Low side high, high side low?

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PeterW76
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Joined: Thu Aug 27, 2020 1:12 pm

Low side high, high side low?

Postby PeterW76 » Thu Aug 27, 2020 1:29 pm

Hi. New to this forum and seeking some help to diagnose a problem. I just bought a 2010 Chevy Aveo and the AC is not blowing cold air. I borrowed the gauge kit from AutoZone and got the following readings with the AC running and the compressor clutch engaged: low side pressure around 90psi, high side pressure around 150psi. Based on ambient temperature around 95F I think that high side pressure is a bit low.

Pressures with the engine off were around 105 on both sides.

From my limited understanding of the system I was initially thinking the expansion valve is faulty. My logic being that that's the "barrier" between the low and high sides. But then I read somewhere that if the expansion valve is stuck open then liquid refrigerant would do directly to the evaporator and cause freezing.

So getting to my question...does anyone know what would cause my low side high, high side low issue?

Thanks in advance,
Peter
Al9
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Location: Southern Europe

Re: Low side high, high side low?

Postby Al9 » Thu Aug 27, 2020 1:53 pm

PeterW76 wrote:But then I read somewhere that if the expansion valve is stuck open then liquid refrigerant would do directly to the evaporator and cause freezing.

In fact it's the other way around. A stuck open TXV prevents the evaporator from freezing up. A nearly jammed closed one can freeze it up instead (but never with a suction pressure controlled variable displacement compressor, which will plainly go fully destroked as soon as the TXV shuts fully or seriously underfeeds). Reason is, in the former case the evaporator gets flooded with refrigerant and as such there's simply no space for any expansion (liquid->vapour transition) to take place within, it will expand elsewhere such as within the suction hose or inside the compressor. Remember it's the refrigerant expansion that cools things down. In the latter case, some refrigerant is still being admitted to the evaporator coil and the evaporator pressure is so low that it's boiling at a very low temperature. In that case it's usually only a part of evaporator that begins freezing up, since it's a starved evaporator, then the rest follows as the developing ice layer prevents heat exchange. But only if the compressor is still pumping.
In your system, the TXV bulb charge is tailored to starve the evaporator a little at high heat loads, and flood it at low heat loads, so that it first pulls suction pressure down fast, and then it's steadily fed with refrigerant so that the V5 compressor's control valve and the compressor's displacement control behaves in a steady manner and never "hunts". Yes, the alternation between a flooded and semi-starved evaporator state is often deliberate in modern MVAC.

4 possible reasons for what you're observing.
1)The system might be terribly low on refrigerant charge, keeping the V5 compressor (almost) fully destroked.
2)The TXV might be bad (either jammed closed or stuck open; in the latter case the suction line/hose will be likely cold and sweating profusely, especially near the compressor; in the first case the compressor is fully destroked) as you guessed
3)The compressor control valve might be bad and keeping the V5 compressor (almost) fully destroked.
4)The compressor piston/bores/piston rings might be terribly worn and out of clearance, bypassing too much refrigerant to the crankcase and again keeping it destroked.

V5+TXV is a really complex MVAC system.

Are you hearing any strange noise inside the car while the compressor is running?
PeterW76
Posts: 4
Joined: Thu Aug 27, 2020 1:12 pm

Re: Low side high, high side low?

Postby PeterW76 » Thu Aug 27, 2020 2:42 pm

Thank you for the very detailed reply. I appreciate you taking the time to write that. Clearly I have some more studying to do!

My initial, basic level thinking was that the exv must be stuck open, allowing the high pressure to bleed through to the low pressure side hence the pressure readings I was seeing. But I think I seriously underestimated the complexity of the system.

The car isn't home right now but I'll spend some more time over the weekend, check temperatures of the suction lines etc. I didn't notice any strange noises when the compressor is engaged. Also, the compressor stays engaged...no cycling on and off.

Thanks,
Peter
Al9
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Location: Southern Europe

Re: Low side high, high side low?

Postby Al9 » Thu Aug 27, 2020 2:56 pm

PeterW76 wrote:Also, the compressor stays engaged...no cycling on and off.

Entirely normal behaviour since it's a variable compressor. The clutch only cycles off and on when high side gets too high, rpm drops too low and the engine is about to stall, WOT.
PeterW76
Posts: 4
Joined: Thu Aug 27, 2020 1:12 pm

Re: Low side high, high side low?

Postby PeterW76 » Thu Aug 27, 2020 3:25 pm

One quick question... Wouldn't low refrigerant level result in low system pressure, either on the the low pressure side or just overall? Would it be worth evacuating, checking for leaks and refilling and take it from there?
Al9
Posts: 207
Joined: Tue Mar 10, 2020 3:26 am
Location: Southern Europe

Re: Low side high, high side low?

Postby Al9 » Fri Aug 28, 2020 1:32 am

Again since this is a variable compressor, pressures aren't really of interest. Refrigerant charge weight is, on the other hand.
Anyway a lower than normal high side definitely means either that the TXV is stuck open or the compressor isn't running at full stroke.

Would it be worth evacuating, checking for leaks and refilling and take it from there?

Definitely. Refill only by weight, with the compressor not running and with the specified amount of refrigerant.
PeterW76
Posts: 4
Joined: Thu Aug 27, 2020 1:12 pm

Re: Low side high, high side low?

Postby PeterW76 » Sat Aug 29, 2020 1:36 pm

Hi. Hooked the gauges up again today and got 65psi on the low side, 100psi on the high side. I understand that pressures don't mean a whole lot with a variable compressor system but just wanted to see what they were today. Feeling around the pipework I feel only mildly warm at the compressor outlet and mildly cool at the evaporator outlet. Would that support lack of refrigerant charge?

You mentioned to recharge with the compressor off... does that mean I'm relying on the vacuum to draw the new refrigerant in?

Finally, do I need to add any oil prior to recharge?

Thanks,
Peter
Al9
Posts: 207
Joined: Tue Mar 10, 2020 3:26 am
Location: Southern Europe

Re: Low side high, high side low?

Postby Al9 » Sat Aug 29, 2020 2:10 pm

At that point, without knowing how much refrigerant is left within the system, the reason is still unknown.

Yes, recharge by weight with the compressor not running and therefore into deep vacuum. That's how AC systems equipped with these compressors are supposed to be serviced. But first carefully check the system for leaks.

If you think about using cans as a refrigerant medium, paying a RRR station equipped pro for a proper service is definitely a better idea. Cans are terribly inaccurate, and V5 compressors are really picky about refrigerant charge amount.

Only add back the same oil amount that came out when the old refrigerant was recovered. Using fresh oil of the proper type of course (typically PAG 150 or PAG Blue with V5 compressors). Anything else is guesswork.

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