AC compressor won't take refrigerant

[Tom]

Hi DrBob

have converted to R134a and the connector pins appeared in good shape. Valves were not leaking per my successful vacuum test.

Expansion valve was not replaced with compressor reseal 6mths ago. I did install a new dryer and flushed the entire system. All was good until the mounting bolt fell out of compressor forcing the recent reseal. I did not replace the dryer this time as six months old, but I capped it off so open to air no longer than if you uncap the new one and installed it. Since the compressor showed no signs of grenading, I made the assumption the dryer was safe to reuse. Fresh PAG oil in the compressor as that was the only component this time that was touched. Six months ago each component got the proper amount of PAG oil.

I strongly suspect a stuck open expansion valve as I think that would lineup with not being able to accept refrigerant and the high low side being basically backwards. Friday, I plan to take the compressor to a shop that can bench tested and confirm it’s still good to use. They also have the proper expansion valve so I plan to pick one up.
If compressor checks out and installing new, TXV and I plan to rent the gauges from a different store to rule out any possible problems there.

Good call getting it bench tested. Your numbers, if accurate, point to something wrong with the compressor in my mind. A wide-open TXV wouldn’t reverse the pressures like that, and the refrigerant would still circulate (and therefore should get sucked in).

Before condemning anything, I’d make absolutely sure the readings are right -- i.e., that the gauges work properly, are actually engaging the Schrader valves, and are being used correctly. You are closing the manifold valves when checking pressures, right?

[dr bob]

FWIW, I saw a loosely similar situation when I had an issue with the vacuum side of the heater control valve. The 928 uses a lot more vacuum actuators (five or six) including the heater control valve. The hot coolant flow into the air handler and the odd airflow damper positions caused a bit of head-scratching at the time. I know your cars depend a lot less on vacuum actuators so less likely a problem you are facing. At least make sure your heater control valve is actually closing.

The borrowed gauge manifold would force me to check out the hose connections, making sure that the high-pressure (red) hose actually connects to the port under the high-side gauge. "Cruel Hoax" if someone decided to swap those.

Last thought goes to your compressor rework, and the potential for debris holding a reed valve open, or a reed is bent and not seating correctly. It only takes one, and a telltale -might- be slightly pulsing gauge needles with the engine idling. Otherwise, the symptom you see on pressures suggests a compressor that isn't compressing.

Is the suction-side pressure really higher than the high-side pressure? Or is it the gauge scales and needle positions that have your attention?

I love a good process and control problem to solve. Car AC is amazingly simple until it doesn't work the way we think it should. More head-scratching, and virtually every episode adds to my list of 500-year event causes. What are the odd of THAT happening? It's back to the basics sometimes -- "It was working great until X happened. Solve for X." X is directly or indirectly related to work I did recently, more often than not.


Last, I'm a tool-a-holic. I sweat zero over justifying tools for projects. If I need it, I just buy it. Charging manifolds and hoses are certainly worth owning and maintaining to me. I think I have at least three in that cabinet drawer right now-- I still have an R12 set, plus three- and four-port R-134A sets. I dusted off and replaced some hose end seals in the R12 set to help another local 928 owner a year or maybe two ago. Seepage at hose hose connection, very slight but enough over time. Old interesting cars present can present some similarly interesting symptoms. Always fun! But in the end, it's simple easy stuff the majority of times.

Of course, maybe the cars are afearin' for their own well-being when I start grabbing tools.

[c5_pilot]

Tom, valves open to add refrigerant , closed otherwise. Pressures never changed open or closed. Low side barely moved when throttle applied.

I will admit that until the past few yrs I would service anything on a vehicle but ac. Well, I am still new to ac work and I’m still learning , but started to understand enough to take it on for three vehicles so far.

Dr Bob
I have to hook the hoses up so it would be on me if that mistake was made. But they are color coded and rather hard to mess up and the connectors are different sizes for the charge ports. I also use a new project as an excuse to buy tools, but I always think I wont to use gauges enough so I’ll just keep renting them.

To my untrained eye I did not see any reed valves that appeared bent. I wore gloves , wiped all parts down, and gave a light coat of PAG oil to gaskets , orings and valves. I’m not sure how small a speck of dirt it would take to cause a valve to be open enough to affect what I am seeing but within the realm of possibilities.

I always check the pressure chart for the current ambient temperature so I know what pressure I should be looking to achieve. Neither was even close. “X” is the missing bolt that started this whole circus as pressures were off when I checked as soon as I got home . It happened before the reseal. But the mounting ears where the bolts go are such a tight fit, I honestly can’t imagine the compressor moving that much to cause damage. But I know it was misaligned some as the belt or pulley squealed. All that went away once I reinstalled a new bolt. But for the O-rings to leak, I would think it would be one or two situations: the compressor over pressurized during misalignment or the case sections had some give when the bolt fell out.

This is all part of classic car ownership. I just prefer it not be rework and be something different.

[dr bob]

Was the belt squeal from a seized or hydro-locked compressor (too much oil maybe)?

Back to your gauges -- did they/do they read ambient-equivalent pressure before you started the engine? On both gauges?

[c5_pilot]

If I had happened to add too much oil
I don’t believe it would have lasted the approx 6mths of flawless performance before bolt fell out.

The gauges… I pulled a vacuum and then I believe I started the car and open up the low side and attempted to add refrigerant. So not sure.

[Petethepug]

Jokers will remove the Schrader depress pin in the charge hose so it can’t charge.

Is there any stop leak type product in the system?

I have always used the free rental gauges from auto parts store. No telling if same set or not. But pressures were similar when I tested aftet the bolt mishap. I did not attempt to charge as ac was still blowing cold.

[c5_pilot]

I am pleased to report my AC is operational again. I had the compressor bench tested at a AC compressor seller. It tested fine and with normal pressures. As to my earlier odd pressures , my suspicion was the TXV so I bought a new TXV and drier to install. I rented gauges from a different auto parts store this time and after having a little trouble with the aluminum crush washer on the compressor drain bolt , got a perfect vacuum that held for hrs. Charging system, the pressures were normal this time. At least for now I’m here to be back in business. And perfect timing as it’s getting hot here on the Gulf Coast.

[chrischrischris]

Any tips for installing the expansion valve? I plan to do the same sometime in the future for the first time and have not worked in that area of the car before.

Also, thank you drbob for the procedure to charge on the high side of the system as the low pressure port on my 87 and 84 are also in terrible locations.

[c5_pilot]

Mine is an 83 , early car. I am sure you already found the procedure for removing evap. But if not there is a good writeup here. https://924s944.com/2022/10/09/more-944 ... ion-valve/
It looks worse than it is, and the worst part is bending over under the dash to get to everything. Thank goodness I have a lift and can raise the vehicle to my height and not have to bend over so much. Carefully remove the drain hose from the rear of the Evaporator before you start. I would say find a small box or something to rest the evap on once you loosen it to make it easier to undo all the wiring without hanging down.

Mine had a foam insulating cover on the valve itself, which I had to be carefully cut into two pieces. Once you install the new valve, you can simply glue the foam halves back together with contact adhesive.
Be careful when reshaping the copper tube on the TXV to mount to the suction line and wrap it good with new Prestite tar tape. Of course use new orings everywhere.
Good luck !