watching AC with gauges question

[JHZR2]

Hi,

Quick question about watching an AC system with gauges. On some cars, the compressor cycles, sucks down to 30 or 25, then goes back up to about 45 or so, then repeats. On other cars, it stays more or less stationary at 30 or 35 psi, operating from 80-83 degrees ambient, and 75 or so percent humidity, with high-side being correct for the temperature outside.

Which is right??? I was always under the impression that compressors should cycle, but the systems that show cycling and suck down are the ones that work the worst (per my other question about ACs the start blowing cold, but after a while go warm).

The one thing that I have not been able to gather is when charging based upon operating pressure (not just by pounds of refrigerant added), what is the true happy medium??? 35F or so on the evaporator? no low side pressure change during operation?

Again, on our toyota and plymouth, both of which blow cold and then get warm after a while, the operating pressure while working is fine, but I do note that the pressure fluctuates as the compressor cycles. Meanwhile, my 134a retrofitted BMW and our acura integra show consistent 35 or so PSI on the low pressure side constantly (45 psi at idle with the fan on high, and about 30 at 1500 RPM with fan on medium, with high pressure at about 200-220 PSI at 80-85F).

So what is the correct happy medium? Ive read the haynes manual on AC charging, read all over the internet, etc... not enough info on how to get the happy medium... plus it seems that every car wants a slightly different operating pressure, but yet it is really tough to get it exactly in some of the specced range.

Any suggestions???

Thanks in advance for any advice!

JMH

 

[Master ACiD]

alot of times gauges wont move because of a poor connection. ive had this happen a few times. be sure to wiggle the connection and check it again.
on my honda crv, if i dont manually hold pressure on the low side connection, the psi stays the same weither or not the compressor is cycling. if i hold the connection down, it goes from 70 to 40. i think those are the unumbers anyways.

also, some cars use an expansion valve to regulate the pressure. my stratus low side pressure goes up and down from 15 to 40 while the compressor is on. just goes up and down, up and down.
car cools fine though.

 

[Merkava_4]

Try jumping the clutch cycling switch on the accumulator with a test lead. Pull the connector and jump the connector, not the switch. If you don't have a test lead, a small paper clip will work.

 

[Dominic]

For R134 I have some links. Its actually best to do a 100% evacuation of the system and refill it with all new R134 based on the lbs. required.

If you don't do that, I try to keep the low side around 35-40psi. If you check the low and the high, you don't want the high side over 270-300 on most cars.