Replaced compressor, failed again 9 days later. Advice sought

[Dougflas]

Do not forget to use the black light and glasses on the evaporator drain tube.

[hcanning]

Just realised the pressure readings are written on the piece of paper I have!

Ambient temp: 21c
Vent temp: 5c
Low pressure S/R: 2.0
High pressure S/R: 7.5

Presumably these readings are in Bar!

[JohnHere]

It took me a little while to make the conversions to US measures and interpret them, but here's what I think is going on.

Given an ambient temp of 21C (70F) and a vent temp of 5C (41F), the LP gauge reading of 2.0BAR (29PSI) seems just about right. However, the HP gauge reading of 7.5BAR (109PSI) is way too low. It should be around 12.1BAR (175PSI) at that ambient temp and at the nearly maximum refrigerant charge of 440g (14.9 oz) that you mentioned earlier.

Therefore, the data above suggest that the rebuilt compressor you had installed was defective from the beginning since it was not building pressure the way it should. It could also be that whatever was causing the pressure issue, plus a compressor leak someplace (shaft seal??), also caused it to lose refrigerant and oil and to disintegrate internally.

At this point, if I was working on this, I would install a brand-new (not rebuilt) compressor; a new condenser and R/D assembly; guesstimate the oil level, adding or removing oil as needed; recharge the system into a well-held vacuum; then run it for a couple of weeks and see how it does. I'll bet that your problem will be solved.

[hcanning]

Thanks for your analysis! This is probably the avenue I’ll take.

Those pressures were measured at idle speed though, not at 1500rpm. Can’t imagine it’ll make much difference though.

[JohnHere]

Right...at 1,500 RPM, the LP reading will be a little lower and the HP a little higher. But I think the HP would still be much too low.
Let us know how you make out.

[hcanning]

Thank you, I will update again once it's been inspected by someone who knows what they're doing

Another theory I was told was that potentially the freewheel clutch pulley on the alternator is failing (not slipping properly in one direction) and putting undue stress on the compressor pulley. Or something else in the aux belt 'drivetrain' is causing it. Does this sound feasible? I slackened the belt and the pulley seemed fine, but I could be wrong. When the engine is running or revved up, there doesn't seem to be any wobbling or undue vibrations.

[JohnHere]

To my knowledge, there is no clutch on the alternator, just a pulley bolted directly onto the shaft and driven by the serpentine belt. I don't think your variable-displacement compressor has a clutch, either. So whenever the engine runs, so do the alternator and compressor. Instead of starting and stopping, the V/D compressor varies its displacement electronically to accommodate the system's heat load.
A too-tight belt certainly can affect the compressor's bearings. But given what you reported, that doesn't seem to be the case.

[hcanning]

Most modern alternators have a freewheeling pulley that disengages in one direction (reverse) so that the inertia of the alternator doesn’t damage other components when the engine speed rapidly decreases. A bit like freewheeling on a bicycle.

[JohnHere]

I readily admit to learning something new. Right after you said that, I went out to examine the alternator on my 2018 Toyota SUV. Unfortunately, the alternator is buried so deeply that I wasn't able to determine whether it has the type of pulley you mention. The vehicle does use R134a and not R1234yf, though. A 1994 Toyota that I've owned since new, and that I still own, doesn't have that type of pulley, probably because it hadn't been introduced yet.

[hcanning]

The AC guy has just come to have a look at it and he seems to think that some free play in the alternator overrunning pulley is causing problems. He checked the pressures (engine off) and said they seem to indicate the right amount of gas, but obviously we’ll know for sure when he removes the gas to change everything. I’m just waiting for another compressor to arrive.

I’m really hoping that the new alternator pulley, compressor, and condenser/dryer will be the end of it!!