R-134a Conversion Tips
Dr. Scott wrote to the list:
>Not being well educated (or mannered), especially in the HVAC department.
>I'd like to ask a few AC questions of the more highly trained.
>1. Is it possible to test for leaks and proper function of the Evaporator,
>the condensor and the compressor while they are OUT of the car??
The evaporator normally runs at a highest pressure of --maybe-- 125psi. You may be able to fashion some connectors and a way to pressure test it. Otherwise, it's a heat exchanger. If it's clean on the outside and doesn't leak, it should be fine. If the car has a grenaded compressor in its history, it -might- be worth your while to flush the thing out and dry it completely. Naturally, the preferred flush is one of those evil carbon-tet' formulas, or try one of the commercial flush chemicals you can find today. the unit must be clean dry, oil-and debris-free when you are done, so if you are using home compressed air be carefull that you have adequate filters on the air ine.
The condenser is the same, except that it can see pressures up to about 400psi in extreme conditions. It's also subject to rock damage and is therefore more liklely to leak than the evaporator which tends to be eaten by acids which corrode the aluminum from the inside. If the compressor has grenaded in the past, the flush is a must to get the aluminum dust out of the condenser.
The compressor cannot be checked realistically while out of the car, except --maybe-- for leaks at low pressure.
>2. How easy is it to eliminate the rear AC of an 87 S4 w/o causing
>perpetual leaks? (this question is ex-post facto by a PO).
The lines for the rear air connect at the bottom rear right of the engine bay. You might be able to cut the old tubes cut and have caps made to fit the tubing there, but the risk of getting metal cuttings in the lines is pretty significant. Consider re-sealing the connections and making the rear air functional if possible.
>3. Does anyone know of a site with detailed writeup on converting R12 to
>R134 (materials and cost)?
I did the conversion myself to my '89, which had a functioning but leaky (recharge twice a year according to PO records) R-12 system.
I bought a kit of o-rings, R-134a expansion valves, and new drier from DR, at a total cost (from memory...) of less than $150, and it may have been closer to $100. I bought a cylinder of R-134a and a gauge set from the local Pep Boys store. The charge port adapters came from Pep Boys also, along with a couple small bottles of synthetic compressor oil type polyolester.
The old gas was recovered by a friend at a local AC shop for free.
The system was disassembled to replace the old o-rings with the new green silicone rings. The o-rings are lubricated with compressor oil befor installation. Avoid overtightening the o-ringed fittings too; the rings seat fine with the light torque specified, and too much just flattens the o-ring and spoils the seal.
The expansion valves were replaced. (front and rear)
The compressor was removed, drained of the old mineral oil, and the o-rings at the little hose manifolds were replaced. This requires removing the little socket-head bolts that hold them, and also required o-rings from my local AC shop that weren't in DR's kit. I flushed by filling it with the new oil (8 oz from memory, see the manual for the total amount needed in the system), turning it over and shaking it, then draining again. New oil was added, and the unit was reinstalled with the hoses attached.
The compressor hoses were rebuilt using barrier hose material by my local shop. They use swedge sleeves at the connections, and the hoses look just like originals except they don't seep R-134a as the old ones might. There are varying opinions on whether the hoses should be rebuilt/replaced, or whether the originals are oil-soaked enough to keep the new gas in. I went the safe route.
The whole system was buttoned up, and the new drier was installed as the last step of that procedure. The new drier required a shim in the bracket, and also required adding a little wire loom wrap on the tubing below, as it was a hair smaller diameter and a fraction of an inch taller than the original.
I attached the new charge fittings, and used the vacuum pump to remove the air and moisture from the system. I left the vacuum pump on overnight, then charged the system the next day with R-134a. I didn't know at the time how much R-134a (by weight) I needed, so I watched the sight glass and charged it slowly until it had just a few bubbles in the glass. This turned out to be a hair over 1000g, vs the 1043g that is the R-12 charge recommendation. It works better undercharged like this, by the way.
The system was leak-checked using an electronic sniffer, the leaks were noted, and the system was cleared and re-sealed where necessary. I re-evacuated and charged again, sniffed for leaks, and declared success.
For those contemplating this at home, consider that I spent the best part of a day dinking with the conversion. My tool collection includes the correct equipment, minus the recovery equipment. In addition to the purchases listed above, I already owned the fitting wrenches, thermometers, and vacuum pump. It's quite practical to do the re-sealing yourself, then take the car to a shop that has the vacuum pump and charging hoses to evacuate and refill the system.
My conversion did not include any repairs, since everything was intact and working OK when I started. It also requires a total devotion to cleanliness in all your work areas, including the area under the hood, the rear evaporator and expansion valve, and the undercar plumbing connections.
Performance of the system is exactly as shown in the repair manual in the GTS section with R-134a. That means it's slightly colder than the R-12 according to the charts supplied. It is capable of freezing your fingers on 85f days, with measured center vent temps of 21f with the anti-freeze switch not functioning. Just what's needed in a black 928 in soCal!
The Technical Reference Manual shows the layout and locations of all the fittings and connections, and should be a good guide to getting them all resealed. The service manual does not give much detail, and while certainly helpful it is not a complete guide to repair or service.
Charlie Griffiths (http://www.griffiths.com/porsac928.htm) sells conversion packages that include barrier hoses, o-rings, drier, etc., with your choice of OEM Nippon-Denso compressors or the more efficient Sanden rotary units available. Several listers have used his kits with the Sanden compressor and report excellent results. Charlie has also contributed may times to AC discussions, and certainly is a valuable resource with a lot more experience than I could ever claim.
I spent in the $450 neighborhood for all the new stuff I needed, including the gauge set (about $125) and the gas cylinder (about $100 at the time) that will be used on other cars.
Hope this helps! Stay cool--
Since the top nut on the dryer on my 79' backed itself off and leaked all the r12 out I went to get it charged yesterday. I asked my AC man here in Lauderdale the process for converting to 134. He said that originally when 134 came out the oil they used was really bad. It coagulated and ate the o-rings if you didn't use the green ones etc. He said that now the Mark VI version of 134 oil is very compatable with the R12 oil. He infact said that since I lost the R 12 charge and most of the oil, that he would simply evacuate, draw in afew ounce of the 134 oil and fill it with 134. He's a sharp guy and the best AC guy I've ever come across. I said go for it. He did it. The pressures numbers look great , it cools great. $50 bucks for 134 conversion. Now we'll see if it lasts or eats a seal or damages the compressor. I think he's right.
>Tomorrow I will be attempting to convert the Euro to R-134 but I have one
>question that I haven't seen addressed. To activate the compressor when the
>system is empty, do you bypass the low pressure safety switch? If yes, where
>is it? TIA
You don't need to activate the compressor while the system is empty. With the engine off, you can dump the first can of refrigerant as a liquid (valve down) into both the low side and the high side. Since the system is under vacuum, virtually all of the refrigerant will enter the system. When the can feels empty, CLOSE THE HIGH SIDE VALVE on the gauge set, crank the engine, and put the A/C on full cold and full fan speed. This will pull the low side down and reduce the pressure in the gauge set and the can. The compressor may short-cycle, but that is no problem.
At this point you can switch cans and start putting in the rest of the refrigerant. If you are very careful, you can put the next can in as liquid also - but go slowly, so as to avoid slugging the compressor.
Remember that you only use about 85% as much R-134a as you do R-12.
BTW, the freon pressure switch is the one on the side of the receiver/dryer - kneel down and look in thru the grille. The switch on the stem is the freon temp switch to trigger the fan.
928 Tips Home Greg's Home