Here's the little wire that burned. It's short and difficult to reach. I had to squeeze my hands in there really tight to get at it, and it hurt. It took hours to get a new connector on it.
Now the pre-resistor is gone and I just bolted the two wires together so the fans run on high speed instead of half speed when the air conditioning is on.
Even after the work I did on bypassing the auxiliary fan pre-resistor, in August the fans were still working intermittently, if at all--which causes the air conditioning to not be cold, and the engine to get too hot.
Apparently the wires in the air conditioning portion of the auxiliary fan harness are too small, and it can be a problem supplying current to the fans and can burn up. So, I bypassed that portion of the wire harness and am using the engine temperature portion of the harness to run the fans.
To do this, I got a different style relay (part number 0015429719) which has a 40 AMP fuse built into it (there is no fuse in the auxiliary fan system). I took an old relay, cut off one of the pins so it can't energize, and then attached a wire between pin 85 of each one by tinning the wires, sliding them down, and then heating them with a soldering iron.
This should be a lot safer than what the factory did. Hopefully this solves the problem.
The old and new relays in place with the jumper wire. I thought this would be better than leaving an empty space with a wire sticking out of it.
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DON'T MESS WITH MY MERCEDES!
1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles