[Mister Pontiac]

A/C experts, I have a question...

My 72 LeMans GT has the factory A/C setup (minus the muffler on the A6). I swapped out the growling A6 compressor this morning, then vacuumed down the system and started charging things up.

I'm still a bit low on R12 at the moment (3 cans are in), but I paused as the old scenario has reared its head.

The line from the POA valve to the back of the compressor is cold, frosting over, but the line between the evaporator to the POA valve is only slightly cool. Consequently, so is the air inside the car, only slightly cool.

I previously swapped the POA valve and had this same issue, but with the noisy compressor, I decided to wait to solve the problem.

So here I am. Rebuilt compressor and rebuilt POA valve, but never seems to get full cold through the evaporator. I'm not sure why. Before I add more valuable R12 to the system, I want to see if there's an easy fix, or do I have an issue with the evaporator?

Someone suggested it's low on R12, but I've filled the system fully before with the same situation. Almost seems like the evaporator is blocked?

Thoughts?

[gto4ben]

Did you rebulld the compressor yourself or buy a rebuilt unit? Today, the rebuilt ones may be prefilled with PAG oil for use with HFC134a. R12 requires mineral oil and the two oils are not compatible. Also, if the system is not flushed, there is oil coating the inside surfaces, roughly 2oz for condensor, evaporator, hoses each. You need to remove the prefill oil by that much in the compressor or there will be too much and that will reduce the cooling.

If the compressor is chattering like you described, it could be an indication of low refrigerant since it's not building pressure on the high side or the expansion or TX valves are stuck open. If so, the high and low sides track. The low side is high and the high side is low. I find that this is sometimes interpreted as a bad compressor but it's not.

Did you change the receiver drier? It's required whenever the compressor in changed. If reused, there's a high chance that debris that was captured inside is dislodged and recirculates in the system, usually clogging the expansion valve screen or POA valve. A clogged receiver/drier will reduce flow, create high condensor pressures, and reduce cooling. Usually, the liquid line going to the expansion valve will feel cold in this case due to the blockage. It also captures remaining moisture that can freeze up in the system that creates blockages. A gauge set will tell you a lot as well. When the POA is working, you'll see 30psi in the low side.

You also need to use a box fan or equivalent in front of the condensor to get the refrigerant to lower its temp there and turn it to a liquid so the expansion valve can regulate the flow.

BTW, both the expansion and POA valve can be bench tested before installation if you have a gauge set and some brass fittings you can buy from the big box stores. if you do have to take it apart again, I recommend you do that again

[Mister Pontiac]

Thanks for the reply, Ben. Sounds like you have a good handle on these systems.

First, I used a reman A6, but one that is slated for use with R12, so should be correct. I installed the same reman unit on my Bonneville last year, and it seems to work proper with R12 in that system.

I did not put on a new receiver/drier this time, but I did install one shortly before I swapped compressors. If I have to pull the system down again, I will install another one. It sure seems something is blocked (or frozen) somewhere.

When I vacuumed down the system it held -28" for a couple hours, then I began the recharge. As I slowly put in the R12, the low side hovered around 30psi while the high side started its climb. With 3 cans in, the high side was around 200, which I understand is still a little low, but I assume should get close to that 300 with the 4th can. I was told 30/300 are ballpark numbers, and about 3.5lbs, so it seemed to be tracking.

Couple questions...

1. How would I test the POA valve with the gauges and brass fittings if it's off the car? I will do that too if I have to pull things down again.

2. Is it worth 'rinsing' out the evaporator with something if I open up the system? What would you use to clean it with? Mineral spirits? Lacquer thinner?

3. What about the expansion valve? Do they go bad? Easy to test? Or simply install a new one when the system is apart?

Anyway, I'll keep fussing with it. If you can think of anything else I'm missing, or if my description seems to point more in a certain direction, please let me know. Also, if my understanding of pressures or weight of R12 seems wrong, let me know that too. I'm still learning the finer points of these systems, but I'd like to get better with them to keep them running for a long time to come. I have no plans to retire my Pontiac's or their R12 systems.

Thanks again!

[gto4ben]

Depending on the outside temperatures, your pressures appear good. Grab a thermometer and measure the ambient air going over the condensor. An R12 temperature-pressure table will show the expected pressure you should see on the high side.

Testing the POA is very straight forward. All you need is a air compressor, your AC gauges, and make some fittings to attach to the POA. I attached a photo of the adapter I made. They sell special adapters as well. You could even test the POA on the car without removing it once you open the system up. Personally, I don't listen to the "pop" since there is some hysteresis. I plot the inlet and outlet temp and look for the "knee" in case I want to readjust the POA pressure setting.

Expansion Valve ( aka TXV) requires removing the capillary bulb so you can immerse it in warm and ice water. This is a pain as 64-67 GTOs will need to remove the inner fender and probably the full fender to get to it. Once out of the car, The TXV will open or close base on temperature.

[gto4ben]

Here are the TXV Hot and cold test setups with the capillary tube immersed in hot and ice water. The displayed pressures at hot show that the TXV has equal inlet and outlet pressures which is fully open. The cold temperature test shows that the outlet pressure is reduced which means the TXV is closing to reduce refrigerant flow. I attached a "On The Car" photo I found that tested the TXV and POA at ambient. I've tried it this way as well. It works for the POA since the TXV is open at ambient.

Regarding flushing the system, do not use mineral spirits since it will leave a residue. I have used lacquer thinner for an initial flush and followed it up with "Supercool A/C Flash Flush". The lacquer thinner evaporates quickly and is cheaper than the dedicated flushes. The Supercool is insurance that the lacquer thinner didn't leave something unknown.

The stock compressor shaft seal is a ceramic disc seal. They are good but seep the oil and leak over time. An easy upgrade is using the available double lip shaft seal instead. I polish the seal contact area on the shaft when rebuilding the compressor to maximize seal life.