So you have 12V at the clutch. You need to check the resistance af the clutch coil with an ohm meter. Check with one lead on each of the terminals. I beleave it should be about 4 ohms. Should be good or it would never engage. Next measure the air gap between the front plate and the clutch coil with some feeler gages, your gap should be about .0025 of an inch. If your gap is too large the coil can't pull the plate in to engage the compressor.

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Kevin
82 300D