Last night I was driving with the a/c off and noticed a whirring sound from under the hood. I turned the a/c on and the noise stopped. The a/c worked fine for the next hour.

Today, the a/c is not working at all. I noticed that when I turn it on, there is a slight slipping noise (probably the belt) for less than a second. Then, when I look at the front of the compressor clutch, I can see it moving at about 1/2" every few seconds.

Any way to tell if it's a bad compressor or just a bad clutch?

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

You have just determined what the problem is by noticing the clutch slippage. When it is not energized, it should never move. This is obviously not the fault of the compressor but of the clutch.

Just to be sure, I would put a volt meter on the clutch lead to make sure there isn't power going to the clutch when this occurs.

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Don't Chrome them; polish them

Thanks for the quick reply. I should have been clearer in my prior post. I only see the front of the clutch turning slightly when I have the a/c selected to on, inside the car. The front of the clutch pulley doesn't turn at all in the "0" position of the a/c selector panel.

I am getting a DTC 6 on socket 8, which is an a/c belt slipping, but when the underhood temps are really hot here in Arizona, I sometimes get a lockout on startup anyway and have to restart the engine to clear it (at least for the past few years). So this code has been a regular event for me in the summer. Maybe this is a sign of a wearing clutch.

Is the clutch something that is either an on/off device or does it weaken with age? I will pull the connector tomorrow and check the voltage.

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

The clutch is essentially an on/off device. It should not slip. Make sure the surfaces aren't greasy. Clean it with brake cleaner. And check the clutch gap to spec. The surfaces can take a real beating from wear and (around here) corrosion. If all that fails, you could just replace the clutch with new parts without having to vent refrigerant.

Thanks. I think I'm going to hit it with a bottle of brake parts cleaner to see if it's slipping because of my leaking p/s system above it. At this point I don't have much to lose.

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

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

From the video, your problem is opposite from what I first thought. The clutch is being dragged when it should be still. Again, it may be dirty and needs cleaning.

I would approach the problem by removing the clutch to inspect the components. Once apart, the problem may present itself.

I have been told that it is difficult to get to the snap ring on the clutch pulley for this car, so I will probably take the whole compressor off and deal with it off the car. Should I be getting voltage to the three-terminal harness going to it?

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

I think I am just going to replace the compressor and clutch. After researching past posts, I think mbdoc probably summed it up best:

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

mbdoc's advice is well repected on this board. But I need clarification with the connection between compressor function and how it affects the clutch. Both share the same shaft - nothing else. How can the insides of the compressor affect the clutch? Is is because compressor seizure puts too much strain and wear from slippage on the clutch?

The OP's situation is clutch slippage when a/c is not called for. How does the condition of the compressor factor into this?

That was my thought--that the compressor was seizing up and putting too much strain on the clutch.

The other reason I would tend to want to change the compressor too is that it seems like there is way too much chance of screwing up the clutch adjustment (for me) and a new compressor already comes with a clutch installed.

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