The GMC Owners Group

j pate said:

my compressor is not pumping ,what will need to be removed to replace it
any help thanks. 99 gmc sierra 5.3

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Before you go off replacing the A/C compressor, make sure it's not electrical, low on R134A, or a faulty A/C clutch that can be replaced with out removing the compressor from the engine accessory mount. Does the compressor clutch activate and what was your A/C high pressure side readings with the clutch engaged??? Post me back on your readings, low R134A in the system will not activate the A/C Compressor clutch and also a bad A/C pressure switch will do this as to a faulty relay.

Feel free to post back, just trying to help you save $600.00 worth of repairs that could only be a simple O ring replacement and recharge.

Thanks for responding , I have checked all the above with clutch pulled in
you can here the engine speed up no presser change on low or high around
28 - 30 equalised or running ,I'm afraid that's all that's left change out ,just didn't know whats involved getting to the compressor vacuum pumps gages
standing by , if you can think of any thing els please let me know ,truck has
328,000 as is perfect just cant put to sleep let me know thanks here or
[email protected]

j pate said:

Thanks for responding , I have checked all the above with clutch pulled in
you can here the engine speed up no presser change on low or high around
28 - 30 equalised or running ,I'm afraid that's all that's left change out ,just didn't know whats involved getting to the compressor vacuum pumps gages
standing by , if you can think of any thing els please let me know ,truck has
328,000 as is perfect just cant put to sleep let me know thanks here or
[email protected]

Click to expand...

Below are normal car AC pressure readings with 134A.

- Normal readings on high and low side with AC OFF (static pressure) – Depends on outside temperature, but normally is between 80-105 PSI

- Normal low side reading with AC on high speed and MAX & engine at 800-1000 RPM’s – Ranges from 25-35 PSI

- Normal high side reading ranges from 200-350 PSI

With the AC on the coldest setting, use a thermometer in a middle vent. Normal vent temperature readings will vary depending on the (ambient) outside temp. The vent temperature should range from around 42-55 degrees in my experience. If normal gauge readings are obtained and the vent air is cold – STOP don’t overcharge the system. The only proper way to remove refrigerant is with a AC recovery machine so if this is being done at home I can’t emphasize enough not to over charge the system. And actually the best way to insure the proper charge is in a system, is to use an AC machine to recover the R143a and then evacuate and recharge the system with the correct amount. Most cars have the specified amount on a decal under the hood.

Here are gauge readings with the engine running and A/C clutch engaged:

- Both low and high side readings are lower than normal, this indicates a cars AC system is low on refrigerant and is under-charged.

- If both low and high side readings are too high, this indicates an overcharged system – too much refrigerant. This also can indicate that the condenser fan is not working, is too slow or the car is overheating and heat is transferring from the radiator to the condenser.

- When the low side goes so low that it’s reading shows it is in a vacuum, the most likely cause is a bad expansion valve or blocked orifice tube. Another possibility is a restricted condenser. Blocked condensers are not as common as they used to be but if a compressor fails and comes apart inside the remnants can end up in the condenser causing it to restrict the flow of refrigerant.

- When the compressor clutch is definitely engaged and the low side is high and the high side is low, the most likely cause is that the compressor is failing – it is not pumping sufficiently. Rarely an AC clutch could be slipping but usually this will be accompanied with a squeal or chirp. Also be sure not to overlook the obvious, like a oil leaking from compressor.

Note: The line going from the compressor to the condenser is the discharge line – it is normal for it to be very hot to the touch. The other line going from the drier or accumulator to the compressor is normally colder. The liquid line can be hot up to the point an orifice tube is in place. Just remember that LOW Pressure = COLD and HIGH Pressure = HOT.

As for the replacement of the compressor, you can have your system vacuumed out at a shop and then do the labor your self to replacing the Compressor. Once you have replaced the compressor, drive back to the shop and have them recharge the system. They will vacuum out any moisture in the system and recharge it for you and the cost should be the same as a A/C recharge service since you did all the labor on replacing the compressor.

When replacing the Compressor, make sure you replace the O rings for the A/C line fittings.

Thanks you know your stuff ,got some checks to do, working and all it may be a few
days , I will let you know what I find thanks jim.