I was mistaken about the clutch not slowly moving when the a/c selector was off. It doesn't matter whether the selector is on or off, the pulley slowly inches along with the motor running.

I took a DC voltmeter to the three terminal connector on the compressor and couldn't get voltage out of any combination of terminals with the selector in on or off. There is a strange hardened grease-like substance that appears to be coming out of the seal in the front of the clutch. I don't know if that's normal. Here's a video of what it's doing. I guess the next logical question would be to find out why there is no voltage getting to the clutch. Any ideas?

VID-20120511-00095.3GP - YouTube

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles