Automotive Air Conditioning Information Forum

The site was hacked around Dec 14th. I've restored it from the 11th. Any posts after the 11th are not going to be listed.

I can't remember when I joined, but apparently my poists are gone. I'll try to start it over. I was getting a lot of help from tbirdtbird. Will try again

Not meaning to get off track but a little background:
This car was one of 36 Corvettes from the 1989 VH-1 sweepstakes, of which one winner won all 36 Corvettes.
Long story short.....the car along with the other 35 Corvettes sat in New York city parking garages for 25 years until they were purchased by a couple of families in New York and were brought back to life and were then entered into a sweepstakes for the National Gaurd Education Fund.
(36 Corvettes.....36 winners) I won the 86 in drawing 1. If you watch the video, I'm the guy wearing the bandanna headband
https://www.thelostcorvettes.com/first-10-winners
That is the only history of this car that I know.

I am back to working on my AC in my 86 vette. I am converting over to 134a and I want to make sure I am not missing anything here. I was convinced to install a new parallel flow condensor. This should work better with the 134a compared to the OE condensor.

Along with the new condensor, I have installed the following:

1. ACDelco LTR4 compressor with clutch
2. Four Seasons Evaporator Core
4. GM Accumulator
5. Main compressor hose assembly
6. Accumulator hose
7. Compressor engine coolant temperature switch (located on evaporator tube/liquid line) Controls the fan
8. Compressor high pressure cycling switch (located on evaporator tube/liquid line)
9. Low pressure clutch cycling cutoff switch (located on evaporator core)
10. AC retrofit kit and o-rings

I am using PAG 150 oil and NYLOG BLUE on all o-rings and connector threads.
The Evaporator tube/liquid line (evaporator to condensor) is the only component of the AC system I did not replace. The only oil in the system at this time is 3 oz in the compressor.

Before installing the compressor, I drained as much oil as possible while rotating the pump. I then let the compressor sit elevated off the bottom of the container I was draining into for several hours to get as much oil drained as possible. The label on the compressor stated the compressor shipped with 3 oz of PAG 150 oil. The oil I drained indeed was just slightly shy of 3 oz. I then filled the compressor slowly with 3 oz of PAG 150 oil while slowly rotating compressor pulley.
I have previously pulled a vacuum on the system using my Manifold gauge set connected to both the hi and low ports with my CPS VG200 Digital Vacuum Gauge connected directly before the low side adapter on the accumulator. I then injected a small amount of refrigerent and used a sniffer to check for leaks. Found and repaired a leaking o-ring.
I pulled another vacuum from the low side line connector beneath the Low pressure switch mounted on the evaporator with the schrader valve removed using a CPS VG200 Digital Vacuum Gauge connected directly to the Appion MGAVCT 1/4" MegaFlow Vacuum-Rated Valve Core Removal Tool and a 3 CFM Two Stage Vacuum Pump. I wanted to isolate my manifold gauge set, hoses, and vacuum pump from this pull.
I originally had an ACDelco variable orifice valve installed but with some help from a forum member was informed that if I wanted performance I should install the correct OT for this application.
From my search, I have found that the correct OT for this application has a orifice size of .072. I have spoken to numerous individuals who have converted their early C4 Corvettes over to R-134a as I have, and they tell me from their experience that the Ford blue OT (part # T38621), which has a orifice size of .067, is the one that performs the best with the smaller size of the R-134a.

So, here I am back to a completely assembled system and am ready to evacuate again. My first question:
I want to pull the vacuum isolating my manifold gauge set, hoses, and vacuum pump as follows:
a. Connect an Appion MGAVCT 1/4" MegaFlow Vacuum-Rated Valve Core Removal Tool with a CPS VG200 Digital Vacuum Gauge attached, to the low side line connector beneath the Low pressure switch mounted on the evaporator with the schrader valve removed.
b. Use a 3/8” vacuum rated hose and a 3 CFM Two Stage Vacuum Pump.
I thought maybe run the vacuum for three hours and see if I can get below 160 microns. Would this be OK?

A few thoughts:

• Installing a parallel-flow condenser in a conversion like yours is a wise move.

• I don't think it will make much difference to the new compressor whether you use PAG-150 or PAG-46.

• Using Nylog Blue is a good thing and will help ensure a leak-free system.

• Not sure about the OT size (0.072 versus 0.067), but you're right—a molecule of R-134a is smaller than a molecule of R-12. Try out each OT to determine which one performs best (as long as you don't mind the extra work).

