There are two wires to the electric cooling fan. One goes to the fan switch in the passenger side head and the other one is what? Ground??

 

The fan switch in the head is the ground. The other is switched power from the fuse panel. (both are 18ga wires leading to the relay)

 

NO-the wires off the cooling fan are both 12 ga. one is a direct to chassis ground-black and it should terminate at the lower core support. the other is 12v feed black/red and this will come off of the switched normally open coolant fan relay. NEITHER of those wires go thru the thermal fan switch. that tfs goes to ground at a pre-determined temp-the one from an 87 buick grand national is the lowest temp switch gm offered-that fits our cylinder heads-and completes of the circuit which pulls the main relay contacts closed thereby supplying power to the fan. the wire size off the switch is18 ga.

 

Thanks, that's what I need to know. I have an aftermarket fan switch that uses a heat probe attached to the radiator. I've used this for 2 years and it works great. When I hooked it up I just simply cut the black/red power wire and spliced my fan switch into it... when the temp probe reaches the designated temp set by the potentiometer the switch closes and completes the circuit sending voltage to the fan. I never did have to touch the other wire so that's why I was wondering what it was. My fan quit working today and I tested all my power wires and I'm getting full voltage to the fan. I thought maybe the other wire was messed up so I cut it and ran a new wire and grounded it directly to the battery but the fan still didn't work. I replaced the motor 2 summers ago and it's under lifetime warranty so no big deal. Maybe if I hooked a relay up between the switch and fan, it would help the lifespan. Any recommendations on what size relay to use??

 

I misunderstood what you asked.. sorry

 

86TPI-yes it would be beneficial to use an aftermarket relay to protect your temperature control switch (potentiometer). You can use any 30 amp relay. You can buy these at most automotive supply stores or at Radio Shack. Good luck with your wiring job.

 

Well, found out today that the motor wasn't dead afterall. Somewhere my wiring had a short. The motor and fan switch are just fine. I recall when I bought the fan switch a couple years ago that it said something in the instructions about it having a built in relay so I'm not too worried about that although it's crossed my mind. Anyhow, I guess after a couple years the wiring has just gone bad somewhere. I had to rewire it and hook it up to a toggle switch temporarily until I can take a better look at things this weekend. Thanks again for the help.