From the time the initial testing was being done on R-134a, before it was used OEM, I had a friend (a former Chrysler parts counterperson) who went to work for Four Seasons. He said their initial testing was not good. The chemical company told them it would take 20% MORE gas in the system, so cooling was not good. It turns out that when they decreased the R-12 gas level to 20% below R-12 levels, cooling returned. So the first recommendations were to start with 20% less gas charge and then add gas until cooling leveled-off at the coolest level, then stop adding gas.
Other side issues surfaced, too, as things progressed. Air flow across the condenser is critical, especially in traffic or at low speeds. We saw this during the GM transition to R-134a in their full-size pickup trucks. Also, GM quietly increased the size and displacement of the R-4 compressor. We sold lots of the cycle switches, which were extremely inexpensive and small.
A related issue was condenser size, too. The condenser on a LH car is usually bigger than the radiator. "Mass flow" condensers were favored over the older "serpentine flow" condenser, too. In a few tests, my associate mentioned that they tried to increase condenser size by stacking them in series on the vehicle, if there was enough room.
In the ackits.com forum that mentioned the EPR adjustment, nothing was mentioned about expansion valves, even on the GM systems where the POA valves or VIR POA valves were adjusted. NO need for the conversion of those systems that ran the compressor constantly to change to cycled-compressor systems. So, as the expansion valve wasn't mentioned in those postings, I didn't worry about it. Be that as it may.
On R-12 systems, the poster mentioned that with the blower motor disconnected and the system operating, the pressure through the evaporator was right at 29.5" Hg. When that pressure was reduced to 26.5" Hg, 40 degree F vent temps happened, and the relationship between "high side" and "low side" came into the same relationship with R-134a as they typically had with R-12 in the system. On the POA valve, which IS adjustable, as with the Chrysler EPR valve, turning the adjustment screw about 1/2 turn CCW would lower the evaporator pressure to the desired level.
My Four Seasons associate noted the need to change the oil in the system, as the old R-12 oil would turn to jelly and settle to the lowest point in the system, usually the condenser, decreasing its capacity and resultantly, cooling performance. The answer was to hook a recycling machine to the system, flush it with pure liquid R-12, then evacuate it. Attach the R-134a machine, evacuate, add the necessary R-134a oil, plus the gas as necessary. When he mentioned this procedure, the old "flush fluid" was becoming very expensive and hard to get, and recycling machines were still in their infancy. As R-12 was still what was seen, that usually was the only recycling machine the shops had, adding the R-134a machines later. At this point in time, finding a shop with a functional R-12 recycling machine might be a trick.
When this deal started, there were THREE different a/c oils, depending upon the nationality of the vehicle and its a/c gas being used. I knew that later on, there would be a universal oil that would work with all gases, which is pretty much where things are now, even synthetic versions, too.
When building new hoses, the old "barb" fittings can't be used with the R-134a-compliant barrier hoses . . . the barbs puncture the barrier, rendering it "failed". The barrier hose is also "stiffer" so you might not be able to conform it to the same places as the prior R-12 hose. Test fit the length prior to finalizing the new hose construction.
The newer R-134a fittings have raised "humps" which seal against the inside of the hose and outside band clamp. That's what works with the barrier hose, so cosmetics are different. Over time, it was determined that if the old hose did not leak with R-12, the refrigerant oil would seal the inner areas of the hose exposed to the gas, so it would probably seal with R-134a. A possibly variable situation? Try it and see. Of course, anything that it's replaced with would need the new barbless fittings, appropriate o-ring seals, etc.
As far as pressure switches, by this time, there should be pressure switches to replace the earlier ones for R-12. Or switches that are adjustable externally, as the cycling switch for my '77 Camaro is. Getting into an a/c parts catalog, with illustrations in the back of the paper catalog, might be what's needed. It will take finding an auto supply that tends to specialize in a/c parts to find these catalogs, although the major suppliers might have them online, too. Check the thread size, the pressure range, and the pigtail plug-in connection to see what might work best for the particular application.
So, some things have changed since the first R-134a systems were prototyped by the OEMs. As I stated previously, send me an PM with your email and I'll sent the file I downloaded with the adjustment posts on R-134a use in R-12 vehicles.
CBODY67
