The fan being switched by temp, not pressure is the case of the setup in my 1981 300SD w126. The pressure switch only kept my AC system compressor from working (back in the days when it leaked). The Temp switch kicks the fan on when it gets terribly hot however as far as my electrical analysis goes, unless the AC system is getting hot, the fan won't turn on. The 603 of course is different but you have to consider you have a bigger, more sensitive engine to cool off. The 617 should have no problem with a good clean system keeping the engine cool under most reasonable circumstances. Also, anyone who realizes how "barely"powered the stock alternator is in these cars (atleast the w126 body) probably realizes how bad of an idea it could be to have the fan running often. The germans probably knew how to properly build their cars so I imagine if you are needing a booster fan with regular driving you probably have other problems (radiator, t-stat, nesessary cleaning)

My engine hasn't broke 100 C yet and the booster fan has only come on, to my recolection, when the AC was on for a long time and I was sitting in the driveway for some time while it was high 90's-low 100's outside.

If you are not daily stuck in hot standstill traffic WIHOUT your AC on, I wouldn't go attacking the wiring before doing some good preventative maintenence to the cooling system. It takes more power (and causes more wear technically) to run the AUX fan when you are using the same amount of engine power, mechanically, to already run the mechanical clutched fan. I'd rather get everything tip top before running that motor an overly large amount.

Just my thoughts


EDIT: Just to make it clear, the 1981 300SD has the 617 turbo in it.