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Compressor wont engage

themow on Wed July 10, 2013 6:04 PM User is offline

Year: 2001
Make: gmc
Model: sierra
Engine Size: 4.8
Refrigerant Type: r134a
Ambient Temp: 80
Pressure Low: 25
Pressure High: 25
Country of Origin: United States

The compressor is not engaging at all at all but i turned it by hand and it is not seized. The readings were taken with the car off. I did not jump the low pressure cutoff and have not checked for voltage at the compressor. I am posting a picture of the gauges to make sure i am reading them correctly. Thanks

Edit. I used the upload image icon to attach a picture but its not showing up.

Edited: Wed July 10, 2013 at 6:05 PM by themow

themow on Wed July 10, 2013 6:19 PM User is offline

 photo gauge_zpsac38f78f.jpg

GM Tech on Wed July 10, 2013 6:34 PM User is offline

It takes 47 psi to engage your compressor- you have about 25-- you have a serious loss of refrigerant- only about 2 oz are ib your 2 lb system-- find your leak- fix it, evacuate and recharge..

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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......

themow on Wed July 10, 2013 6:38 PM User is offline

themow on Wed July 10, 2013 6:40 PM User is offline

the truck sat unused for 2+ years. The AC worked well prior. Would the sitting have any effect on the loss of refrigerant?

mk378 on Wed July 10, 2013 6:44 PM User is offline

When you first hook up the gauges with the car off (engine and compressor have not been run for a considerable time, it is called a static reading), the pressure should correspond to the car's ambient temperature, looking at the outer scale on the gauge under R-134a. If it is less, a serious leak has left no liquid refrigerant in the system, and it isn't going to work. That is the only time that you use that scale.

As you can see by the reading in the picture, 25 psi would be normal only at a zero C ambient temperature. The compressor will not engage then, since no one needs A/C.

themow on Wed July 10, 2013 6:52 PM User is offline

Ok the outer scale corresponds to ambient temp. Makes sense. So at 0 c a full system would read 25psi but at 70 or 80 here today should be in the 80 psi range. That makes sense. So sitting for extended periods should have no effect.

themow on Wed July 31, 2013 8:21 PM User is offline

themow on Wed July 31, 2013 9:02 PM User is offline

I am back to trying to get this unit working again. I figure before I find the leak I should make sure the compressor clutch is good because if its bad Ill probably just leave it alone. I am going to jump the ac relay because the power connector on the compressor could not be in any worse of a location. I am a bit confused as to why there is another connector on the back of the compressor though. I have a connector on the dryer(low side i assume) Then one on the high side line, one on the front of the compressor(clutch power?) then one on the back of the comrpessor? I know very little here and am just looking to learn a bit.

Jag987 on Wed July 31, 2013 9:19 PM User is offline

The one on back of the compressor is a high pressure switch if I am not mistaken, it is sometimes called a clutch cycling switch.


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I bought a can of 134a at w**-mart that had a stop leak, oil, and dye in it. It also had a hose and a gauge, so now I'm an AC pro!

GM Tech on Wed July 31, 2013 10:10 PM User is offline

One the back of compressor is indeed a high pressure cut-out switch-- never does it cycle a system nor is it ever called a cycling switch- that name refers to the evap inlet switch that is used to avoid evaporator freeze-up

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

mk378 on Wed July 31, 2013 11:06 PM User is offline

Do not jump compressor with the engine running. There's no protection against under or over pressure.

OK to do it with the engine stopped just to see if the plate pulls in.

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