Shorted connector took out MAS relay
 

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

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.