AC SYMPTOMS Pressure equalizing valve?.
 

Don't mean to pester you all again with AC problems but 'tis the season. Have read numerous threads, printed comments, procedures, circuits diagrams, etc. Have not had any luck. Recently I have noticed this with my low pressure readings that I have not noticed prior. When initially activating the AC, the clutch cuts in and out continuously so looked for low refrigerant. When first turning on the AC it appears to be low.. The clutch would cycle on/off as the pressure initially dropped to below the green "OK" indicator on the dial. Then for a short while, 3-5 minutes the pressure would come up to the green and the compressor was working as it should. Then in another 3 minutes or so the pressure would get too high and into the yellow and approaching red and then the clutch would disengage as it should and the pressure would drop. So now the compressor is cycling on and off. I am assuming the high pressure switch doing its job. Am I assuming this correctly? If so, anyone have a quick answer to what is happening here and what the problem might be? I have read about an equalizing valve in other compressors and was wondering if the one on this compressor is replaceable. Have not found diagrams yet. Thanks. Been in the hundreds here and getting sweaty and dusty...
Oh yeah. It's a 2002 with manual AC...

The first thing I would do is slap a set of gauges on it. That is the only way of telling what is really going on.
You can get a set for around $50 at Harbor Freight.

When the system is at rest the pressures on both the low and high side are equal and are dictated by a 134a temp/pressure chart. When the compressor starts the high side pressure starts climbing and the low side starts dropping. The two sides are separated by an orifice tube in front of the evaporator. The pressures once stable are dictated by the charge level, outside temp and humidity and the load (temp and humidity) in the evaporator/cabin. As the cabin approaches the set point the low side starts dropping and eventually falls below the low side cycling switch trigger point and the compressor will cycle. There is a high side cutoff switch behind the compressor which only switches off under a fault condition.A low charge will cause excess cycling or a no start condition if low enough. A high charge can cause a high pressure cutout but this would normally be due to another fault such as poor condenser cooling or a plugged orifice tube.

You need the following to know what’s going on:

outside temp/humidity
center vent temp
interior temp
high and low side pressures

Do not vent refrigerant into the atmosphere, it’s illegal.

George

And that is why you need a set of manifold gauges..

This. You can't properly diagnose the system without knowing what's happening on the high side (and what the actual pressure is on the low side).

Got it...

So here is the skinny on my NOW FUNCTIONING A.C. The cooling of the A.C. condenser is partially reliant on forced air from the motor fan to keep it cool, especially at idle. Well it seems that the past owner of the vehicle installed electric fans that did not include an A.C. relay to turn them on when the A.C. was activated, causing the system to get too hot, raise the pressure, and shut down the compressor intermittently. A simple jumper wire from the compressor input hot wire to the fan input hot wire has solved the problem. ( Not to mention that I was given an old set of gauges that leaked half the refrigerant out of the system and had to charge it again.) I now can get it down to about 53 degrees at idle in 90 degree heat. Not perfect, but not bad either...

Good to hear. And a perfect example of why you also need a high side gauge for diagnosis.

Make sure that jumper wire doesn't feed back and turn the air conditioning on every time the fans switch on.

Good point...

Mitigated
 

Put in a LITTELFUSE 1 amp diode that does the trick.


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