Learn me old car A/C diagnosis - Porsche 924S

I can do most things mechanical on most cars, but A/C is really a dark art for me, and I've never really had to deal with it other than replacing a hard-line to the rear A/C on my Sequioa a couple years ago. 

So when I got this car ('88 Porsche 924S), it had been sitting for at least a year with a spun bearing. After I had already paid the guy and loaded the car, I asked him for an honest answer about whether the A/C worked. He had been totally straight-up with me about all the car's issues (I paid under $1k for it...) so he had no reason to lie I wouldn't think. Anyhow, he said that before he blew the engine, the A/C worked fine. 

So fast forward, I finally am getting around to checking out the system. It has already been refitted with r134 valve (only on the low-pressure side, oddly enough), so I assume at some point it may have run r134 - but I know that can leak out of r12 systems over time. All other components appear original.

First thing I replaced the dryer with a new r134 dryer (with new o-rings there), and added an r134 valve on the high side as well.

With the car running, if you turn on the A/C it does activate the compressor clutch (but of course there is no r134 in the system right now)

I evacuated the system with a vacuum pump and it's been holding constant vacuum for about half a day now. I have full o-ring set, but if the system is holding vacuum, I may want to hold off on accidentally causing any new leaks replacing o-rings, to be honest.

Next I plan to pull the compressor to drain the oil and add PAG oil to it (r134 compliant).

But what other ways can I test the system before I dump r134 in? I have a manifold gauge set - can I run the system with no r134 in it to get compressor pressures? Or will that not work with an empty system?

Note : I have 4-5 tall cans of r134 that I got from an estate sale for a couple bucks, so I don't necessarily mind wasting some of that in the cause of testing the system, but would love if someone has a good plan of action to see what does/doesn't work.

Find the quantity it is supposed to hold and put that much in.  If it's cold after, it works.

The compressor can't make pressure on a vacuum.

If it's not leaking down, you should be good to go.  The low pressure switch should keep the compressor from turning without refrigerant. You will likely burn up the compressor pretty quick if it's turning without oil/refrigerant. Are you sure it's not just the pulley turning and the clutch is not engaged? If not, then that's definitely an issue. That old of a car may not have a low pressure switch though (?)

The real question you should have is:  How much oil should I put in it?  Theoretically, the oil should still be in it (it will settle in the evaporator and condenser).  Was there oil in the dryer (there should be)?  If so, then you should add what was in the dryer. A bit too much oil is better then too little.

If you trust the guy, then add the missing oil and charge it.  You charge through the low side, so that is why that was the only side changed.  The compressor should kick on as you start adding the first can. How much you add is a bit of a guess since the specs will be for R12.

Most R12 system hoses have become impregnated enough to not leak 134, so you should be able to get away without replacing the hoses.

Others will likely have some more specific info.  Not sure if you will need more or less 134 than 12.  You can get pretty damn close just by adding and measuring the outlet temps though (make sure to put a fan on the radiator / condenser).

Don't worry about running out of 134, there are a number of replacements that work fine (I have used Envirosafe).

aircooled said:

If it's not leaking down, you should be good to go.  The low pressure switch should keep the compressor from turning without refrigerant. You will likely burn up the compressor pretty quick if it's turning without oil/refrigerant. Are you sure it's not just the pulley turning and the clutch is not engaged? If not, then that's definitely an issue. That old of a car may not have a low pressure switch though (?)

The real question you should have is:  How much oil should I put in it?  Theoretically, the oil should still be in it (it will settle in the evaporator and condenser).  Was there oil in the dryer (there should be)?  If so, then you should add what was in the dryer. A bit too much oil is better then too little.

If you trust the guy, then add the missing oil and charge it.  You charge through the low side, so that is why that was the only side changed.  The compressor should kick on as you start adding the first can. How much you add is a bit of a guess since the specs will be for R12.

Most R12 system hoses have become impregnated enough to not leak 134, so you should be able to get away without replacing the hoses.

Others will likely have some more specific info.  Not sure if you will need more or less 134 than 12.  You can get pretty damn close just by adding and measuring the outlet temps though (make sure to put a fan on the radiator / condenser).

Don't worry about running out of 134, there are a number of replacements that work fine (I have used Envirosafe).

Before I vacuumed the system I put in half a can of r134 just to test the system and it was enough that the compressor clutch engaged (and disengaged with the A/C switch off). It's easily visible. So while I don't know if "the system" is good, I do know the compressor clutch is at least engaging. And yes, it has a low pressure switch right next to the drier.

Oil amount to put in is prety well-documented in the Porsche forums (2-3oz) so that's not an issue, as is the r134 system capacity vs. r12. (IIRC it's about 80%)

Not too worried about running out of it lol, but buying more costs money. And I'm a 924 guy, not a 911 guy ;)

OK, well, that fact that it turned off the LP switch means the compressor is working.  I guess you could charge it to see if it will develop full pressure.  Of course, you could just add the oil and charge it then.  All you are out is some PAG oil.

You will need a full charge to see full pressure.  More charge = more pressure.  The "system" is working if cold air comes out.  Checking pressures etc. is really only if the air is not cold or not cold enough.  The system is actually pretty simple (build pressure and heat, cool hot/pressurized charge, expand hot/pressurized charge = cold, extract cold, repeat)

I would probably just do the oil swap, charge, and see if it's cold.  Not sure what the issue is with the potential of mixing oils, but these sort of swaps have been done many times.

I am honestly no super expert, but I did make an old Mazda run for many years just by swapping in some blend.  I recently flushed the entire system (the evaporator is a pain) to replace a bad compressor and went with PAG and Envirosafe. It was blowing sub 40 deg today and it was in the mid 90's outside.

I wouldn’t drain the compressor. Add a small amount of oil if you removed the dryer and the fill with refrigerant. Sounds like it’s going to work fine. 

Well, it seemed to hold vacuum fine (and I did end up draining the compressor of the old mineral oil and adding PAG 46, as well as new o-rings on the lines), so I just charged it up with about ~25oz of r134a (this is a 30 oz. system, and from what I read you're supposed to do 85% r134a vs. original r12.

Anyhow, happy to report that in my garage at 80 degrees ambient temp, running at 2000 rpm, the vents are now blowing 38-degree air. So, hopefully no leaks and it keeps doing this :)

Thanks for the advice, all. 

Nice.  Air conditioning is an awesome thing to have.