need help to fix auxilary fan (front of condenser) 450sl 1977
 

It looks like my aux fan does not come on when engine temp goes to red line after a/c on in short runs. How can I check the fan terminals after seperating the connector plug at the radiator? Tried hook up to plus on battery connection and negative to ground, fan did not turn over is this a good test? Where is 212 degree swtich on radiator? Engine runs at normal temp when a/c off. Cools fine. fuses ok. regular fan clutch OK. Any help appreciated. Regards to all, Abe G PS: PFred can you help?