2nd Gen Non-Engine/Transmission What does the PCM control of the A/C

[esamuelson]

I am wondering what the PCM does to the signals it is getting from the dash, low pressure, and high pressure switches? I guess my main question is what tells the compressor to cycle when you are driving down the road and then tells it to stay on while you are at idle? My service manual tells you that the pcm could be a problem if your compressor rapidly cycles. My problem is my ac vent temps are ok at idle but should be colder while driving but are not really much better ( idle 56, driving 50). I have checked pressures and they are ok. AC is colder with higher rpms but while you are driving it seems like the compressor is cycling to fast and not letting the AC get cold before it switches off. I am thinking of replacing the orifice tube (can you do this or do you have to change the whole line?), and the accumulator. Any suggestions would be great.

 

[Missouri Mule]

The PCM controls the compressor clutch.

 

[B.G. Smith]

The clutch cycling switch which is mounted in the side of the receiver dryer delivers the signal which causes the compressor clutch to engage or disengage. It is a pressure operated switch and the contacts in it open at 23# and close at 45#, bg + or - a pound or so. If the compressor is cycling off and on frequently while driving I would add about a half can (7 oz. ) of 134-A and see if that improves the cooling. bg

 

[fest3er]

For what it's worth, the A/C in my '98 has been misbehaving for years. Several years ago, the local A/C shop removed the PCM from the circuit, because the A/C just stopped working one day. (They ran a wire from the PDC to the firewall side of the pressure switch harness. ) That got it working again, but it was never really cold (or cool) on hot days, and it really hadn't been since '98 or so.



Recently, I added a couple cans of R134A; enough so that the low pressure stayed up around 35-40 PSI on high-fan, fresh air, and around 1100RPM. The compressor no longer cycles much at all, but it may shot off when the low side pressure rises about about 45 PSI. For the first time in about 7 years, the A/C is cool when it's hot and humid outside. And when it's 7 and dry-ish, I have to turn the temp up some to keep the truck from turning into a rolling deep freeze.



One thing to check is your low-pressure switch. Mine was not working very well. After replacing it (a year or so ago), the sysem worked better. Take it off and bang it a few times; it might be sticking. I would expect it should rattle a *tiny* bit when you shake it hard. If not, you might consider replacing it.



Consider the PCM as effectively another relay in the clutch pressure switch circuit. The low pressure switch disengages the clutch when the low pressure side drops below around 25 PSI. The high pressure switch cuts it off when the low pressure side rises above about 45 PSI. The PCM cuts it off when the throttle is WO for some period of time. This is really needed for gassers that lose 10-25% of their output to the A/C compressor; diesels don't have that problem, so the PCM part of the circuit really isn't needed.



And when you are playing with the switches, don't shunt the high-pressure switch very long. You will evoke a very loud screech down near the compressor if the pressure gets too high.