Haven't seen this here before so....

Turned on the A/C first time this season the other day. Blew cold for about fve minutes then compressor shut down for good and outlet temp came up to ambient.

I noticed the clutch was not engaging. I immediately thought, "Oh, no! Dreaded leaky evaporator! (You know, the one MB disavows any knowledge of). I ruled out low pressure/low-pressure switch as the problem. Whew!

Since we have two 300Es, I swapped the MAS relays and sure enough, the clutch engaged and all was fine. EXCEPT, I just happened to notice a flashing, like arcing, at the compressor electrical connector. I shut it down before it took out the other MAS relay. I disassembled the connector and the little resistor, etc. I could see nothing that was shorting out, ie no bare wire or anything.

However, the connector area was quite dirty/greasy from the infamous oil leak on the front of the 103 engine (You know, the one MB disavows any knowledge of). I cleaned it all up with carburetor cleaner and all has been fine since. I assume it was actually arcing across the grease/dirt to ground. Be nice if there was a fuse to take this instead of losing the MAS relay.

Anyway, problem solved.

Brew

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2012 E350
2006 Callaway SC560

Sounds more like the circuit was bad and the arc was between pieces that normally would be a tight fit. Because the circuit connects a coil of wire (inductor) openning and closing the circuit will draw voltage spikes maybe over a hundred volts. A fuse might not have been much use if I'm right about the cause. Only current takes out a fuse not voltage.

The MAS replaced the OVP and does some of its jobs. I'm guessing it gave its life swallowing those spikes.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician