My turn, need the wizards

[tbirdtbird]

BACKGROUND:We retrofit AC to vintage vehicles here, almost always using the same exact components, with excellent results. Have fitted about 8 cars.We also work on modern cars with faulty AC and fix whatever is needed, with good results. I also do residential.

CURRENT problem:1959 Cadillac.This car never had AC. So we have fitted the usual generic components as we have done many times.Thus, the make, model, year of the car is immaterial. Initially we used an A6 knockoff, known as an S6 for compressor since that is what Old Air suggested. We have had miserable luck with that unit. They lock up after 100 miles and are junk. I believe one reason they lock up is that no matter what we do, we cannot get the low side over 15 psi, which I believe means inadequate oil return

NEW COMPONENTS TODAY:
Comp  changed to Sanden 508 knockoff (Japanese, from Tim). No more S6s here.
Cond   14x 24 from Omega (largest that will fit)
Drier from UAC
Evap equiv. to Old Air products # IP-300L-CF
TXV 1.5 Ton from Ranshu
Premium PAG from Tim
Condenser installed with air dams
Evap and hoses have been flushed with a pressure gun (100 psi) connected to my shop air (we paint cars here so need a clean/dry air supply. We use  a refrigerated unit ( commercial dryer) to wring the moisture from the compressed air. Flush agent is naphtha. Ran 2 gallons thru hoses and evap until evap was clean, then shop air til dry. That naphtha dries out quickly and leaves no residue. We use it in painting cars all the time. Last flush revealed clear naptha with no metal particles.

Comp, cond, and drier are brand new.

SPECIFICATIONS TODAY:
Vacuumed to 1800 x 3 and did a triple evac each time
Because this is a custom setup, we do not have a known weight of 134 and thus meter it in the old fashioned way.
Ambient  90     Humidity 50%   (North Texas)
Low         15
High        230   (I went a bit higher here than I wanted to to try to bring up the low side, to no avail)
Vent         32
RPM        1800
This was a Full load test all windows down, no recirc
It can't be the TXV, I have replaced it 3 times with the same results.


No matter what we do or change, we CANNOT GET THE LOW SIDE OVER 15 PSI
I consulted with an established local industry expert who does field work for Omega, and has a traveling van he takes from dealer to dealer etc. He is quite sharp.and experienced.He is stumped, and I am stumped.
Any takers? Thanks

[JohnHere]

Several things come to mind. Here are a few:

The first concerns the TXV sensing bulb. Are you sure that it's securely attached to the outlet tube of the evaporator and that it's completely covered by Prestite tape? I agree with you...I don't think the TXV is otherwise malfunctioning.

With a low pressure of only 15 PSI, the evaporator temperature is well below freezing, which sounds like there might not be anything to limit how cold the evaporator gets. Are you running some sort of frost switch to cycle the clutch, or else an EPR valve calibrated for R-134a for continuous compressor operation?

The SD-508 knock-off that you're using is a fixed-displacement compressor, correct?

[tbirdtbird]

Thank you for your reply.

The first concerns the TXV sensing bulb. Are you sure that it's securely attached to the outlet tube of the evaporator and that it's completely covered by Prestite tape? I agree with you...I don't think the TXV is otherwise malfunctioning.
Dead certain bulb is installed properly

With a low pressure of only 15 PSI, the evaporator temperature is well below freezing, which sounds like there might not be anything to limit how cold the evaporator gets. Are you running some sort of frost switch to cycle the clutch, or else an EPR valve calibrated for R-134a for continuous compressor operation?
Yes there is a thermostatic control with a probe in the evap. While charging, I have it set to max cold (because this is Texas) and it usually will not cycle. I can dial it back and it will cycle but as soon as the clutch engages the low drops right away to 15 psi
All the other identical systems we have installed show 30-35 on the low side under identical conditions

The SD-508 knock-off that you're using is a fixed-displacement compressor, correct?
Yes

I should add that the pipe in and the pipe out are the same cold temp when felt by hand

[DetroitAC]

Vent Temps? 3 of them ambient and 1 slightly cooler? I'm thinking defective evap short circuiting liquid right to the outlet, I've encountered them a few times.

[tbirdtbird]

Thanks for reply

Hmm, interesting. Never heard of that.
I will check and get back.

Is my assumption that such a low side pressure will not allow adequate oil return?

[JohnHere]

I'm thinking about the TXV again. Perhaps the one you're using from Ranchu just isn't calibrated for this application and not metering properly. It's possible. I've had problems with non-OEM TXV's in the past. Might be worth trying an OEM unit (if you can find one) or a correct OEM replacement, which are available in the market.

[tbirdtbird]

Thanks, John, there really isn't an OEM TXV per se, all the components are aftermarket, the car never had AC.
I have tried 3 different TXVs (sourced differently with stated different manufacturers). Same results. All are 1.5 ton rated

[JohnHere]

Thanks, John, there really isn't an OEM TXV per se, all the components are aftermarket, the car never had AC.
I have tried 3 different TXVs (sourced differently with stated different manufacturers). Same results. All are 1.5 ton rated

Yes, I remembered your saying that the car never had A/C. I was thinking that an OEM or OEM-replacement TXV might make a difference. But since you've already tried three TXV's from different manufacturers, it appears that the "stuck on 15 PSI" problem lies someplace else.

I never experienced a short-circuited evaporator that DetroitAC described, either. But it certainly merits investigating. I think you mentioned that the vent temps are 32 degrees, too, which is mighty cold.

How much refrigerant is in it now?

[tbirdtbird]

I did not weigh it in, but I am gonna guess about 24 oz.
I started with a 12oz can liquid into the high side, as always, then topped off on the low with my jug.

Not sure this will help, but while charging, the low reads about 30 until I shut the jug off, then dips right back to 15.
Seems to me on charging other cars the low reads about 60-70 while charging, then drops off to 35 when you close the tank valve. But I can;t be certain my memory is correct on this.

I have a refrigerant scale, should I weigh it in next time? would that matter?

[JohnHere]

I did not weigh it in, but I am gonna guess about 24 oz.
I started with a 12oz can liquid into the high side, as always, then topped off on the low with my jug.

The charge amount of ~24 ounces does sound a little light to me. For a custom system like that, just off the top of my head, I'd expect it to take about 30 ounces. But that's mostly guesswork on my part. I believe you mentioned that you've installed systems like this before. Do you recall how much refrigerant you dispensed into them?

Not sure this will help, but while charging, the low reads about 30 until I shut the jug off, then dips right back to 15.
Seems to me on charging other cars the low reads about 60-70 while charging, then drops off to 35 when you close the tank valve. But I can;t be certain my memory is correct on this.

I think your memory is essentially correct on the low-side pressures while charging, but they will vary somewhat depending on the system in question, engine RPM, ambient, and heat load. The actual charging pressures you're seeing seem a bit odd to me, though. How about the accuracy of your Manifold Gauge Set? Have you tried a different one? Is your refrigerant cylinder approaching (or is it) empty perhaps?

I have a refrigerant scale, should I weigh it in next time? would that matter?

You could, but it's still going to be somewhat experimental getting the charge just right for a custom system.