Compressor Short Cycles with Correct Charge

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JimnJudi
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Compressor Short Cycles with Correct Charge

Postby JimnJudi » Tue Sep 29, 2020 2:41 pm

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
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Re: Compressor Short Cycles with Correct Charge

Postby JimnJudi » Tue Sep 29, 2020 3:17 pm

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.
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JohnHere
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Re: Compressor Short Cycles with Correct Charge

Postby JohnHere » Tue Sep 29, 2020 3:31 pm

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.
Al9
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Re: Compressor Short Cycles with Correct Charge

Postby Al9 » Tue Sep 29, 2020 4:04 pm

You wanted some ideas and here they are.

1)Forget about cans. Not an accurate charging method, especially with modern TXV MVAC systems. Have it charged by a pro.

Never release any refrigerant into the atmosphere. Also never remove an amount of refrigerant from a system hoping that a cooling issue somehow fixes itself, this can harm the compressor by hampering oil return.

2)A condenser (and any heat exchanger) can look clean yet have its fins plugged full of dirt. You want to see light shining through as many fins as possible. Even worse, it may also have clean fins yet another heat exchanger behind it might be dirty and blocking air flow.

3)A good radiator/condenser fan and especially its electric motor or clutch goes a long way towards correct AC operation and can be seriously worn even with no obvious engine overheating or noise/vibration/harshness issues. In fact, a brand new radiator/cond fan can be louder and harder on the engine than one that is about to kick the bucket.

4)And lastly, about the suction going to 0 once you froze the power head off. Personal experience coming, though someone might consider this utter junk.

The Denso valve likely snaps shut when you do that, for some reason. I had a look at part numbers and your valve is likely a Denso 447500-1600 derivative, and i have this one in my possession (used it as an evaporator flushing rig when getting my compressor replaced, since its attachments are compatible with my evaporator and removing the metering pin so that it stays wide open all the time is really easy on this one, and played with it a lot). This one simply snaps shut in a heartbeat once you get freeze spray on its disc. I wish i had a dedicated testing rig that allowed me to simulate different evaporator pressures to check how this one throttles refrigerant inside an actual system rather than at atmospheric pressure, but since this valve and its revised versions (these ones basically feature a thin metal rod instead of the original thick resin-sheathed hollow metal sensing tube, and supposedly "hunt", that is, react excessively to minute refrigerant output temperature changes, way less than the -1600) have a number of clutchless variable displacement compressor applications, i suppose that what really happens is that it's still modulating (i.e. never closing fully) at very low heat loads (i.e. defroster on a cold and wet day kind of situation). It might never see the freeze spray kind of temps during system operation (after all it's AC, not a meat locker).
The TGK made valve that came from my car, and its revised (thicker and plastic encased shaft that won't bend like the old one did) second hand replacement now in place, doesn't do that. Freeze the power head off with cold spray and it will never seat like the Denso does. Just throttles a little and that's it. Larger modulating range perhaps?

One plausible explanation for that could be that for some reason Denso uses a lot of adsorbent (i.e. activated charcoal) in their power head charges (in order to eliminate hunting) and at freeze spray temperatures the whole charge gets adsorbed (heat clearly works against adsorption, so adsorbing stuff and not "let it go" is pretty easy when things are frozen, pun intended), especially since i noticed it takes a while for the valve to unseat (desorption is a slow process), but i might be wrong...
JimnJudi
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Re: Compressor Short Cycles with Correct Charge

Postby JimnJudi » Wed Sep 30, 2020 7:32 am

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?
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bohica2xo
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Re: Compressor Short Cycles with Correct Charge

Postby bohica2xo » Wed Sep 30, 2020 6:35 pm

What is the high side reading while it cycles?
JimnJudi
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Re: Compressor Short Cycles with Correct Charge

Postby JimnJudi » Thu Oct 01, 2020 4:48 am

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.
Al9
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Re: Compressor Short Cycles with Correct Charge

Postby Al9 » Thu Oct 01, 2020 5:30 am

No, improper evaporator airflow drops suction.

You wouldn't be seeing those pressures with a jammed wide open TXV. One completely unable to modulate.

I'll restate idea #1.
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bohica2xo
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Re: Compressor Short Cycles with Correct Charge

Postby bohica2xo » Thu Oct 01, 2020 7:23 am

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
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Re: Compressor Short Cycles with Correct Charge

Postby JimnJudi » Thu Oct 01, 2020 7:30 am

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?

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