A stand alone switch is for the fan. It closes at 235f. This switch, along with one built into the A/C controls, completes a relay coil circuit to ground. The relay contacts actually supply the large current needed by the fan. This relay is located in the LH front corner of the front compartment. This fan switch is notorious for not operating. If you find your car (this goes for other GM's as well) overheating when not in motion, but cools down at speed, its a good chance this switch is bad.
coolant fan switch dk grn/wht
84-86 two speed 3050223 lt grn/blk L4-LH front of head
84-86 non A/C 3040674
85-88 V6 all 3040674 dk grn/wht V6-RH top of engine
near thermostat
From: Tin Man
The quickest and easiest way to check the coolant fan circuits is to turn on the A/C. If the fan comes on (the ignition must be ON), then everything is OK and the only thing not proved is our buddy the coolant fan switch back on the engine.
If the fan doesn't come on when activating the A/C, check the C/H fuse. There is a relay in the lower left of the front trunk that actually completes the circuit to the fan. All the control wiring that engauges the relay is Dark Green/wht.
From: Tin Man
Removing the fan switch is very simple. Once you find it it is obvious what hoses and lines that have to be moved/disconnected in order to get a socket down onto the switch. No, I don't remember what size it is, something like 3/4" (I think its SAE, not metric), and no you don't have to drain anything. Have the new switch threads lightly coated with sealant (not Teflon tape) and at the ready. As soon as you unthread the old, install the new. Coolant won't gush out since there's no real way for air to enter to displace the coolant. Don't do this hot! Some coolant will come out, but not enough to warrant fooling with draining the system. Don't forget to squeese the single lead connector on the switch to release it.
From: Tin Man