You will not receive 12V at the plug without the speed sensor in the loop AND the vehicle running.

Look at the ETM for a schematic of that circuit on the 1990+ cars with the MAS relay or search 'KLIMA relay' on the earlier cars. Same deal.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

I assumed as much which is why I tested with jumped leads from the plug and socket for the two sensor wires and then added 12v direct from the battery- no response. This lead me to believe that there is something wrong with the plug.

It's a bit cramped in there to unbolt the plug from the holder while the compressor is still in place, but when I managed to do it, I noticed a problem... the wire from the magnet to the plug/pin that comes from beneath the plug bracket had worn out insulation over a 1" area and the exposed wires started to fray! It must have been grounding out as it would touch the case from time to time. This must have been the reason why my #7 fuse would blow at times.

After adding some solder to the exposed wires and covering the bare area with electrical tape, and before the bolted everything back up again, I re-tested with 12v direct from the battery... the clutch clicked and moved!

It's a PITA to re-install the ground wire beneath the plug bracket. The other side of the bracket, a wire with the resistor that is bolted to it. It is impossible to re-attach so I tastefully attached an extended ground wire that I attached to it.

Fired it up again... no clutch action....what?

So know I knew that the problem had to be on the MAS relay end.
After testing with 2 more MAS units (I've accumulated 5 spares overall), I found a MAS that worked!

Lessons learned:
1) if you don't get the clutch to move after applying a direct 12v line from the battery, check the plug itself (assuming that your clutch is still freely moving when turned by hand)

2) Check the continuity of the power line from the plug end to the MAS end, if it's good than it's likely a MAS problem

3) you can test across the tabs from the low pressure sensor to make sure that there is continuity or jump the leads themselves.

Thanks all.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio