Re:Re:Re:2002 Silverado Compressor Not Cycling Off

Hoping someone with some experience with these '02s has some input. I've got this Silverado in here that keeps freezing the evaporator and all the lines on the firewall after a few minutes worth of driving. Did some looking and saw the compressor was not cycling at all, just stuck on, and had a low side reading of 22-26 psi (high side hovered around 200-220) while the A/C is on. Checked charge weight, was a little low but not low enough to cause this issue. Happened to have a spare cycling switch handy and tossed it on there with no change. Unplugged the cycling switch and the recirc high pressure switch coming off the condenser and that doesn't stop it. High pressure cutoff on back of compressor works normally. Pull out the scanner and saw the Low Pressure Cycle Switch PID did not change state from 'normal' no matter the condition of the switch be it plugged in or unplugged. Didn't measure any shorts on the wire going to the switch either.

Is there some very specific issue with these systems that I'm overlooking, or could this potentially be a faulty PCM?

Check to see if the clutch is cycling? Not much of a AC guy myself but generally the clutch cycling in and out can rule out if things are moving as they should cycling, etc

If the pressures stay the same all the time then you could have a faulty clutch

Also it could be one of many things, wiring, relay, frozen evap coil, dirty condenser & so forth.

Does the freezing/frost start before, at, or after the orifice tube?

After.

Should have mentioned I did flush the lines and both evaporator and condenser in the same service as adjusting refrigerant charge, thinking a possible restriction could be the culprit. No dice.

Low side sound low. I don't mess with auto ac much, but if you have a call to cool it will not cut out until temp is reached or a safety switch is triggered. I would want to see something above 30psi

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Verify wiring integrity from the module controlling the ac to the compressor clutch. Monitor the 12v at the clutch while you turn the AC OFF. is 12v present when the ac is off? (Probably not. ) so now we know the control is good and can control it is down to an input that the pcm uses to satisfy a condition to switch the compressor off. 26 psi on low ain't bad 220 seems a little high but since I don't know the temp that day or humidity it's a mystery. Can you simulate cycle switch manually. Eliminate that?

22 psi is about -5 degrees centigrade so the pipes will be freezing!
This just sounds like an underfill to me? What is the pipe temperature coming through the firewall from the evaporator?
What is the airflow out of the vents?
Post your ambient temperature and fan speeds when giving pressure readings.

In refridgeration we are taught to look at 4 major factors. 1. Condenser split, this is the difference between the condensing temperature (based on pressure) and air entering the condensing coil. 2. Subcooling, the difference between condensing temp and liquid line temp leaving the condenser. 3. Evaporator temperature difference, compare boiling temp to ambient air entering evaporator (in AC we use air delta) 4. Super-heat, the difference between boiling temp and suction line temp leaving evaporator. (This shows if the coil is starving or flooded just like subcooling)

The OP gave.200-220psi this is 130-137°f condensing/saturation temp (assumed 134a)
This does not seem low. A simple subcooling test would prove if it is low. This might be on the high side. I have no idea what the condenser split is designed to be or what the ambient temp was.

I would not rule out a restriction in the txv or elsewhere.
Cheers

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Thanks for the help all. Wound up being the PCM in the end after all.

The system itself was performing as intended, all pressures nominal for that vehicle and a vent temperature delta of over 40f vs ambient (about 80f). Issue was that even though the compressor cycling switch (low pressure cutoff switch, mounted on accumulator, which is downstream of evaporator core) was operating normally and all voltages on that sensing circuit operated as normal the PCM was not changing that cycling switch state from 'normal' to 'low'. This was causing the icing, since the PCM never saw the cycling switch open to shut off the compressor clutch and low side pressures would be sub 20psi.

Don't know what could have caused the internal failure or what that even was, but it has had a tuner poking around in it in the past since a load of EVAP components have been disabled.

That sounds wrong. Again I am not a car guy, but I am surprised that the vehicle was designed to cycle on a low pressure cutout and not the thermostat. The delta sounds high. Sounds like you still have a starving evaporator. (Low charge, or restriction etc)

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In a vehicle application (and depending on climate) a ~30 degree delta under ambient is a good vent temperature. I was getting well under that temperature here in a fault condition where the compressor just kept going and ice was forming on the evaporator core on a 75-80 degree day. Much longer and it would have completely frozen over and stopped working altogether. That has since been fixed with the PCM swap.

These systems are designed fairly simply. The HVAC head unit A/C button sends a 12v signal to the PCM for an A/C request and that signal runs through the high pressure cutoff switch on the back of the compressor. Should the pressure rise over 450psi the switch opens and cuts the A/C request. The PCM then checks for voltage on the low pressure/cycling switch, and if no voltage is present in the circuit that indicates a closed switch and the PCM energizes the compressor clutch relay. That switch opens around 25 psi and closes around 40 psi. Newer vehicles do take much more into account with A/C operation, especially automatic systems.

ok, thanks for the explanation. I didn't realize there is a cut in and cut out feature in addition to the safety limit.
I guess this allows them to use a fixed orfice of some sort, and save $20.
Lol cheers.


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They're absolutely running a fixed orifice tube. But hey, if it's good enough to ice over an evaporator in a hot Alabama summer it's good enough for me.