My auxcillary fan does not operate on my 79 450SLC. The fan does run if I hot wire it to the battery, so it is not the fan itself. It does not run in conjunction with the AC or by any other method that I can discover.
HOWEVER, on my previous engine and my current engine in this car there is an "Auxcillary Fan Switch" located on the water pump but nothing is connected to it. It looks like an oversize stereo plug.
I have tested it with an ohmmeter and it registers no resistance when cold and high resistance when hot. This would indicate to me that it functions with some sort of biased relay that turns on the fan when the bias is reduced and the fan runs until the temp reaches some value preset by the relay/switch combination.
Does anyone know anything about this "phantom" circuit. There are no loose wires or plugs around that would appear to connect to this "switch".