According to ALLDATA DIY for my 94 SL500 there is no relay. The A/C compressor clutch is fed through the engine management controller. As the A/C system is one of the few things I have not had to repair on my SL I can not speak from experience on that particular system but some general information that may be pertinent. In all likelyhood there are several sub systems that can cause no clutch engagement. The clutch itself could be bad, it should be a two wire connector 12v+ and ground. Jumper the system and see if the clutch engages. If it does then the problem culd be a low pressure cut off switch that will not allow clutch engagement if the refrigerant has been lost. Possible causes, low charge, bad switch and of course wiring. High pressure cut off switch will disable clutch if refrigerant pressure gets to high for some reason. Again possible bad switch or wiring. Full throttle cut off, This will disengage the clutch to prevent compressor over revving. Knowing Mercedes love of overkill and redundancy there could also be a vehicle speed sensor in the system. Unfortunately the wiring diagrams I have access to do not show circuitry inside any of the control modules so I can not check where or even if any of these systems exist. The operational description in all data (really some data) is scant at best so all I can offer is general information of systems that do exist in the industry. Good luck on this one.
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