After riding my new (to me) 2000 trail boss, I noticed the fan NEVER ran, even after letting the quad idle for 15 minutes. I thought I had a problem with my fan or my thermister, so I did some checking and found both were working as they were designed...turning on at 260 degrees F. A further check on the various forums found that others noticed their fans not turning on.
I believe that this is a design error. The thermister, sitting on the downstream side of the radiator, is waiting for the oil to literaly cook before turning on the fan. I mean, if the oil has reached 260 degrees after going through the radiator, how hot is it in my engine? My belief is that the best oil temps are just above boiling (215 to 245 degrees F or so), so as to help eliminate any water build up, but keeps the oil from cooking.
I also noticed that the fan blows air through the radiator, then across the cooling fins of the head/jug. I feel it would be nice if the fan would come on at lower temps, to help with cylinder cooling as well.
So I came up with a mod that has really seemed to do the trick. Basically, I just placed at 10K ohm resister across the the thermister wires, so that it is wired in parallel (from one wire to the other). The result:
Before: Fan would only turn on at 260 degrees F, off at 240 F. Engine temp light activates at 290 degrees F.
After: Fan turns on at 245 degrees F, off at 225. Engine temp light activates at (estimated) 285 degrees F.
After making the mod, I tested it, and after idling for 10 minutes, my fan turned on!!!!
Oh, and when I placed the resister jumper, I was able to keep everything water tight. I pulled the pins out of the harness that attaches to the thermister wires. Once the pins were out, I used a needle to poke a small hole through each of the two "sealing bushings" that were attached to the wires. I then threaded the resister leads through these small holes, and soldered them to the back ends of the pins. Then I put the pins back into the harness, and for good measure covered the back of the sealing bushings with silicone, also covering the resister in the process.
If the temp figures are not low enough for ya, you could use a lower value resister. Go with a 5.6 K, and your fan would turn on at approximately 230 degrees, off at around 200.
If you already have a soldering gun, solder, and silicone, the mod only costs .99c for the resister!
This mod was done on a 2000 Trail boss. I'm sure it would work for other years and/or models. Check your owners supplement...if your thermister activates your fan at 260 degrees with 2519 ohms across, then it will work.