This one really has me stumped. 1987 300D. I basically replaced the entire system 2 years ago and it has worked perfectly until now. New compressor,evaporator,condenser,drier,TXV,all servos in the dash. System still cools perfectly as long as the the compressor stays engaged,but that's my problem. Randomly the compressor kicks off and I discovered that if I turn the key off and then back on it works again. It may then work for hours or sometimes 5 minutes, but if I power the car down and then back on, it works. I have recently replaced the evaporator temp sensor, the green coolant sensor, both sensors in the drier and condition remained the same. I swapped a known good pushbutton unit and N1 relay from my 1990 300D and that did not help either. I've run out of components to replace. I took the electrical schematic of the AC system and listed all the sensors,switches,relays etc. that control the compressor and I am stumped. What is it about powering down the car that would reset everything ?

if its like the 95 w124 then it does shut down the compressor if slip is detected by the compressor rpm sensor and is only reset by restarting, this can be caused by a bad belt or a bad compressor clutch hub.

i read on the benzworld forum of a 300e facing such problems that was caused by a compressor clutch air gap that was too large

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

My guess? A bad Klima relay. My 92 300D was exhibiting almost exactly the same symptoms. Replaced the Klima now all is well. If your layout is like mine, it sits behind the battery.

Really, the same issue as what Zulfiqar said....but the fault is in the relay. If you could find a known good one to swap out, it would be easy to check.

Not cheap....~$120 new. One on Ebay right now for $55.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

What you describe is likely caused by a bad compressor clutch, slipping belt, or worn compressor.

These systems have a rotational sensor on the back of the compressor. Whenever it does not send a rotational pulse to the controls, the clutch is shut down and will not reengage until the ignition switch is cycled and the engine restarted.

The purpose is to prevent a catastrophically failing compressor from scattering debris throughout the system that will destroy a new compressor.

Of the three possible causes, the slipping belt and slipping clutch are easy to understand. The worn compressor might not be so intuitive to figure out. What happens sometimes with an aged compressor is that it wears such that there is fore/aft movement of the inner workings which moves them far enough away from the sensor that it no longer picks up a signal. IF THIS is your problem, the cure is compressor replacement.

From what I've seen, when a compressor wears to this point, the seal is probably leaking and the clutch has seen its better days anyway. Replacing the compressor at this stage BEFORE catastrophic failure makes it a MUCH simpler repair, since total system dismantling and flushing is not necessary.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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