[ferricsteed]

I need troubleshooting skills: Is my compressor bad? Suction prerssure will not drop below 50 psig. Temperature from air vent doesn't get below 60F. Discharge will run 200 to 220 psig. I can shut radiator fan off, discharge climbs to 300 psig. Regardless, the suction side will not drop. If the compressor can build 300 psig, it dshould be OK? I have replaced orifice tube, tried both a variable tube and fixed orifice to no avail. I use bathroom scale to add R134a, (converted 10 years ago but never was really happy with results, cool air but not sharp cold. A cycling clutch is too simple a system but apparently I am missing something. Much appreciative of tech advice.

[Hotrod]

Don't know what ambient temps were when you took the pressure readings but, sounds like your compressor is bad or at least one of the reed valves on the suction side. Short of replacing the compressor, have you tried cleaning the condenser fins. Its also possible you over charged it, but I see you weighed the charge. It should be 80% of the R-12 charge. Conversions never really work that well, the condensers just can't dissipate the heat. The fin count on a R-134a condenser is much higher than the fin count on a R-12 condenser, plus the fact that the corvette has poor air flow thru the cond and rad. You really need to push the air thru these things. Does the discharge air temp drop when you take it on the highway? Also, did you run the engine at 1500RPMs while checking the pressures? You should have a suction pressure of around 20psi to 45 psi again depending on temperature. Did you install a new pressure switch suitable for the 134a Refrig.? Also, did you check the temperature blend door and were the temps you recorded in Max AC or Norm AC. The heater valve may be bad allowing the heater core to circulate coolant thru it, if it even has one.

[toobroketoretire]

Originally Posted by ferricsteed

I need troubleshooting skills: Is my compressor bad? Suction pressure will not drop below 50 psi. Temperature from air vent doesn't get below 60F. Discharge will run 200 to 220 psi. I can shut radiator fan off, discharge climbs to 300 psi. Regardless, the suction side will not drop. If the compressor can build 300 psi, it should be OK? I have replaced orifice tube, tried both a variable tube and fixed orifice to no avail. I use bathroom scale to add R134a, (converted 10 years ago but never was really happy with results, cool air but not sharp cold. A cycling clutch is too simple a system but apparently I am missing something. Much appreciative of tech advice.

Do you happen to have a sight glass installed anywhere in the hot side? Although not necessary they are an aid in troubleshooting.

When your compressor is running are both of the gauges needles steady without any fluttering up-and-down?

And did you remember to put the o-ring on the orifice tube? Without the o-ring your suction pressure will never drop much because too much refrigerant will be flowing thru the orifice. The high side pressures you have seem awfully low for R-134 as I usually shoot for around 240-260 psi

[ferricsteed]

Here are the details: Discharged system through leaky schrader. Replaced both schrader valves. Pulled Murray 38902 variable orifice (standard, not severe duty, I don't know orifice diameter), inspected, looked good, it was solidly installed. Reinstalled. Vacuum system for an hour. Filled with R134A with 3 lbs using scales. (roughly 85% of R12 fill if I have initial fill info correct.) Started car in garage, 1500 rpm. Ambient temperature in Southwest Louisiana 90F, 70F wet bulb (humid). 230 psig discharge, 30 psig suction pressure. System would not pull down to 20 psig to cycle compressor off. Revved engine to 2000 rpm, suction dropped maybe another psi, still not cycling because it won't pull suction pressure down to trip point. Discharge climbs to 280 psig, cuts on radiator fan, discharge drops to 180 psig and fan trips off. I disconnect HP switch which keeps fan running. Discharge pressure stabilizes at 230 psig. Center air vent temperature dwon to 44F in max full recycle mode, windows rolled up. Return engine to idle, suction climbs to 50 psig, air vent temperature climbs to 60F. Is this a bad compressor? Help me understand why or why not. Orifice allowing too much flow? It is marketed to reduce air vent temp (at idle!) 5 to 11F. I assume by reucing flow and therfore pressure to allow for cycling of compressor at idle. It ain't happening. Thanks for the reply in advance.

