Im still trying to get some info on my 450sl 77 aux fan. Can someone supply simple way to bypass switch and install on-off to aux fan? Would be appreciated. Regards, Abe G

Aux fan switch is on the reciever/drier of a/c system.. the one with the pigtail leads ..

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1D00YU6RY1JK12H7B3&year=1977&make=MB&model=450-SL-002&category=R&part=A%2FC+Temp.+Switch&appChassis=_any

Just run a couple of wires from these to a cabin toggle sw..

Jumpering these leads will also verify the fan circuit.

Dear Arthur: My 450 sl 1977 has a switch on thermostat housing with single connection. Picture shows one that goes on receiver-drier. Does this model have 2 temp switches, one on thermostat and one on rec-drier? Confused, but thanks for your reply, Abe G

If you want the fan to come on with a manual cabin sw. as you stated , you do as my last posting.
Don't confuse other sensors with the aux fan circuit.
The single wire sensor you are now asking is simply the temp coolant sensor for the temp gauge on the dash....nothing to do with where we are at on your request..

The load resistor for the fan is a handy spot. If you put power to one side you're at half speed, power to the other side and you're at full speed.

Jim
That is another good spot. However , if one does that, you would want to go with heavy wire and a 30Amp switch, as the fan circuit draw is quite high.
You are no longer using the relay with that circuit.
With the drier sw. jumper , you are using the primary circuit of the relay, so there is very little current [ the relay secondary does the load ] allowing light wire and switch, with the added benefit of still having a fused circuit cuz yu are still using the existing relay/load circuit..
Either way...