Another round of testing, and still no success.

Ambient Air Temp: 85°
Ambient Humidity: 45%

Engine starts, AC runs. Five minutes after startup, engine temp is running slightly above 80°C, engine oil pressue is not quite pegged, idle RPM is about 900. I can see the compressor is engaged, vent temps are down to 63°F (from an original 122°F sitting in the sun all day - original internal humidity was 29%), and internal humidity is at about 30%. The beginnings of a condensation puddle is forming on the passenger side of the transmission. All is well.

I run the car in a high idle (RPM 2500).

Ten minutes after startup, engine temp is just below the unlabled tic-mark between 80°C and 120°C (I take this to be about 100°C). When I return the engine to idle, RPMs drop to below 1000 and the oil pressue drops to below pegged. AC Compressor is still engaged, and there is a fair bit of condensation forming along the low-pressue lines - as well as in two puddles beneath the car. Vent temps are down to 58°F, and humidity is staying around 30%.

Fifteen minutes after startup, once again following a high-idle run, engine temp is not significantly changed, engine idle is down to about 800, and oil pressue is down to about the middle of the guage (between the 1 and the 2). Vent temps are down to 54°F and humidity is down to 27%.

Eighteen minutes after startup (right on time) the compressor shuts off. Engine temp is unchanged, idle oil pressue is slightly higher (just under the 2), and RPM has returned to 900. Vent temps spike quickly into the 70s, and humidity is above 48%.

An immediate engine shut-off and restart does not re-engage the compressor.

I shut the engine off, and remove the Klima Relay, and jumper terminals 15 and 87, and reinstall it.

Start the engine, and the compressor does not re-engage. Vent temps are running above 80°F and humidity is running above 60%. Engine was off for about 5 minutes during this jumpering.

Engine temp has fallen to about 90°C, and RPMs are running about 900 at idle, with oil pressue at the 2 mark.

I have removed the jumper, and will re-try the AC system in a few minutes - giving it a 30 minute break for a re-start.

I can clearly see in photos I took during this procedure (I will post in a bit) the compressor clutch engaged and dis-engaged. This indicates to me (I may be in error) that it is not a slipping clutch. I never saw the RPMs fall below 600, though they did get down to about 800 at one point. If it was just the RPM cutoff, the jumpering should have allowed the compressor to operate regardless. It was not even attempting to run.

Any other suggestions?

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE