tveltman

If you are doing the conversion, typically people swap out expansion valves too. My A/C ended up working, and I stuck with R-12, but because I had to track down a leak, I ended up swapping all the seals, so essentially the car is 134-ready. IDK if the new replacement refrigerant will be backwards compatible with those seals, or even what the refrigerant might be.

However (nerd alert) I will make one comment about r12 vs r134 from an environmental standpoint. The ozone layer shields us from a great deal of UV radiation, which contributes a not-insignificant amount of heat to the atmosphere. In fact, the greenhouse effect works because the earth absorbs higher energy radiation (like UV, vis, or NIR), and then radiates long wave IR. The atmosphere is relatively transparent to the higher energy radiation, but not to the far IR, and so it ends up reflecting much of the radiated heat back to the earth, producing a warming cycle. I won't claim to know how much each gas affects the atmosphere, but poking a hole in the ozone layer and adding more greenhouse emissions effectively do the same thing (though again, I don't know to what degree one is worse than the other). From an environmental standpoint, removing both of them is a good thing. You are right, however, to point out the law of unintended consequences. The gov't thought they were making the problem better, but actually they were just transferring it elsewhere. I recall that the same thing happened when they tried to clean up diesel soot emissions. They got a lot of the soot out of the air, but it turns out that that soot was acting like a filter to remove some of the heat coming from the sun, and so by removing it, they ended up intensifying the greenhouse effect. While many would argue that the soot needed to be gotten rid of, I think most would tell you that they would rather have the soot and reduced climate change compared to what we are dealing with now. That's just my 0.02

underdog928

Thanks for the heads-up on the expansion valve. Makes sense. I'll see what my mechanic says but a new expansion valve is cheap and WYAIT.....

tveltman

be aware that PO may have stripped the head out on the allen bolt, which means the only way to get it out (for me at least) was to drill a hole through the head and stick a bar through it to provide adequate torque to remove the bolt. I replaced with a hex head bolt to avoid this problem in the future. Allen bolts are poor choices of fasteners until they get reasonably large, IMO.

WallyP

"On cost, I checked today at a local independent parts dealer. $8.95 per 12oz can of R12. Need to sign-up online first for a permit ( I think the charge is $25)."


I have VERY serious doubts that anyone will sell you R-12 for $8.95 a can. That stuff is almost certainly one of the "drop in" replacements for R-12, and I won't use it in any car that I service...

roberth58

A few things I have done to make my R-134A conversions easier.

1. Nylog - great stuff, thin coating on all o-rings and threaded fittings.

2. Make sure replacement hoses are barrier hose. If they are not available for your car barrier hose is available in bulk along with numerous fittings, just get them crimped at a local shop.

3. New TXV or orifice tube.

4. New drier.

5. Flush everything you can. I use Isopropyl 91% and lots of air to remove any moisture.

6. BVA Auto 100, very good oil.

7. Good set of gauges.

8. Vacuum system, at least 3 hours longer if you can. Check gauge 30 minutes after you turn off pump. If it moved you have a leak.

9. Add 134 service ports and conversion label.

10. Add a high pressure switch if you dont have one. (An EPA rule)

11. Dont rush and you will end up with a 134 conversion that will make brass monkeys run away


Bob

tveltman

Bob has excellent advice here, however I will add that I think it is not worth it to try to charge R134 yourself. If you can complete the conversion, go to an A/C shop (or the dealer), tell them it's been converted already and that you found and fixed a leak, and that you want the system flushed and recharged. You need some serious equipment in order to do this properly, and unless you plan on servicing other automotive A/C systems, or you are like me and prefer to accumulate tools in the event that they will be useful later, I strongly recommend against a DIY here. The actual conversion is worth doing yourself because the dealer (or anyone) will charge you an arm and a leg, and you won't know what you are getting. Remember that these cars are very rare, so very few Porsche wrenches (and fewer A/C shop ones) know anything about the car. You'll be paying some guy $120 an hour to learn on your car. If you study the posts here, you will find all the information you need to do the conversion, but in the end, let someone else flush and fill the system, especially since they can probably pressure test the system with dry nitrogen or argon which will immediately find leaks that vacuum won't pick up.

dprantl

Flushing A/C system components is very easy. Just get a $30 flush cylinder attached to an air compressor and your favorite flushing agent (I use mineral spirits). You can use a simple rubber adapter to hold on the condenser and evaporator, then flush away and purge with compressed air when done.

Charging is not so difficult either if you understand what you are doing. Just get a detailed description of an A/C system and the charging process and just study it for a week or so. You need a set of gauges ($50) and a good vacuum pump ($200). You can get away with using small cans so you will not need a scale and a 30lb tank. Just remember to always purge the air out of the lines when attaching the new can.

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

tveltman

Dan, where might one find one of these flushing cylinders?

dprantl

Here's one:

http://www.tooldiscounter.com/ItemDi...kup=MAS91046-A

And here's a vacuum pump that's good enough for A/C work:

http://www.tooldiscounter.com/ItemDi...ookup=MAS90066

Do not get a cheaper pump, trust me. Many people/mechanics use a $50 pump that attaches to an air compressor. These pumps are crap and can never pull a good enough vacuum to boil off all the water vapor in the system properly.

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

GlennD

At the risk of getting flamed ...

The AC did not work on the car I purchased. I took it to a AC service person and he confirmed that the system had no loss of integrity (it passed his vacuum test) and hence appeared to still be moisture free.

Most of the R-12 was gone (insufficient amount to trigger the switch on the receiver).

He then suggested that rather doing the full conversion from R-12 to R-134a, that he purge the system, flush a couple of times and then refill with oil. He stated that the o-rings would hold the R-134a. If I leak some refrigerant, the cost of topping up each year would be less than doing the conversion (and the associated headache of replacing all the o-rings, etc.)

I proceeded with his recommendation ... got it filled up - beginning of last driving session in 2008. Today, the AC still works. I may have to top up the R-134a next year ...

Just another approach on the subject.

dprantl

The above is certainly one way to go, but replacing o-rings is relatively easy and is cheap. Why not do it?

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

tveltman

I agree. I already had a really nice vacuum pump (well, a 25 micron pump from Harbor Freight). My A/C is working fine even though I didn't end up flushing it, but if I ever have to do it in the future I'd like to knwo where to get a flushing canister. Thanks!

dr bob

Not a flame, but maybe a heads-up--


Generally, the loss of refrigerant means that the old stuff leaked out.

A 'vacuum test' is a test for a gross leak, like a loose connection or a missing o-ring. It's possible to develop close to 15PSI of diffrerential pressure with the vacuum pump, hardly comparable to typical 200+ PSI system pressures. A small leak will go unnoticed in the 30 minute test using the bottom of the low-pressure scale on typical gauges.

I don't see the connection between 'passing the vacuum test' and 'moisture free'.

Flushing the system always sounds like a good idea, especially since each component is flushed separately when done correctly. Great opportunity to change all the old original cracked black o-rings to new green polyolester oil compatible rings. My guess, based on your description, is that your 'expert' forgot to do it 'correctly' and missed the o-ring replacement step as a result.

The compressor MUST be removed to get the old oil out. The original mineral oil turns to jelly when exposed to R-134a. It's actually possible to put the new oil in by vacuuming it through the low-side port, but you MUST cycle the compressor by hand to clear the cylinders. Might as well just put it in when the compressor is out on the bench for flushing.

New expansion valve(s) and drier are mandatory. Expansion valve(s) have slightly different characteristics for R-134a, and will work a lot better with the new gas. The original drier media is not R-134a compatible, plus it's saturated with that mineral oil that turns to jelly stuff. So it's partially plugged with jelly, and will no longer do any drying. Sounds ideal!

----

For a system that leaks some small amount every year, you are depending on some sort of luck to help you decide how much oil to out back in when you 'top up' from those cute little cans. How much oil was carried out through the leak? It varies depending on where the leak is in the system. Not enough oil means a compressor failure is in your future. Too much means the heat exchangers have oil film on the insides, limiting transfer.


You are living on borrowed time my friend. Enjoy it! The fix after a system failure from jellied lubricant is significant. Compressor grenades, scattering aluminum dust and then shrapnel through the whole system. Cost of commercial repair for that approaches the current market value of some cars, unfortunately.

GlennD

Thanks for the information Dr. Bob - it is much appreciated.

Note that the system evacuation was done in a AC shop and the old oil in the compressor was evacuated (although the compressor was not removed). The new oil that was added was compatible with both R-12 and R-134a (in the past, this was not the case).

The attached link provides some additional information that appears to follow the procedure used by the fellow who did my work. When I get time I will replace the o-rings and expansion valve. My first pass was just to see the shape of the AC system in the car and hence this apparent shortcut.

http://www.aa1car.com/library/retrofit.htm

Seems to be varied opinions on changing from R12 to R134a available on the internet. Above is one that is consistent with the shops I talked with here.

Side note: there was some previous comment on using the cheap refrigerant replacement (hydrocarbon based). I looked into this as well to to satisfy my curiosity. Once one adds this stuff into their system, no AC shop (here) would touch the car for AC work as this hydrocarbon based system would contaminate their equipment. Using the Redtec (or similar) refrigerant is a bit of a one way street.