I think it is the law of entropy in science that says matter in a high energy state will seek a low energy state. It has been a long time since I was in a science class, but I believe this law because I can see it working in my 73 450 SL.

Like many owners, my cooling system does not perform well with the added load of the AC that MB bootstrapped to satisfy the American market. My AC is marginal but at least it decreases my mileage and makes my engine run hot. After shutdown, my aux fan did not run like modern cars do. If I looked at the temperature gauge with the ignition at position I, it approached 240F (red zone). This led me to the wiring diagram for my AC system and the rest of the electrical system.

MB designed the aux fan to run under two conditions:
1. >100C coolant temp at the aux fan switch on the water neck
2. >67C refrigerant temp at the reciever drier intake.
Both conditions require ignition accessory power at terminal 15 (ingition switch postion I or II).

I did not enjoy sitting in my hot car controlling the aux fan with the ignition switch so I wanted to add a third condition:
3. >100C coolant temp at the aux fan switch with igintion power off

This is done with a decision relay and a switching relay (pictured outer right and left in thumbnails of original post). The decision relay issues the ground signal from the 100C switch to one of two places depending on the state of terminal 15 power (fuse 9 in my car). If terminal 15 is powered, the ground signal is sent to terminal 85 of relay 18 as MB designed it. If terminal 15 is unpowered, the ground signal is sent to terminal 85 of the switching relay which is powered by terminal 30 (fuse 1 in my car).

I am happy with the results and safety of this modification. Temps are controlled near 200F after shutdown and the relays are MB spare parts mounted in an MB relay rack. All switching is done on the low signal side (ground switching). Assembly required one solder reassignment at terminal 85 of relay 18, the rest was pin movement and additional wiring using MB wire, pins and color conventions.

I created color wiring diagrams of this modification and the entire electrical system of my car using MS Visio (reader is a free download). I am happy to share these rather large files via PM upon request.

Cush