Portions of this are plagiarized from my review of the
Hayden 3651 Fan Controller on Amazon. It's not very complimentary...
I have a 1996 GMC Sonoma 2.2 with electric fans from a 1994 Dodge Intrepid. This conversion ROCKS. Much quieter now, and a bit better gas mileage as a bonus.
The "mistake" part of the experience:
I initially wanted an adjustable fan controller, so I purchased and installed the Hayden 3651 fan controller, which provides precise control of the "on" temperature for your electric fans, as well as an "all on" setting when your A/C clutch is activated. Then, after about three weeks, it quit. The relay didn't fail, the controller electronics did. I didn't think a nicely potted bit of electronics only three weeks old would fail, so I wasted a bit of time troubleshooting fans and electrical connections, but after all that checked out and the relay tested functional, I had to toss in the towel.
Solution:
Went to the local NAPA parts store, dragged out the specifications list for thermostatic switches, got one with the desired temperature range, salvaged the relay from the junk controller, set that up as the "low" circuit, and added another relay triggered by the A/C clutch to run the "high" circuit.
Runs fine, with a "fan on" range of 215F-195F.
Details:
I purchased an inline drain adapter from Jegs (p/n 555-51154) to serve as a sensor mount, and installed the temperature sensor in the upper radiator hose. This drain adapter comes threaded 1/4NPT, so if you use the sensor I chose you'll need to either enlarge the hole to 3/8NPT or drill another hole and tap it - this is the route I chose, using a 9/16" drill (because I can't say "37/64ths", Sylvester) and it worked well.
I then went to the NAPA store and got an Echlin FS118 switch (219F, 8deg tolerance, threaded stud connector, sub Wells SW510 if needed) and installed it in my modified drain adapter.
You have two options for grounding this sort of thermo switch, and I chose the more complicated version (of course). The simple way would be to install a grounding screw on the body of the drain adapter. The complicated way (
click here for visual aid) is to solder a male QD tab to a copper flat washer big enough to land on the rim of the switch and use a nylon shoulder washer to isolate it from the center stud. Add a nylon flat washer, a brass flat washer, a ring tongue terminal for the center post, and the right size nut (I think it's M4, but the stainless nut I tried didn't work well, so I used a nylon one instead) to compress the whole mess together.
OK, so now we've got a pretty little fan controller switch, installed. I've chosen to not use ignition-switched power for the fans, so the relay wiring is really simple.
I'm using the Intrepid/Concorde/etc fan that's popular for the S-10/Sonoma, and said fan has three wires; black for ground, tan for low, and yellow for high. I am using the thermo switch for the low side relay and the A/C clutch signal for the high side.
The relays and fuses are tucked away in a Radio Shack project box sitting between the ECM and the A/C blower housing.
Grounds:
Use 12G or heavier ground wire for fans, ground to chassis.
Use 18G for relay signal grounds, ground to chassis.
Low (thermal) circuit:
+12v to pins 30 and 85 of relay (powers fan and supplies signal, use 12G or heavier wire for pin 30, and fuse to 30A).
Pin 86 of relay to center of thermo switch (signal level only, 18G wire OK).
Ground thermo switch body or drain adapter as you see fit.
Pin 87 of relay to tan fan wire (use 12G or heavier wire, and fuse to 20A).
High (A/C) circuit:
Jumper from pin 30 of low circuit to pin 30 of high circuit to provide fan power, using 12G or heavier wire.
Tap the A/C fan clutch positive wire and connect to pin 85 of relay.
Pin 86 of relay to ground.
Pin 87 of relay to yellow fan wire (use 12G or heavier wire, and fuse to 25A).
Bonus:
Run a pair of wires from the dash to the A/C fan clutch positive wire upstream of the tap, and add a rocker switch on the dash - now you have an A/C compressor shutoff switch just like a Toyota! I'd suggest an indicator switch so you know when you've got it off.
Have fun making spaghetti!
ThomCat