[Hotrod]

If the low side pressure won't pull down to cut-off the compressor: then either you have too much charge, or the compressor is shot or the orifice is too large. I would be leaning towards the compressor. But first try removing some refrigerant and see if the suction pressure drops. All in all though it's not too far off from being ideal.

High side pressure should be 2 to 2.5 X ambient temp. depending on the humidity the higher the humidity the higher the multipier(2.5) Mind you, this is not an exact number here, its just to get you in the ballpark.

[toobroketoretire]

Originally Posted by Hotrod

I would be leaning towards the compressor.

Yeah, me too as the A4's use aluminum cylinders and valve plates so they will wear out quickly. If you can get 100,000 to 150,000 miles out of an A4 consider yourself lucky it lasted that long.

"At rest" the systems hold around 75 psi on a 90 degree day (normalized with the engine shut off). Then when the compressor is engaged the high side will increase and the low side will decrease; the difference between the two sides being determined by the volume of compressor output and the rate of leakage thru the orifice. Any time you see any compressor functioning only at elevated speeds it usually means it's reed valves aren't sealing as well as they used to and that's very typical of a high-mileage R4.

Oh yeah, and I would never use a bathroom scale to weigh refrigerant as "spring" scales are only accurate when they're loaded to at least 1/2 their maximum capacity or about 150 pounds. If you end up replacing your compressor put a sight glass somewhere in the high side at the same time as it'll aid you in future charging or troubleshooting. When I recently replaced my receiver/dryer in my '71 I was delighted to see it had a sight glass in it. And when I charged it I used that sight glass to determine how much refrigerant I put in it. When using a sight glass you just put in enough refrigerant to clear the bubbles and then stop.

[ferricsteed]

More troubleshooting info needed. I pulled compressor, dismantled it. There aren't any suction valves. Would discharge reed or plate valves cause the same problem I have? My problem is the suction side will not pull down below 30 psig at any rpm. A too big orifice tube is a possibility, a leaky orifice tube (bad o-ring). The tube looks good, o-ring is intact, looked down the pipe bore, OT fits snuggly. Answers: both suction and discharge readings are rock steady. I can force discharge up to 300 psig by shutting off radiator fan. When I do, the suction pressure rises to 50 psig. I like the idea of installing a sight glass. Good suggestion. Agreed, bathroom scales are not ideal. I played with bleeding some freon, adding freon to no avail until I get too low in freon.

[toobroketoretire]

All A/C compressors have suction and discharge valves and they're right next to each other.

But with the R4's the problem is usually worn-out piston rings and cylinder walls.

I suggest simply buying a new one and throw yours in the garbage can (unless you need it for a core).

[ferricsteed]

I am taking your advice and am buying a new compressor. I have the existing compressor dismantled, sitting in the jaws of my vice. I don't have the expertise to or the desire at this point to try to overhaul this thing versus $200 for a new one. Just for your info, I cannot identify the suction valves. There are four discharge valves mounted above the four pistons. It is almost as if the gas is supposed to bypass the piston on the downstroke. Also, the local parts store has stock pressure cycling switch for R12, then a separate part number for this switch for R134A. He couldn't tell me the pressure setting. Do you know or advise me where to find out what the pressure settings are? I would like to get one set to cycle the compressor off at a low pressure, say 20 to 22 psig. The service manual states 26 psig and that is just too high for R134A. Would you comment on orifice tubes? I have read where a smaller orifice tube (red one) from Ford with a 0.067 inch versus the stock GM 0.072 would give cooler air. Fixed versus the new variable OT? Thanks.

[Hotrod]

The cut-out pressure is about 20psi and the cut-in pressure is about 45 psi, this is for R134a. If you are using R-134a refrigerant don't get an R-12 pressure switch, they have higher cut-out pressures and are designed for the characteristics of R-12 refrigerant. And yes I have heard that a Ford orifice tube will drop your Discharge air temp a few degrees. Never tried it though. Also, Pep Boys has R-4 compressors new from Factory -Air for about $150. I just bought one for my truck. The old one had over 200k on it and it would only perform well at highway speedss, no idle performance at all, none.