Has anyone serviced the Calsonic control valve (R-134a)?

[3Q Jay]

Performed A/C service on BlacQ sunday.
Had been over 8 years since last, a/c efficiency gradually reduced in last year, seemed to vary between cooling and ambient-ish air over course of drive cycle not forward speed or RPM dependent*
Checked manifold pressures before service, high side was 125ish, low side was 50ish, figured was just refrigerant depleted.
evacuated system (didn't measure exact removal amount, but was less than full charge for sure), replaced the drier (used 4-seasons cheap chinese part--see amazon review--have used this before on Silver, my logic is it's just dessicant, not a part to worry about pedigree--reused the robust nissan OEM triple pressure switch of course). Pumped down got a HARD 30+in of vac (good), injected 1 oz PAG-46 on low side, continued pumpdown achieving another HARD 30+, pumped 30 min left sitting 1hr, no detectable loss of vac (good).
got out my scale and put in exactly 810g fresh refrigerant.
now the results:
from about 300g on up, the low side read 50 ish and the high side read 130ish. compressor ran full time, control set to max cold, ambient conditions ~77F, 60% RH. shaded garage (sunload sensor would be inactive). The pressure readings at idle did not significantly change all the way up to full weight load.
vent thermometer read ~64F. Condensor fan never came on (I will need to look into that possibly, unless the pressure switch is telling it to do exactly what it should be). *When i raised the throttle to 1200-1500 all else same, I COULD see the low side come down a bit (say 45ish), and the high side go up to about 175. test drive Sunday (now with sun load, ambient ~73F) showed vent temp go as low as 48F.
Drove it for about 30 min this afternoon, ambient 78-81F, setting at 78F, some sunload, vent temp pretty steady at 64F. Both this afternoon and before service, it seems that the vent temp 'hunts' for lack of a better term when in the morning (cooler temps, less sun load), and is relatively stable when the ambient is higher than the setpoint and there is some sunload.
so.
It's tolerable (maybe slightly better than before), but not perfect. the FSM suggests bad compressor for low side too high and high side too low. which after 21.5 yrs it may be. But, there is also a pressure control valve in the compressor, which could cause the observable symptoms, if clogged or otherwise poorly regulating.
From some cursory research it would appear that this is the valve, and that it is mechanical only (some are electro-mechanical).
http://www.mattsdiscountac.com/AC-Part/ ... 19104.html
Has anyone attempted this? I'm assuming that the circlip holds it into the compressor, and that the system will have to be evacuated and re-recharged. Does anyone know if compressor oil comes out with it? and if so, can fresh be installed to the low side line via vacuum injector, or must it be put into the compressor directly?
other thoughts?

[3Q Jay]

Update:
Appears to be working nominally again.
Checked today and low side pressure was 36 and high side was 180. engine warm idle, OAT=71F. Perhaps the bellows control valve is (was) sticking.
Did replace a JRA core on the low side--seems that even the replacements (fleabay specials) burp slightly when removing the service cap.
looking back thru my records an interesting note. both the 7/93 and 11/94 G50 builds are R-134a systems, but the 7/93 calls for 750g and the 11/94 calls for 800g.
systems do not 'appear' any different however....

[elwesso]

So basically you moved your evaporator temperature down to 41°F (saturation temp) from 54°F. SInce you were 6°F warmer OAT (77°F first go, 71°F second go), let's say at the same condition you may have been around 48-50°F evap temperature.

to me your low side still seems a little too high. According to the FSM you should be more along the lines of 28-35 PSI (32-40°F saturation, which makes sense to me.), however those numbers are at 2k RPM I believe and not idle.. Of course for full "meat locker" effect we'd want the evaporator to be as close to 32°F as possible. If you were doing 180 PSI on the condenser side it would seem like the compressor is healthy (i'm kind of surprised that at 77°F ambient the high side is doing roughly 122°F saturation).

Maybe you should go get an electronic expansion valve [sporlan or danfoss] and put some fancy controls on there to maintain about 34°F evaporator temperature...

I wouldn't think that 50G of refrigerant would make any difference, especially since the filter drier also appears to be sort of like an accumulator which can store liquid refrigerant.. Wonder if they found in some cases they were seeing too much superheat through the evaporator and decided to add a smidge more refrigerant to ensure 100% liquid at the inlet to the expansion valve?

[3Q Jay]

now we're gettin somewhere!.....
See that guys--we got an ENGINEER interested in this!
excellent discussion.
I neglected to mention that a slight throttle raise today pulled the low side down to 31 and the high side went up to 220.
I never thought about an electronic expansion valve before. more trouble than I need at the moment, but could be the shizzle for sure.
Condensor fan not coming on even at 220psig, so I need to look in to that.
yes, upon reflection the 4-seasons drier is about 3 cm longer than the nissan 67U10 that i installed last time. could hold even a bit more refrigerant i suppose, and yes that explanation makes sense. As 94 model year was the first for r-134a, the service departments may have been seeing a lot of traffic on the early facelift cars.

[elwesso]

You know what would be interesting is at idle if you forced on the condenser fan, what that would do to your low side. Of course if you lower your condensing temperature the system's mass flow rate is going to increase and you'll get more capacity, so it should lower your evaporating temp.. At <80F ambient, 220PSI (or about 138F saturation) seems excessive to me. The FSM says 120-130F saturation (178-208 PSI), so in general it appears we're running at about a 40°F approach temperature on that heat exchanger [seems really high to me, but I only work with water-to-water heat pumps). I don't see any disadvantage by making the condensing temperature as low as possible. Maybe you could make a set of water sprayers on the front of the condenser to make it like a wet cooling tower...

Theoretically this should work fine because according to the FSM, the compressor's capacity is determined by the suction pressure. Not exactly sure how it works (dont feel like digging into it) but it appears that the compressor capacity control is to maintain the suction pressure regardless of the ambient conditions. Looking at the chart in the FSM it does a pretty good job at doing that [only an 8F swing in evaporator temps over a roughly 30F swing in ambient].

You should measure temperature at the same place where you measure the pressure (so we could see what kind of superheat you're getting to the compressor). I would imagine if you're seeing less than 20F superheat then using a different expansion device might not be worth while.