overheating

my 86 is running hot!! the head gasket was replaced under a year ago. i noticed the small fan on the right side of the shroud is not turning on even when the a/c is on, the temp gauge works, i took the fan out and ran a jump to it and it works. is there a sensor gone bad? temp switch? and if so witch one??

i would check the overheating fan relay, i'm sure there's one there somewhere, check fuses etc
i can't remember which exact one is for the fan thats in the thermostat housing, i would just find another two and replace them both
also check the wiring going to them, they may be degraded etc

my 2c :)