AC overhaul pressure problem

[dave5701]

1990 K1500 Chevy. Replaced R4 compressor with Sanden 508. Used Red orfice tube. All other parts are the original parts. Pulled vacuum for 45 min. Put in 3 cans 134 (original R12 says 2.5 lbs=40oz, 3 cans =36 oz). Compressor comes on and wont shut off. Outside temp 79-83F and estimate humidity 60-70%. System equalized at 40psi low and ~225psi high. Manual says pressure switch cutoff at 21psi. Removed some gas. Not much pressure change. Removed a little more gas. Felling by hand inside truck says this a little cooler than start, good sweat on accumulator. Not much change in pressures. Removed more gas. Got it down to ~30-35 low ~190-210 high. Cooling inside truck not as good. Since I cant get pressures down to ~21psi, do I have problem? Bad pressure sw? I even took a glass of water and poured over condenser and it dropped some, but not enough to cycle clutch. revved engine, same, small pressure drop, not enough to matter. I have used the blue tube before on another conversion with R4 compressor and all went well. Is the red tube causing me issues? What to do now????

[bohica2xo]

Doors open & cabin fan on highest speed.
Raise the engine RPM to 2200, hold for at least 1 minute @ 2200 then record pressures at that RPM

[dave5701]

Will do. Another question. I have always seen the compressor cycle. Is it possible, with temp, humidity, and this is not a standard system, could the compressor not cycle??

[Jag987]

I don't know why, but I am always bothered when someone thinks there is a problem just because a compressor is not cycling. If the high side is not too high, low side is not too low, and evap is not too cold, why would or should a system cycle off and on?

Trying to get the pressure low enough to cycle by removing (venting I'm sure) refrigerant is starving the compressor of refrigerant and oil while effectively trying to decrease compressor run time and the amount of cold air it produces. If it never cycles off, and everything is working properly, it never stops making cold air.

Just my thoughts, someone please correct me if I am wrong.

[Yallrun]

Jag I agree, when it's super hot and or humid your compressor is not going to cycle much if at all when at idle. I even went as for as to put a go pro under my hood and filmed the compressor while I drove.

Dave5701 try this. Cold start up with the AC on, crank her over and watch the compressor. If it cycles before warm up even a couple times it's not your switches. If your still not sure then get a volt meter and test your switches. Let us know. See if anyone else weighs in, I'm no expert.

[Cusser]

Jag I agree, when it's super hot and or humid your compressor is not going to cycle much if at all when at idle. I even went as for as to put a go pro under my hood and filmed the compressor while I drove.

When I was having a clutch slippage issue a decade ago, on one of my trucks I installed a dash indicator light so I could see when the AC clutch was getting voltage. I just soldered its wire to the single positive lead going to my compressor clutch, still works fine.

The AC clutch slippage was actually due to the compressor nut not being tight enough anymore, causing the "gap" to be slightly too large.

In Arizona, the AC clutch is engaged the vast majority of the time....

[wptski]

In Arizona, the AC clutch is engaged the vast majority of the time....

I've read in certain climates where some have installed clutch cycling bypass switches where possible.

[Cusser]

In Arizona, the AC clutch is engaged the vast majority of the time....

I've read in certain climates where some have installed clutch cycling bypass switches where possible.

Not sure why such bypass would be needed, as factory system would have AC clutch engaged all that time anyway, unless malfunctioning. If there's moisture in the air, a too-cold evaporator would freeze moisture condensed from the air, blocking the air flow unless compressor was allowed to cycle so such ice would melt.

Question for professionals: if such system was allowed to have compressor run but little evaporation of refrigerant occurred in the evaporator (due to low air flow), wouldn't that mean that possibly liquid refrigerant would get back to compressor, and subsequently damage that ???

[bohica2xo]

Cusser

I run two switches in parallel with a switch here. When the summer humidity is 2% I run the lower set point of 18psi. Even when humid, the low switch is fine for short trips - the evaporator is processing 100f inlet air so ice is not a problem.

If I have icing, or am on the freeway for more than a few minutes I switch back to the factory setpoint switch. Never had an issue with liquid return.

[GM Tech]

I worked in a vehicle test lab at Delphi- if I wanted to destroy HT-6 compressors- I would bypass the cycling switch- it only took 10 miles running at 10 psi to weld together the shoes to the axial plate and the front thrust bearings overheated due to lack of lube. Oil does not flow well at 10 psi- and everything on the low side is frosted over- the a/d was twice its size and snow white with ice/frost. The suction line to compressor was 3 inches in diameter with frost- and the compressor sounded like a chainsaw-- did this dozens of times- to test what happens when cycling switches fail in the closed position. Granted it only happens when low ambient- low 70 deg or less-- but still why chance it.