Compressor Short Cycles with Correct Charge

[JimnJudi]

New member here with a difficult issue. I inherited a 2008 Subaru Outback (57,000 miles) with weak cooling. The compressor cycled off/on every five seconds and the vent temps were around 70 F on an 85 degree day. The condenser was clean of debris and the cabin filter new. I had my mechanic remove the refrigerant and I installed a new expansion valve (Denso). After a three hour vacuum (and verifying no leaks) I hooked a can of 134a, purged the yellow hose, then added 11oz of the 13-15 oz required for my car. As the low side hit 40 (high side 175) the short cycles began (the Subaru specs say the pressures should be 18-28 low side, 213-242 high side when temps are 86-95 F). I could get it to stop the short cycles by releasing some of the gas but the vent temps stayed in the high 60's. The ambient temp at the time was 81F. I then replaced the compressor with a new one, and after repeating the procedure got the same results. I also tried a trick I saw on Rick's Free AC Info site where I sprayed the expansion valve disc to get it to frost over and the pressures dropped to 0 low side and 125 high side. Also, when I shut off the car the pressures equalize in about 15 seconds. Any ideas would be appreciated.
Jim

[JimnJudi]

Forgot to add that I brought the total charge to 14 oz on the first charge and the short cycles continued with tepid vent temps.

[JohnHere]

Your car holds less than a pound of refrigerant, a very small amount. If the charge is off by only a couple of ounces, it's likely that the system won't operate properly. Lets start by weighing-in the exact amount of pure R-134a (no sealers or other additives) using an accurate refrigerant scale...or have it done. You might very well have less than 14 ounces in there now. Then test the pressures and vent temps again and see what you get while monitoring the compressor's operation. I presume you added the correct amount and type of oil as well.

[JimnJudi]

I feel confident the amount of charge is correct by weighing the cans before and after and taking into account the two seconds of purge from the yellow hose. I can only assume the contents of the can is 100% R134a. Regardless, the compressor kicks on when only a small amount has been added, and stays on until the can is about 3/4 empty, at which point it begins to cycle. Also at this point the low side has reached 40 when, if things were working properly, it should never have risen above 20 or so. What would allow the low side to reach 40 other than an open expansion valve?

[bohica2xo]

What is the high side reading while it cycles?

[JimnJudi]

When outside temp is 81F, 700 rpm, various readings over 1/2 hour, low side 42, high side 175, 43-190, 44-200, 40-150
At 1500 rpm 35 and 240, 33-175, 34-198.
Outside temp 72F, 700 rpm, 30-145, 26-135, 24-130.
These are all with approximately 11 oz of refrigerant (ie: when the first 12 oz can is almost empty).
The only other thing I can think of is the evaporator covered with debris. Would that raise the low side pressure? When I got the car the cabin filter was a mess (leaves, twigs, etc) and I immediately replaced it.

[bohica2xo]

Try another gauge set.

Your control system appears to cycle off at all sorts of different low side pressures. Either the switches / sensors are bad or your gauges do not repeat.

Before you go after the control system, try a second set of gauges to make sure you are not chasing your tail.

You have reported high side pressures with a range of 130 to 240 psi. That makes me wonder if your service valve connector is actually opening the high side port properly. Your system could actually be cycling on the high side cutoff.

[JimnJudi]

The 130 was on a dry 72 degree day at 700 rpm and the 240 was a humid 81 at 1500 rpm. Would that explain the wide range?

[JimnJudi]

The only person I know who has a set of gauges is about an hour away. I'll hook his set up the next time I'm out his way.
A couple more bits of info:
- Drained 1.4 oz oil from original compressor, after draining oil from new one added 2.0 oz.
- Added 1 oz oil to new desiccant bag.

[Cusser]

The only person I know who has a set of gauges is about an hour away.

Time to obtain your own set !!!

In my area, I know Autozone has free loaner AC gauge sets, had to borrow a set a few years ago because my own gauge set was 100 miles away.

Even a DIY needs a correct set of gauges for a/c.

https://www.ackits.com/mt1416-santech-r ... oses-24615