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High Low side Pressure

dji on Wed April 25, 2012 10:31 AM User is offline

Year: 1994
Make: Chevy
Model: Silverado
Engine Size: 350
Refrigerant Type: R-134
Ambient Temp: 85
Pressure Low: 80
Pressure High: 100
Country of Origin: United States

I replaced my high side service value (small leak). Vacuum was good and added some oil to the low side. I have added 2.5 cans (12oz) of R134 and I am not sure what these readings are telling me?

Would adding to much oil (1/2 can) or maybe not bleeding the air out of the lines before adding R134 cause this?

Not sure what to do next except to start over?


Static test reading 85
Compressor is cycling on/off
Line to evaporator does get cold

GM Tech on Wed April 25, 2012 2:39 PM User is offline

One of three things:

1.. Are both valves on gage set closed?

2. Do you know for sure you have an orifice tube in the system? - it is in condenser outlet- if condenser has been replaced- it could have been forgotten.

3. Your compressor is shot

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......

dji on Wed April 25, 2012 4:31 PM User is offline

Both valves on the gauge were closed but I will check again.

The orifice tube was in there when I bought the truck in 94 and last summer it has been working. I did have the passenger fender replaced 6 months ago. No reason for them to touch the condenser but you never know.

It must then be the compressor? I was hoping that since it was cycling and the line to the evaporator was cold it was something else like air in the line or too much oil.

I will check everything 1 more time and look and see if the condenser may have been removed.

If the compressor is bad will you always see a high low side reading like 80?

Thanks for your help..

Dougflas on Thu April 26, 2012 10:32 AM User is offline

Yes, low side could be high because you are reading ambient temperature influence. The compressor if bad is not "sucking" the low side down and not pumping the high side upwards. Too much oil would still give you better guage readings but cooling capacity would be reduced so I don't think that is your problem. If you pulled a good vacuum as you stated and purged the hoses before opening the handwheels, you should not have air in the system. Air in the system would increase the high side so I don't think that is your problem. It appears the R4 compressor has taken a dump. Your low side and high side are too close to being the same psi. Are your gauges zeroed out? If not, you may be at the same pressures also indicating the compressor is not engaged. Make sure the clutch plate is pulled in and spinning.

dji on Thu April 26, 2012 7:54 PM User is offline

It looks like the gauges I was using had some issues. Good news they were borrowed from the parts store. With a new set of gauges I have 43 on the low side and 125 on the high side (at 800 rpms only and 92 humid deg's). This makes more sense since I only added 2 (12 oz) cans. Compressor (clutch) is kicking on/off but now I am assuming it is just a little low. I will buy another can and see what happens.

Thanks for all the information. It was a great help for a non professionals like me.

mk378 on Thu April 26, 2012 8:17 PM User is offline

Since this is a factory R-134a system, charge it with exactly the specified weight of refrigerant. The extra oil is not advisable but probably not a deal killer either. Compressor should cycle off at 20-25 psi and back on before 50 psi but with a low charge it may be cycling too fast to tell exactly what the pressure switch is doing. Do not run much with a low charge because the oil doesn't circulate like it should.

Edited: Thu April 26, 2012 at 8:18 PM by mk378

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