Ok... did some more checking this evening. Crawling under the car, I found the drain hoses - quite deteriorated. I know that one was at least partially obstructed, but doing some gentle probing I cleared any evident blockages. When I idled the car, I started getting condensation drip (quite a bit, actually) at 10 minutes, but the system still stopped functioning at about 20 minutes. I actually watched the compressor stop, and it never came back on. I stopped the engine, and restarted it. The AC compressor did not engage. I stopped the engine, and let it sit for a few minutes, and when I restarted, the AC compressor still did not start.

I did not get any additional time to check by jumpering the relay as suggested, due to a pending thunderstorm (don't have a garage here like I did in Kansas). Nor did I get to do more than more fully examine the belt. The belt has about 3/4" of deflection midway between the coolant pump and belt pulley. I can feel the tensioner resisting my push/pull. I did locate the tension adjustment control, but didn't have a chance to try increasing it.

During the test run, I never saw any indication of the compressor slipping, it simply ran until it was shut off. The engine temp never really got high (I kept it in idle, and had my wife occasionally throttle the engine. During this test run, the ambient air temp was about 90°F with about 45% humidity. Vent temps barely got down into the 50s, but humidity in the cabin got down to about 35%. Once the compressor shut off, vent temps spiked quickly into the 70s and 80s, and humidity in the cabin climbed above 60%.

Unless someone tells me not to, I will jumper the relay in the morning, and re-test. I don't get the feeling it's a loose belt, but I will deferr to the expertise on this forum.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE