tricky A/C question

rigitrite · 09-03-2005, 03:50 PM

Here's the deal: 1984 944 that just got back up and running after being dead for about 2 years. After the engine problems were sorted, I went to fix the A/C. I knew that it wasn't working and had been switched to 134a. I disconnected and flushed all the lines and compressor, replaced all the o-rings and the receiver/dryer. sucked it down to vacuum and filled it with 2oz. of oil on the low side on the compressor. Here's the wierd part, it would only take about 1/2 the bottle of 134a and the compressor wouldn't come on. Huh....I tried another can, and same deal, it took about 1/2 the can and then no more plus no compressor. I then activated the compressor remotely on the rest of the 2nd can, and it took a little more, but not all. it was now cooling a little bit, but not like it should, and I didn't want to push the compressor, since the low pressure switch was obviously saying "don't run the compressor" or it was faulty.
I spent the rest of a nice afternoon trying to figure out what could be going on here...it sucked.
there are no blockages in the system, since the flush came right through under air pressure. The compressor appears to work from when I had it out of the car: it spit flushing fluid all over my head when I rotated the clutch (no fun there), and rotates smoothly. The magnetic clutch works, no question. So here's what I'm thinking:
compressor is bad (reed valves or something not obvious)
Switches are bad
Expansion valve is screwed up (this is a ***** to get to...thanks porsche.)
The pressure reads:
low side: 145 psi
high side: 30 psi

One last thing: From all the diagrams I've seen, there is no mention of which is the high side and which is the low side on the compressor. I'm 99.9% sure I have it right, but there are two service ports on the compressor and one more by the reciever/dryer, which is weird. I'm using the service port on the reciever/dryer as high side and the one on the compressor closest to the pulley as the low side.
thanks in advance!

bearone2 · 09-03-2005, 04:29 PM

rd side is hi pressure.

how long did it hold vacuum pressure?

after being evacuated it should take just under 3#.

145/30 shouldn't trip the low pressure switch.

AaronM · 09-03-2005, 05:23 PM

145 PSI on the low side?! Something is definitely wrong. 30 PSI low side when running is what you should be seeing.

rigitrite · 09-03-2005, 08:51 PM

Well, that's good to know (about the oil). I poured about 5oz of oil in the compressor before I hooked it all back up. The pressure readings indicate a bad compressor, but it seems to work. I had a compressor go out on our volvo last year, and that was REALLY obvious: shear failure in the drive plate material caused it to lock up and send tiny pieces of aluminum into the orifice tube. I really don't want to have to buy a new compressor just to test a theory, nor do I want to rip out the entire dash just to get to the expansion valve (again: THANKS SO MUCH PORSCHE!!!) What I really really really don't want to have to do is take it to the stealership to get it fixxed.

Wolf1 · 09-04-2005, 11:30 AM

Quote:

Originally posted by rigitrite
The pressure reads:
low side: 145 psi
high side: 30 psi

This is either a typo....
Or, you are reading/charging the high side instead of the low side.

bazar01 · bazar01's Garage

On a 90F outside temp, here are the approximate system pressures.

Low side: 40-50 psi

High side: 200-225 psi (both cooling fans running)

Looks like you are still under charged.

rigitrite · 09-04-2005, 05:30 PM

I have it hooked up right ( I checked again), and yeah, it probably is undercharged, but it won't suck any refrigerant in. I really think the compressor has a bad valve or two, that's the only thing that makes sense.

bazar01 · bazar01's Garage

Check to make sure the R134A schrader conversion valve adapter is properly seated on the R12 valve so that the charging hose quick connect fitting is pushing on the valve stem to open it for charging. Maybe the charging valve is not opening all the way that is why it is not sucking in any refrigerant.

rigitrite · 09-05-2005, 04:13 AM

Yeah, it's opening, that's why I get a pressure reading, and it does take a little refrigerant, just not nearly what it should.

rigitrite · 09-09-2005, 06:08 PM

Ok, I found the culprit: the low side valve was stuck shut. I don't need to buy a new compressor though, since I was able to fix it. I took the charge and works fine now. Thanks for your help guys!