aux.fan switch
 

I noticed on my 86 300sl that the thermal switch going to my aux. fan crumbled apart when I tried to tighten it so I thought I'd wire a toggle switch to it . I wired in into the coil and the fan comes on when I activate it but it kills the engine. I must have wired it wrong. Any suggestions? Thanks,Greg.

Why not replace the temp switch? An '86 300E temp switch is $35 (Behr)/$48 (MB) from Pelican Parts, the site sponsor. Or be conservative and get the 006-545-42-24 red top switch with 100/110*C setpoints rather than the 300E's 105/115*C. What color is the plastic on the original switch on your car?

Sixto
87 300D

It's kind of beige in color. I looked on e-bay an at Eckler's's but couldnt find one so I wired it to a switch that I put in my glove box. It looks and works good and when I get a factory replacement one I'll change it over. Thanks for the information.

ballast resistor
 

My mistake,what I thought was a switch of some kind is the ballast resistor.I found it at Pellican for $16. What does it do and how does it work? Thanks. Greg.

The fan runs at two speeds - low when triggered by a temp switch in the AC circuit and high when triggered by the coolant temp switch described earlier; the lower setpoint triggers the fan, the upper setpoint cuts the AC compressor. There are separate relays for low- and high speed. Low speed goes through the resistor you found. You can bridge the resistor for a single speed fan until you can replace the resistor.

Sixto
87 300D