A/C Problem No High Side Pressure

rlchv70 on Wed February 20, 2013 4:22 PM

Year: 2004
Make: Pontiac
Model: GTO
Engine Size: 5.7L
Refrigerant Type: R134a
Ambient Temp: 40
Pressure Low: 70
Pressure High: 70
Country of Origin: United States

Working on my personal vehicle in my garage. Not a technician by trade, just a backyard mechanic. Limited experience with A/C systems.

A/C doesn't work. A/C clutch won't come on. If I command on with scan tool, it will work. However, still doesn't blow cold. I attached gauge manifold. Has plenty of low side pressure.

However, high side pressure is identical to low side pressure, even with compressor commanded on.

Scan tool shows pressure sensor reads accurately. No codes.

Any ideas? Thanks.

GM Tech on Wed February 20, 2013 6:47 PM

Two possibilities:

Pump is shot, broken reeds, or broken shaft

or your gage set is open - be sure both valves are closed CW

turn front of compressor by hand- doe it feel like it is turning pump? or is it really loose?

Or.....70 psi is a very common pressure- for an ambient exposure to refrigerant trapped in a gage set- with no communication to the vehicle--are your gage taps open (CW) at the gage access ports on the vehicle?

-------------------------
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......

Edited: Wed February 20, 2013 at 7:58 PM by GM Tech

NickD on Thu February 21, 2013 7:30 AM

Testing pressures are done at 1,500-2,000 rpm, doors open, AC on, blower at max and at an ambient temperature of at least 80*F. At 40*F, assume F, only way to know if the system is properly charged is to recover, measure, and put the correct amount of refrigerant back in.

Are you even measuring a single vent opened vent temperature? At 40*F, would only expect a couple of degrees drop with the AC on. Also at 40*F marginal if the high side temperature sensor would even turn on the AC. Should turn on with the hood closed and the engine at operating temperature.

Wait for a warmer day.

GM Tech on Thu February 21, 2013 11:04 AM

Yes - did not see the 40 degf ambient-- your system is monitored by a high side pressure transducer which must see at least 47 psi to activate compressor relay--at 40 degf amb- you have about 40 psi in the system- (look at your P/T chart for 134a) - you basically can't evaluate a/c unless it is at least in a 70 degf ambient.

-------------------------
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......

rlchv70 on Fri February 22, 2013 9:18 AM

I think it is the compressor. Spun it by hand and it turned very easily.

NickD on Fri February 22, 2013 9:55 AM

It better turn easily by hand, even the slightest bit of binding tells you your compressor will be a part of the history books.

GM Tech on Fri February 22, 2013 11:45 AM

A variable CVC compressor as is yours will turn very easily- especially in low ambient- since it is destroked.....

Put the car in a warm environment and test it-then pull out the refrigerant and weigh it- this will tell you a lot- the number one a/c failure mode is loss of charge- due to a leak.

-------------------------
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......