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Question on high pressure on high-side

nlt99 on Wed June 24, 2009 1:05 PM User is offline

Year: 2003
Make: Buick
Model: Rendezvous
Engine Size: 3.4L
Refrigerant Type: R-134a
Ambient Temp: 100
Pressure Low: 22
Pressure High: 420
Country of Origin: United States

At idle, the pressures are: Low- 38, High - 220 and outlet temp is 59 degrees at ambient temp of 100 degrees and 17% humidity.... all about normal per factory documentation. As I increase RPM however, things start to go haywire: @ 1500 RPM, low side goes to 20 and high side to 350. outlet temp to 53 degrees. @2000 RPM, low side stays about 20 but high side increases slowly to 420 before hi-pressure cutoff switch opens and clutch disengages. Outlet temp decreases a bit and then goes warm of course once the clutch disengages. I have also seen low side go a low as 15 at high RPM. The problem seems to worsen the longer the A/C is on during testing.

Could this problem be due to overcharge, or am I more likely looking at contamination in the system, or some other problem?

I would appreciate any input. Thank you! Nick

HVargas on Wed June 24, 2009 2:57 PM User is offlineView users profile

What have you had done recently. Have you recharged it or had any sort of work done?


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bohica2xo on Wed June 24, 2009 4:16 PM User is offline

Check the condensor fans operation.

When the high side crosses 190 psi, the BCM/PCM commands low speed fans. WHne the high side gets to 240 psi, the command is for high speed fans.

Both fans should be turning, and they should be operating @ high speed over 240 psi.

Check the underhood fuse block, fuse 39 & 40 are for the left & right fans. Relays 59, 61 & 62 control the fans. The fans operate in series for low speed, and parallel for high speed. If relay 59 craps out, the left fan will stop running when they switch to high speed, and the right fan will speed up. Visiually check to see that they BOTH speed up.

Also check the air path on the heat exchanger stack. It may be filthy down inside the fins.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

nlt99 on Thu June 25, 2009 2:46 AM User is offline

Thanks bohica2xo!! Before I read your response I got the "brilliant" idea to check outlet temp on the road. At 110 ambient & steady 2000 RPM, outlet temp ran at a steady 43 degrees, so I figured I might have an airflow problem when standing still, although I didn't have pressure gauges on it of course. I appreciate your detailed and complete response and will check all of that carefully. I wasn't aware of the 2-speed fan function at all.

Regarding the first response, I just bought this SUV so have no knowledge of previous work other than that it had been in a front-end accident a few years ago. Thanks for your response also! Nick

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