I finally got around to getting the a/c system in order. I flushed the lines, the condensor, replaced the receiver dryer, put in a used evaporator, kept the expansion valve because it was clean inside, and replaced the compressor. Using the quantities chart in the original shop manual, i filled each component with the proper amount of oil. Because I will be using R134, I used PAG 525 viscosity oil. All new o-rings and the system is holding vacuum overnight, so I think I'm ready to go.

First Question:
The original shop manual for the 72 lists the a-body freon charge to be 4 pounds. Is this right?!?!? That seems like so much freon, considering our new traverse with two evaporators only is using 2.5 pounds. I can't find any other mention of the charge load for the system.

Second Question:
Save for the receiver dryer and the compressor, I am using all old factory parts - including the POA valve. I read on other threads here, that you need to calibrate it? I always thought it was sorta a universal function. Will I be jeopardizing my system if I just charge it up and start using it?

Any advisement on this would be appreciated.

 

I would say if you can get R-12 recharge it with that and the proper oil. It will work so much better since you seemed to have a good vacuum.

 

I can't really go with the R12, simply based on cost . I have lots of r134 cans in my garage just waiting to be used. I'm trying to keep the costs down.

What about the question about charge? Is 4 pounds right?

It is holding vacuum, but it is only going down to -22, not the specified -29. I think maybe the pump I borrowed from my neighbor isn't strong enough.

 

The design of the newer systems 134a allows for much less refrigerant than the old R-12 systems. The expansion valve system differs from that capillary tube or orifice type system. I cannot say for sure how much 134 will be required but you will need to adjust the charge according to pressures while operating. The evap or low side should maintain 35-38 degrees. I don't have the conversion chart here to give psi required. I would think 2-3 lbs will be required.

 

Rule of thumb is 80% of the R12 charge when doing a conversion, so ~51 oz of R134a. These old systems are quite large.
The POA is adjustable. There's a sticky on here with some of the info. It's a little difficult, but not impossible, to do on the car. That modification will get you a few extra degrees by lowering the low side pressure, which aligns with R134a's properties better. It's fine to run it as-is, the outlet temp will be maybe 5 to 10 degrees higher than it could be otherwise.

 

... And don't fill the system until you adjust the POA, because you need to disconnect the lines from the POQ in order to do it.

- Eric

 

So the question here is then, how do you adjust the POA?

I don't mind opening up the system for a short time as long as I know what to do....

 

It's all in a sticky:

https://classicoldsmobile.com/forums...2-cutlass.html

- Eric

 

Ok, seems straightforward! I will just have to do the 1/4 turn, as I'm not sure how exactly I would hook up the test air hose to the inlet side. I have two spare POA valves, and I could try and practice.

Unfortunately, I have this weekend booked and won't be able to get to it for a while.

On a side note - It is really interesting that you guys are even talking about this. I really didn't find this anywhere on the Internet being discussed at all.

Thanks guys!

 

We are the internet's internet.

- Eric

 

I have a 72 GM with a converted (connections and rings only) system running R134A in an otherwise R12 system. It will put out mid 60s air when upper 90s and humid. It is not ideal, but acceptable. This is without POA mods. I say this to give you a performance data point. My car will go back to R12 during some far off restoration (10 years plus).