There are a lot of reasons that a compressor won't operate. One is that it is not getting power because the ACC is not sending it. The compressor should come on in DEF, though. It is pretty much hard-wired in that setting but ithe current still needs to get through a couple of relays and the pressure switch. If the system is low on or out of R12 or R134, you will need to jump the pressure switch to briefly test the compressor.

Removing the compressor is not a big deal but because of its position, it's nasty. Disconnect the battery, capture the gas from the system, undo the belt, unhook the wire(s), disconnect the manifold pipe, undo the bolts that hold the compressor in, and remove the compressor. Install in reverse order with new seals.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe