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A/C low side pressure very high

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Old 04-15-2008, 12:32 PM   #1
RoccoC5
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Default A/C low side pressure very high

My air conditioning is no longer blowing cold air. I can see that the clutch is engaging and that the compressor is running, but the low-side pressure does not cycle like it should. It remains at 100 psi or so.

Does this mean my compressor is shot? Anything else it could be?
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Old 04-15-2008, 01:13 PM   #2
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These are the norms:
A/C System Pressure Readings
Ambient Temperature Low Side Pressure High Side Pressure Center Vent Temperature
Low Hi
60 F 28-38 psi 130-190 psi 44-46 F
70 F 30-40 psi 190-220 psi 44-48 F
80 F 30-40 psi 190-220 psi 43-48 F
90 F 35-40 psi 190-225 psi 44-50 F
100 F 40-50 psi 200-250 psi 52-60 F
110 F 50-60 psi 250-300 psi 68-74 F
120 F 55-65 psi 320-350 psi 70-75 F
Low Compressor Discharge Pressure
If the pressure on the low side is high the following may be the problem.

1. Leak in system Repair leak in system
2. Defective expansion valve Replace valve
3. Suction valve closed Open valve
4. Freon shortage Add freon
5. Plugged receiver drier Replace drier
6. Compressor suction valve leaking Replace valve
7. Bad reed valves in compressor Replace reed valves

Usually The following applies:
Low Pressure Low
High Pressure Low Low refrigerant Charge

Low side normal
High Side Low High Side Restriction

Low side normal/high
High Side Normal/High System Overcharged


Low side high
High Side high Moisture in the system


Low side Low
High Side Normal Faulty Cycling Switch

System will not operate Stuck open Cycling Switch

ow side high
High Side high Inoperative cooling fan, blocked Condenser

Rent or buy a Vacuum pump hook it to a compressor and evacuate the ssytem.
Evacuating the System:
Evacuation is performed with a Vacuum Pump which literally sucks the air and moisture out of the system. To properly evacuate a system, the pump must be capable of producing 29"hg of vacuum. At 29"hg any moisture in the system begins to vaporize sufficiently to be drawn from the system. The industry norm is to leave the vacuum at29"hg for 30-45 minutes.
Once the connections have been made, turn on the evacuation equipment, open both high and low side service valves on the gauge set.After approx 5 minutes, the gauge readings should be steady at 29&quothg. Continue evacuating for another 45 minutes. After this period, shut off the valves in your gauge set, turn off the vacuum pump. Note:It is now that I recommend checking for leaks. Your gauges should hold this vacuum point for at LEAST 20 minutes. If there is any reduction in vacuum readings there is a good possibility you have a leak.Again, with coolant as expensive as it is, you can't afford not to check for leaks.
Just replaced a condensor on my neighbors daughters car and had to evacuate the system afterwards. Both the suction(low) and pressure(high) sides were high. Suction pressure should not exceed 70 psi.
Evacuate and see if it hold pressure for at least 20 -25 minutes. If there is a leak, pressure will go away.
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Old 04-17-2008, 12:54 PM   #3
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Thanks for the reply. I just evacuated and refilled the system and the problem remains. I'm told that there are some valves in the compressor that are likely stuck and that it needs to be replaced. The tech said that he didn't see any material come out during evacuation so it's doubtful that there's any blockage in the system.

Rather than pay $1200 to have it replaced, I think I'll have them evacuate the freon again and I'll replace it myself. Looks like I have some work ahead of me!
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