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!