1984 300 cd cooling problem

hello, i own a 1984 300 cd and the temp is going way past 100c. when i turn the a/c on the aux fan turns on the way it should. when the a/c is off the fan never comes on. i have checked the fuses, and also grounded the single wire that goes to the sender above the thermostat housing. this single black wire has juice when tested with a test light. is this normal? any ideas? thanks for any help!

[psfred]

Bad thermostat and/or bad fan relay. The wire is always hot, the switch grounds the coil connection to switch the relay and turn on the fan.

Fuse is probably not in the fusebox, instead in a small brown plastic box in the engine compartment. I'm not familiar enough with the W123 to tell you where the relay is, but I'm pretty sure it's bad.

If you get that fixed and the aux fan runs with the wire grounded, but never when the engine is above 100C, probably a bad thermostat not opening (althoug this will usually cause it to boil coolant out).

Other things to check:

Visco clutch must engage.

RAdiator must be fairly clean, you must have sufficient coolant.

pressure cap must hold pressure (I'd replace it anyway, they are cheap).

No leaks.

Water pump must be good.

Peter