VehiGAZ

I recently had my A/C compressor replaced with a rebuilt unit due to a very slow leak and a desire to revert back to R12 from 134a. I also had the rear A/C line solenoid and rear control relay replaced, and the rear A/C was working again. Yay!

Before I go on, I'll add that the A/C system - when topped off with 134a for the summer - worked well and blew nice, cold air when running on 134a (chalk one up in the "if it ain't broke..." column).

Anyway, the work was done just fine, the system was evacuated and charged with R12 (no oil added since the compressor came pre-oiled), but the cooling wasn't very impressive. Moderately cool, I'll call it. I would estimate that it was blowing in the 50*F range. So I called my mech and he took another look at it today.

He agreed that it was not blowing very cold, so he evacuated the system again (a little oil came out with the R12, as expected), recharged it with R12 by the weight spec, and the pressures were spot-on: 20-30 psi low side and 240-250 psi high side at 1200-1500 rpm and around 87*F ambient temperature. The hot water valve does not appear to be open or leaking (he crimped the line without noticeable results). He got about 58*F from the center vents and oddly about 56*F (slightly colder) at the rear A/C vent.

I recently vacuumed out the evaporator fins from the plenum side, and I have very good air flow, and it was working well a couple of weeks ago with 134a in it, so I don't think that's the problem.

He is stumped and offered to put 134a back into it, but I don't want him to do that since I might have the same problem afterward, with even poorer cooling.

All I can think of is that he might be working with a crappy tank of R12. From what I understand, most if not all R12 on the market these days is recovered/recycled, so maybe it's a not-so-well-reprocessed batch.

Anyone have any other ideas? Thanks!!

Ed Scherer

Two oil-related things to consider:

• When the system was R-134a, what oil was being used? If it was PAG, you might be in trouble now with R-12 unless the oil was flushed out (and it doesn't sound like it was). What kind of oil was in the rebuilt compressor that was installed? If everything was and is ester (a.k.a. Polyolester, a.k.a. POE), you're probably OK.
• It's possible you have too much oil in your system, regardless of what kind it is. Too much oil starts to kill performance.

I assume that the system was properly evacuated and purged to assure that all (well, at least down to 500 microns) air and moisture was eliminated from the system.

BTW, I believe new R-12 is still manufactured (legally) in Mexico and (legally) imported into the U.S. where you can (legally) buy it. I've gotten it at NAPA, off eBay, from our favorite 928 vendors, etc. You'll have to have your EPA section 609 certification card handy, though.


FWIW, I just worked on my R-12 A/C (to fix a "too much oil" problem, actually, and also switched from ester to mineral oil) a few weeks ago and upon completion was seeing about 45 °F at the center vent on a 100 °F day (this was while driving).

dprantl

Did you flush all the old oil out of the existing components when the compressor was replaced?

Dan
'91 928GT S/C 475hp/460lb.ft

blown 87

Seen it time and time again, system not flushed with the right flush or not flushed at all.
Who ever did the 134a conversion just dumped 8 or 9 OZ of oil in it, now you have put a new compressor on it that was loaded with oil, no mystery here.

Take it all apart, front and rear, get some wurth brake clean and flush the hell out of everything, every line and component, take the compressor off and drain the oil, all of it.
This is about a solid day and half worth of work, longer if you have never done it before.

We just got done with one today with front and rear AC, and now my mind is made up, I am installing a rear AC in mine, it is cold, on a hot Georgia day.
Use mineral oil, not ester and put R12 back in it.

My take on it.

borland

Most likely too much oil. Excess oil reduces the efficiency of the cooling because it displaces the refrigerant which would transfer heat to the condenser.

There's no way of telling how much oil is left in the system after leak repairs, so a complete flush of the lines, replacement of the filter/drier, and an inverted drain of the compressor would be in order. That way, when you add mineral oil for R12, you know you have the right amount of oil for the sizing of the system components.

Have you used the temperature/pressure charts in the WSM for comparision? Those should be used for R12.

Ed Scherer

(just wanted to add my own editorial comments, having done it twice on my car over the last couple of years)

BTW, interesting to note that it appears we're all guessing the same thing on this one. Can't we find some controversy here somehow?

borland

Well, I'm not sure how much a vacuum you need to safely remove brake cleaner? Could require less than 500 microns. I'd say only use approved AC flushing solutions.

Ed Scherer

I've been using Four Seasons Dura II; you can get it at NAPA. Expensive as hell (like $30 a quart), but it's good stuff. Super volatile; you can just watch it evaporate.

borland

Brake cleaner may contain additives that could leave a reside. It could also be that brake cleaner is not compatible on any component seals, like 0-rings. So a flushing solution like Four Seasons Dura II would likely be formulated for R12 o-rings.

blown 87

You can not get what I said to use in CA, so you may have to use the four Seasons product or something similar, which I really do not think does a very good job.

I have used just about every commercial flush out there, never had a problem with the Wurth brake clean as a flush, but I have had issues with some commercial flushes.

As far as the O-rings, I always remove them before a flush and they get replaced.

Never flush a expansion valve, compressor, or a drier, although on mine i do flush the compressor with a spray can of the Wurth brake clean while holding it to drain and turning it.

You guys use what you are comfortable with, as I will.

PS, this is nothing like the brake cleaner you get at a auto parts house, I would not use that either.

WallyP

R-12 at 30 psig is 32 deg F. Your temps make me think that your tech got screwed, and that isn't R-12 in your system.

And yes, oil reduces the efficiency - but from 32 deg to 56 deg is a big drop...

Ed Scherer

Hey, Greg, that's cool.

I'm just a DIYer, and you've probably done hundreds more of these jobs than I have. When I was working on mine, I just tried to research the flush issue as best I could, and there's a ton of conflicting information out there. "Use this!", "No, don't use this, use that!" etc. etc. So I just went with what appeared to be designed as a flush and tried not to lose too much sleep over it. I did make an extra effort to go through two or three vacuum/purge cycles last time I worked on mine, hoping that I got every last bit of flush out.

I'm perfectly comfortable deferring to those who have BTDT a zillion times.

VehiGAZ

Thanks for the input, guys! I'll let him know that the forum consensus is either excess oil or bad R12. If I'm not the first charge off his R12, we should be able to eliminate that possibility quickly.

Tampa 928s

I am sure you replaced your accumulator it and the compressor holds the most oil. The condenser and evaporator will have oil trapped in the bottom and needs to be flushed throughly. R-134 and R-12 Oils do not mix, by going back to R-12 you may have started a cascading issue with corrosion and lumping of the oil remaining in your system. If it was me I would remove the condenser to flush it out completely replace the accumulator again flush all lines and evaporator. I would go so far and remove the compressor and drain if the oil is in doubt.

VehiGAZ

That's the plan right now - replace the drier (accumulator) altogether, drain the compressor, flush the lines and components, put fresh oil in, and try again.

By the way, it's not R12, it's FREEZE-12 that he's using. The tank is fairly new so he is going to check if he's charged any other systems with it.

Does the fact that he's using FREEZE-12 change anything?