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compressor cycling at fast idle normal?

ron7001 on Thu June 28, 2012 9:11 AM User is offline

Year: 2006
Make: GMC
Model: Sierra 2500HD
Engine Size: 8.1L
Refrigerant Type: r134a
Ambient Temp: 75
Pressure Low: 30
Pressure High: 150

does anyone know what type of ac compressor is on my truck?
and where can i find info about AC compressors for what type they are such as clutch cycling or variable stroke and all that?
Actually, would like to find info on auto ac compressor designs.

bought this truck used, has 75k miles, and yesterday idling in my driveway was first time i have checked the AC.
It was fairly cool out and late evening no sun and low humidity.
Air out the vents was always pretty cold, I'd say 55F or less.
At 600 rpm idle I turn the AC on, compressor kicks on and low pressure holds steady at 30psi and compressor stays engaged and does not cycle.
If I hold idle at 1500 rpm the low pressure side never seems to go below 30 psi but the compressor will cycle on and off. It'll kick on, low side will drop to 30psi and stay there for not more than 5 seconds then compressor will cycle off. The radiator fan is driven by the engine, not electric fans.
Is the compressor cycling normal? Or should I wait for a hotter day and recheck? My first thought since it's used and 6 years old is the system is low, but from what I observed i'm not 100% positive it's low so i'm hesitant to add refrig.

bueller on Thu June 28, 2012 1:29 PM User is offline

What was the high side pressure doing when you went to 1500 rpm?

GM Tech on Thu June 28, 2012 1:56 PM User is offline

You have a fixed displacement Sanden compressor--- cycling could easily be high pressure cut-outs - what is your high side pressure when this happens-- use a real gage set, not a filller gage. Spray water on condenser from a garden hose when it is cycling-- does cycling stop? if so , you definitely have an air flow across condenser issue....

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

ron7001 on Thu June 28, 2012 2:24 PM User is offline

thanks.
high side was around 150psi, definitely under 200 psi the whole time whether it was 600 rpm idle or 1500 rpm idle.
using a general manifold gauge set from HF.
now that i think about it, this system is set up with the high and low service ports just before and after the orifice tube, not sure where the low pressure switch is I think it might be at the compressor inlet. so maybe i'm seeing near 30 psi at the orifice tube but the switch further downstream near the compressor is seeing much lower and cycling because the system is low in refrig ?
I have the gm service manual for the truck and it shows normal low side pressure to be 35-45 psi, maybe because of where it's being measured?

having a fixed displacement compressor then, does that mean it is clutch cycling as in CCOT ? should i expect the compressor to cycle off under cool conditions as normal operation?

Edited: Thu June 28, 2012 at 2:26 PM by ron7001

GM Tech on Thu June 28, 2012 4:31 PM User is offline

Cycling switch is on accumulator.

Cycling is normal to avert freeze-up in low ambient conditions

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

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