The 1987 300D has a circuit which compares the RPM of the compressor to the engine. If the two differ enough the compressor is disabled until the car is restarted.

There are about a half dozen possible causes for this problem. The two simplest and most common are these - 1) Grease/oil on the compressor clutch. Hey, it's an old car and these diesels get grimy under the hood. Get a spray can of degreaser or brake cleaner and spray liberally into the compressor clutch gap. Rinse with a hose, if necessary. Sometimes this is enough the cure the problem. 2) Excessive clutch gap due to normal wear and tear. Measure the clutch gap with a set of feeler guages. Should be 0.50mm. If substantially greater than that, you've found your problem. My personal observation is that clutch engagement becomes flaky when the gap reaches 0.80-0.90mm. Gap can be corrected by removing the clutch plate (special puller required, available at Autozone) and removing or swapping out the shims behind it. Not too difficult a job.

- JimY