• Any evacuation below 500 microns is considered a deep vacuum and is more than satisfactory in my humble opinion.

"I want to pull the vacuum isolating my manifold gauge set"

OK, I am slightly confused here and suspect you mean something else. Not sure there is a way to pull vacuum without involving your manifold gauge set (you said you were going to isolate it)

And strongly suggest you pull from both sides this time.
If your 'vette buddies have determined an optimal OT size, I would start with that one

And totally agree with John (who is sharper than I am) that 500 microns is adequate

You have acquired some decent AC equipment and done a lot of research. Many of our posters would be so much better off it they followed your example

Keep us posted, and good luck

Oh, I'm not done with you guys yet
As far as the low pressure switch is concerned, it is my understanding that the stock R12 switch is set to cycle at 25 PSI, which is about 25.8 degrees with R12, and that R134a will only drop to about 29 degrees at the stock 25 PSI cycling pressure. To get down to the same 25.8 degrees with R134a the switch needs to be set to cycle at about 23 PSI.

I will pull the vacuum from both sides this time as you suggest, tbirdtbird.
As far as isolating my gauges during the vacuum pull, the only reason I wanted to do that was to get maybe a more acurate reading from my micron gauge after letting it sit in vacuum overnight.
I'm prolly just letting my OCD get in the way and over thinking it, huh?

"As far as isolating my gauges during the vacuum pull, the only reason I wanted to do that was to get maybe a more acurate reading from my micron gauge after letting it sit in vacuum overnight."

There is no need to do this. If you isolate your gauge set from the micron gauge, then you are not reading the vacuum of the AC system.
If you have puffed in some 134 on top of a good vacuum, and used a leak sniffer, as I think you said you did, then once you get back down to 500 microns there is no need to let it sit any longer. Just charge it. Hopefully your ambient is high enough that the 134 will come out of the container. You may need to use a warm water bath to set the cans in to boost the internal pressure. Or, just as we advised another recent poster, leave a partial charge in there until spring. You never want to leave a system in vacuum for an extended period of time. If you have a known weight to charge in , then you can get an accurate charge even in cooler weather. BUT if there is no known amount, then you can easily overcharge by guestimating in cooler weather if you try to get to a desired endpoint. In the spring wait for an 80F day before finishing the charge

As far as adjusting the low pressure switch, on an OT system there is nothing to adjust. Unfortunately you missed an installment I wrote about this before the hack. Whoever told you about that mistakenly assumed you had a POA valve, which is the old way GM did things. But you don't have a POA so there is nothing to adjust. The switch on the accumulator will take care of everything, if it has been wired correctly.

kenlou wrote: ↑Thu Dec 15, 2022 4:25 pm As far as the low pressure switch is concerned, it is my understanding that the stock R12 switch is set to cycle at 25 PSI, which is about 25.8 degrees with R12, and that R134a will only drop to about 29 degrees at the stock 25 PSI cycling pressure. To get down to the same 25.8 degrees with R134a the switch needs to be set to cycle at about 23 PSI.

If I'm understanding correctly, I think what you're driving at is that you want the LPCO switch to turn off the compressor at 23 PSI instead of 25 PSI so that you can achieve approximately the same evaporator temperature of 25.8°F (that you would get with R-12) but with R-134a.

However, if you use a later-model LPCO switch designed for R-134a, which I think you might already have, it will be factory-calibrated to open and close at the optimum pressures and temperatures for R-134a.

An evaporator temperature of 25.8°F is pretty cold anyway and might cause evaporator icing problems under certain operating conditions. So I think that your concern about the pressures and temperatures are probably unfounded.

The LPCO switch I bought from Corvette Central was set @ 25 PSI. I confirmed this when I pressurized with nitrogen and using a multimeter set to measure continuity. I then pressurized again and adjusted it down to 22.5 PSI.

Thanks,
Kenny

Tbirdtbird, I did not miss the installment you wrote concerning adjusting the low pressure switch, on an OT system before the hack.
When you mention the switch on the accumulator will take care of everything, if it has been wired correctly.
This is what I have as it came from factory: Thanks,
Kenny

The diagram is most useful.

OK, so the LPCO is not mounted on the accumulator, but on the low side line as it exits the evap. Basically the same idea. Do not think I have seen modern adjustable LPCO switches. I agree with John that adjusting it prolly was not necessary, especially since they are usually fixed. I guess you will be OK, but be aware of the cautions he gave you, ie, evap icing