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85 Chevy R4 compressor cycles off extended time

TNshadetree on Tue August 12, 2008 9:14 AM User is offline

Year: 1985
Make: Chevy
Model: Monte Carlo SS
Engine Size: 305
Refrigerant Type: R134a
Ambient Temp: 86
Pressure Low: 30
Pressure High: 175
Country of Origin: United States

I retrofitted my Monte Carlo with R134a and it works well as long as the compressor clutch is activated. But it tends to cycle off and stay off a very long time. I put the gauges on it last night a saw these conditions at idle.

ambient = 86F
Low = 30
High =175

When revved to around 3,000 rpm the pressures change to
Low =20
High =200

I waited some time hoping the compressor would cycle off while I watched the gauges, but no luck. But I could force the issue by turning the selector to off for a second and then putting it back on max. Although turned back to max, the compressor clutch stayed off. The system then stabilized at low = 83, high =83 and then of course climbed as the system absorbed heat ending up approx 95-100 each side.

I suspect there is an additional electrical switch in the system that can disengage the clutch. My service manual refers to some models having cutoffs for low idle speed and low discharge temps. But the manual I have doesn't include the section on AC electrical systems even thought the stupid thing is 4" thick.
I have noticed that when I turn the system to off and back to max trying to trick the clutch to into engaging, I hear the radio pick up a "pop" if it doesn't engage.

Any ideas?

TRB on Tue August 12, 2008 10:27 AM User is offlineView users profile

At what point "PSI" does the compressor cycle? May just need to adjust the cycling switch on the accumulator a little.


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mk378 on Tue August 12, 2008 10:39 AM User is offline

Hit the clutch plate when it is not engaged but should be. If that makes it snap in, close up the air gap. If not, check where the power is being lost (an "electrical problem" is anything that can't be fixed with a hammer). Start by jumping the cycling switch. If it's not the cycling switch you'll probably need to find a diagram and trace through everything. I doubt it was real complicated in 1985 but the command for the compressor may pass thru the computer which activates a relay to power it up (the low idle and wide open throttle kills are implemented in software).

TRB on Tue August 12, 2008 10:43 AM User is offlineView users profile

I need more caffeine this morning. Missed the point of cycling off and staying off.


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iceman2555 on Tue August 12, 2008 10:59 AM User is offlineView users profile

Not sure about the 'pop' sound...but this system is seriously undercharged. A standard 134a system operated at 2000 rpm, 86 ambient temp, and humidity factor of 30% should be closer to 30-35 low and 210-265 high. With a conversion this pressure should be much greater than those expressed in the post.
Charging with an attempt to maintain a perceived pressure/temp is a direct approach to a system failure...esp the compressor... pressures...esp with a retro fit...are not a true indication of a properly charged system. This procedure is a 'hit/miss' for the most seasoned of techs...for a novice....almost impossible.
Try getting the system properly charged and, if desired, adjust the LPCO. Although the company/tech department that originally suggested this as a part of retro fitting has long since reversed this suggestion....stating that the change was of such low value that the procedure did not contribute a significant value to system operation.
Making sure the system is correctly charged is the primary contributor to proper system operation.
Try this: Max Cool, High Blower, Doors Open, operate the system for app 5-7 minutes to allow the system to stabilize. The measure the inlet and outlet temps of the evap. Inlet is after the orifice tube...and outlet is before the accumulator inlet. Adjust charge until these temps are the same or within 3-5 degrees of each other. As adjustments are made...the system must be allow to stabilize after each change. Once the correct charge level is determined.....recheck pressures....if the high side is extremely elevated....removal of refrigerant is not the solution....additional cooling for the condenser is necessary. It may be necessary to add a extra electrical fan for the condenser...or perhaps, a simple change of the worn fan clutch will remove the extra heat necessary.
Only after the system is totally charged would an adjustment of the LPCO be considered....but the system should cool without this adjustment.
Good luck!!!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

TNshadetree on Tue August 12, 2008 3:54 PM User is offline

Thanks Iceman. You may have nailed it, because last night when I had it running I noticed with my calibrated hands that there was a significant difference between the evaporator inlet and outlet. I didn't know enough then to realize it was an issue. I just was looking around and noticed the inlet was frosted.

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