Ok, I live here in hot, humid Orlando, Florida and my S4 has no ac. I NEED AC. Today I took my shark over to a friend who works on ac systems for all cars and he checked my system out and he said that my ac compressor was going and that I had a minor leak from the evaporator. Now he is no 928 expert, but rather a person who knows ac systems. My question to all is, how does this system work? I understand there is an outside temp sensor imbedded in the alternator cooling hose, an interior sensor on dash which by the way is presently covered by a dash mat. What does this expansion valve do?? I understand that they fail a lot. What is the system pressure suppose to be? He charged my car free of charge and now I have ac. Its cold but I think not as cold as it could be. When I slide the front slide anywhere but 65 degrees, instant heat. A month ago at an Orlando shark owners meeting at Steak and Shake I met a fellow who said he would look at my system and fix it the right way. Well I lost his card. He lives in West Florida or works there. if you know of him please email with directions.
Thanks for your listening ears and let the responses come on in

OK, here is the Reader's Digest Condensed version.

The AC works by boiling refrigerant in the evaporator coil, which is located under the dash. As it boils, the refrigerant absorbs heat, cooling the coil. Air is forced through the evap coil, and the now cool air is blown into the car. The  refrigerant then goes to a compressor, that compresses it up to around 300psi. The now compressed (and hot) gas is sent to the condenser coil, which is mounted in front of your radiator. Air flowing through that coil cools the gas, which under high pressure, liquefies in the coil. This evap/condensing cycle is what makes the system so efficient. The liquid refrigerant now goes to the dryer, where dirt and moisture (water) are removed. After the dryer, the refrigerant goes to a small orifice (hole). This is the expansion valve.
The effect is similar to what happens in a fuel injector. It sprays a fine mist.  Expansion valves don't break, BTW, they clog.  And since this is a closed system, the pressure is really low on the far side of the orifice (about 100psi). This causes the refrigerant (which by this time is back in the evaporator coil where we started) to evaporate very quickly.  So the main components are the compressor, the condenser, the dryer, the expansion valve, and the evaporator. The compressor can fail, which will cause pressure loss and no cooling. The condenser and evaporator can leak. Think of the dryer as a kind of dry sponge. Once it is wet, it swells, and since there is no place for the water to go, it stays wet. If its clogged, its trashed, and must be replaced. Any problem that allows outside air to get into the system for more that a day or so will clog the dryer with water. The system must be evacuated with a vacuum pump to remove the air before it can be charged with refrigerant. The refrigerant in your system is contaminated with air because of the leak, so your cooling is reduced. All we have left now is the control. The compressor is cycled on and off by a pressure switch to maintain a high side pressure of about 300psi. The pressure switch is usually mounted on the dryer. If the switch fails, the compressor's clutch will not engage, and the compressor will not run. A small leak in the system will cause the compressor to run continuously, icing over the evaporator. If you notice less and less airflow, which can be fixed by turning the system off for a few minutes, you need refrigerant. As the leak causes the pressure to go down, eventually condensation will no longer take place and the system will quit. The in dash controls regulate the amount of cold air that is sent to the cabin. I don't think they control compressor cycling. They are in series; either sensor failing will send the system to full hot. You can find info about the sensors (and a lot of other stuff), at Greg Nichols excellent website,
Hope this helps.

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 careful that you have adequate filters on the airline.
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 likely 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 polyester.

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 before installation.  Avoid over tightening 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 under car 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 ( 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--

dr bob