WEIRD ac low side pressure readings

[roncoinc]

With the air on high/recirculate and the probe on my fluke down the vent the air temp was 64* at ambient 80*

so i used a hose and gauge from a DIY fill kit.

put it on the low side fitting and ran the car 15 min or so and went and looked at the gauge..

right at min press,,28 lbs or so..

thinking,ok low on coolant..

then i see the needle on the gauge wiggle,up and down a couple pounds,,then the needle goes up,and up,and up,,reaches 60 lbs and the comp shuts off,press drops and it cycles again..

then it hold at low press awhile,,then cycles again..

last year about this time i had 54* air and i could live with that.

so it lost 10* in a year.

i googled untill my eyes got googley,,talked to an HVAC guy,mechanic ,several people and no good answers about THAT much fluctuation on pressure.

tomorrow will borrow a manifold set and get better readings..

WTH could make the low side press fluctuate 30 lb's ??

 

[joegr]

Coolant goes in the radiator. Refrigerant goes in the AC system.

You do need both gauges to accurately charge it. Most likely the pressure variation is due to the thermal expansion valve trying to do its thing with too little refrigerant, coupled with the way the variable displacement scroll compressor works. However, you can't be sure without having the high side reading at the same time. Also, you need to look for and find the leak. A tiny bit of out-gassing through the hoses in normal, but it sounds like you are beyond that. It's very important to determine if the leak is somewhere low in the system, because if it is, you have probably lost a lot of oil too.

 

[oddball]

A/C work is a deep and twisty maze. If you really want to do it, then fine, but there's a steep learning curve. I can get systems working, but I'm still no expert. Can't get my semi-custom system on my Olds to cool sufficiently yet.

Anyway, what I expect you are seeing is that when the compressor is ENGAGED the low side falls to the regulated low side pressure, which is in the neighborhood of 28psi for R-134a systems. The LS uses a cycling clutch compressor which means that when the evap gets near freezing it disengages the compressor. Once this happens the high and low side will start to equalize with each other. So the low side rises. Once the evap warms above a certain point the compressor engages, low side drops back down, repeat. So it sounds to me like you just thought the compressor was doing the opposite of what it was actually doing. That cycling and pressure change is normal.

The other option is if the low side protection trips and causes the compressor to disengage. I don't recall offhand which method the LS uses for that.

In any case, first make sure the radiator fan is pulling enough air. Put a LARGE box fan or shop fan in front of the car while doing this kind of work. Make sure the condenser, radiator and the gaps between them aren't clogged with crud.
It's by far best to do a purge and complete refill when something is suspect, but now you're talking about a new receiver/drier and pulling a vacuum on the system. Some Autozone's actually rent a small vacuum pump. It's the bottom-of-the-list import model, but it works OK if it hasn't been abused. Always use fresh vacuum pump oil. Don't use the "freeze" or whatever fancy junk refrigerant at the parts store. That has propane replacing some of the R-143a which is NOT helpful. Look for the plain, boring looking cans.

 

[joegr]

Actually, the LS is not a cycling clutch system. Those systems are on cars with fixed orifice, instead of TXV and variable displacement compressors.
The LS only monitors the high side pressure. If the high side is too low, then the compressor won't engage. There is also an evap temperature sensor. If the evaporator comes too close to freezing temperature, then it will disengage the clutch till it warms. However, this should only happen with very low cooling loads, as the compressor can adjust down to a very low pumping level to normally keep it from needing to shut down.