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Recently I have noticed that the air from ac vents sometimes cold, but sometime less cool/fan mode for a while, before return to cold. I also noticed almost continuous "hissing" sound from around the place where cabin filter is located. I checked the low pressure when AC is on max, it shows 32 PSI on ambient temp of 33C (91F) at 48% humidity. Refrigerant is R134a.

I have visited repair shop once, and they suspect the selenoid valve in ac compressor gets dirty, so they cleaned it. But I could feel the problem persist, but it does feel better, (the "fan mode" get shorter). They said, the clutch is engaging when its not cold, so its not a clutch or belt issue.

I wonder if is it caused by not enough refrigerant?

Note : When AC is off, the low pressure is 80 PSI.

Thanks!

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I would suggest few areas to inspect. When you notice no cold air from the vent outlets, go and check the A/C tubing under the hood. At least some of them must be freezing cold. Depending on the A/C system type, you might or might not have an orifice valve inside one of the A/C tubes - this will make one part of the tube hot and the other part freezing cold. You can have the refrigerant amount checked by A/C shop - let them suck everything out and refill it back and watch for the difference in amounts. It will cost you few bucks for the job and for some of the refrigerant as not everything can be recovered from the system. If you suspect leak, let the system be tested for leaks. Be aware though, that the vacuum test done by A/C machine doesn't necessarily find a leak unless it's big enough. Best if they have some sort of leak detectors.

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When diagnosing AC, always begin by identifying the type and model of the compressor. You mentioned a solenoid valve and it probably means it's an externally controlled variable displacement swash/wobble compressor. With these compressors, checking for pressures is pretty nonsense, because they can adjust the pressures in the system by themselves according to the cooling demand, all they have to do is reduce the swash plate/driver angle.

Anyways, it's better if you identify the compressor model (reach for the compressor in the engine bay and read the label, it's written here) and post it here, so that we know what kind of compressor we're talking about.

OTs are most likely out of the question here (yes, modern systems, especially ones featuring variable compressors, are pretty always based on a TXV, because orifice tubes are obsolete technology that can't allow for any superheat adjustment. How can you know it's TXV or orifice tube, without even looking for anything resembling one of these two devices or pulling any drawing at all? Look for where the accumulator is located. If it's on the high pressure side right before the evaporator, it's a TXV system and the accumulator is actually a liquid receiver, otherwise it's an OT system. Variable compressors don't really care if it's OT or TXV because they can adjust their flow any moment so no risk of compressor slugging even without a low pressure cut out switch, but TXV is always preferred because it's a more energy efficient solution).

The hissing sound from the evaporator's area, therefore, means one thing: refrigerant vapor is reaching the TXV inlet (only liquid refrigerant should make it there). This shouldn't happen. That means you're most likely on a low refrigerant charge and the receiver isn't storing enough liquid refrigerant.

Low refrigerant causes evaporator frost to happen fast (basically that little liquid that makes it through can't cover the entire coil extension, so that it boils only inside one part of the evaporator, near its inlet; the evaporator progressively develops frost across all its surface as the actual boiling location progresses along the evaporator's coils). Evaporator frost stops heat exchange and clogs airflow through the evaporator fins so that, more than no cold air, you should actually temporarily feel a reduced airflow. However, the AC system senses the evaporator frost (temperature sensor on the evaporator's fins in the case of the ECVDC) and reduces the compressor's cooling capacity (swash plate angle in that case, provided we're talking about an ECVDC) and this means less cooling and frost soon melting. And everything repeats like that. And that might be what you're experiencing.

Compressors don't like low refrigerant levels too: it means less oil flow around the system, so there's a risk of the compressor seizing due to a reduced oil return rate. It also implies higher superheat because that little liquid trickle inside the evaporator is absorbing more heat than it should. So the compressor is not receiving the same cooling effect from the refrigerant that reaches it. Add the lowered oil return, and the compressor is getting hotter than it should. And this can mean compressor damage.

Bottom line, go follow Jan's recommendations as soon as you can. Have the system inspected and serviced (which means having the refrigerant evacuated, system leak tested, vacuumed if no leaks found, then correct gas quantity inserted along with all the oil that the recovery machine pulled out of the lines). Having the system serviced by a pro is the only way of not doing any damage when dealing with variable displacement compressors. AC systems featuring one of these must be charged by refrigerant weight, not by pressure.

  • Thanks for the comprehensive explanation. I am new to this, so will check if anything I could find on compressor type. My car is Ford Ecosport 1.5 TDCI Petrol 2014. Do you know what type it uses? As my current low side pressure is 32 PSI when AC MAX, do you think its still low? – hudarsono May 16 '18 at 3:30
  • Unfortunately i don't. You need to look for the label on your compressor. Post it here if you can. 32 PSI (guess you actually mean psig) low side matches to a R134A boiling point (close to the actual evaporator temperature with newer evaporators) of 2.71°C, slightly above water freeze point, and won't mean much until we know if it's variable displacement or not. – Al_ May 16 '18 at 9:42
  • The most i could find online for that car is a Visteon VS16 compressor and it is a clutch-equipped externally controlled variable displacement compressor (it has a solenoid valve that allows a computer to control refrigerant flow). – Al_ May 16 '18 at 9:50
  • Hi Al_, yes the compressor label quite hard to see, but it is written Ford and VS 16. here is image i could take dropbox.com/s/v4n1bwppk3el2dl/20180522_192156.jpg?dl=0 – hudarsono May 22 '18 at 12:26
  • Thanks for the pic. Definitely a VS16. Hissing can be caused by low charge on TXV systems. As said before 32PSIG nearly (we're talking about a small car AC system here so negligible pressure drop due to line lenght) matches to a 2.71°C boiling point inside the evap (practically evap temp with high efficiency evap). I guess you set it on max cold (with manual climate control: no heat, AC on, max blower and recirc on) and if so the evap should have immediately reached 0°C (27.7 PSIG suction for R134A) on max charge and then stayed there (variable displacement compressor so it doesn't cycle). – Al_ May 22 '18 at 12:41

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