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Update on the 2004 Safari Trek:
I spent a couple of days doing careful tests of AC exit air temp vs. engine and condenser temps. Previously I had measured the condenser fin temp at about 150 when the AC exit air was essentially at ambient (hot!). I did the new test starting each one with a cold engine. This time I found a few places I could measure condenser and trans cooler temps from top to bottom (the refrigerant runs from top to bottom in the condenser).
What I discovered was that the AC quit functioning effectively well before the condenser reached a high temperature, and further that the bottom of the condenser never got warm enough to explain the AC not working. The system worked noticeably better on a cold day (ambient in the mid 60s) but still went from a delta in-out of 25 degrees shortly after startup to 10 degrees after about 15 minutes of idling.
So I found an AC mechanic that was willing to dig into it a bit. On cold startup the pressures quickly ran to 400/100 and the compressor then shut down. After the pressures dropped it would come on momentarily, pressures would go back up within seconds, and it would again shut down. As the engine and condenser heated up, the compressor spent most of the time shut off.
We then checked the refrigerant for contamination (100% 134a, no air or other) and pumped it all from the system, all 4.26 lbs of it. The sticker on the evaporator says 32 oz., but it is from SCS and I was suspicious of that figure. I called Monoco, they suggested 2.5 - 3.0 lbs. So we put 2.5 lbs in and tried again. Now the pressures ran 225/30 and got only up to 275/40 at fast idle with the engine temp stabilized. The intake/ext temps were now 80/46, better than ever before. I have driven it around a bit and the exit air stays below 48 degrees.
It is baffling that when Monoco in Oregon took it to their Workhorse repair facility with the specific complaint that the AC did not work and there might be a fault in the overpressure circuit, it was returned from Workhorse and Monoco with a statement that it was functioning normally. I suggest anyone with a problem with the AC find an expert willing to spend some time diagnosing the system, rather than just pumping more refrigerant in (or whatever they did at Workhorse).
I hope it continues to work as well as this. Saves me the work of reworking the whole cooling system. I'll see in a couple of weeks when we drive back to Oregon through some very hot country.
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