011 6:59 PM

70 low side is bad--Your V-5 compressor is designed to run all the time - but I'd venture to say you have a broken suction or discharge reed in compressor. Is compressor noisy- if so, that pretty much confirms a discharge reed issue.

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Bosterfeld on Fri May 13, 2011 10:38 PM

The compressor does have a slight rattle. I just chalked it up to the car having 168K. Just so I'm not replacing parts with out understanding why......What is the sequence of events taking place with a bad discharge reed that causes the condition I'm experiencing?? What's going wrong inside the system from this failed component. Also, I would assume that this is a problem only fixed by replacing the compressor? If so, what other components do you recommend replacing while the system is open (considering the year/mileage)???

Thanks!

mk378 on Sat May 14, 2011 4:20 AM

The reeds are one-way valves that control refrigerant flow in and out of the compressor's cylinders. Compressor not working on all cylinders means that it can't remove gas from the evaporator fast enough to keep the low side pressure down around 30 where it should be. Higher pressure raises the evaporation temperature of the refrigerant, and the evaporator doesn't get cold.