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 water boiling temp(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. High heat light would activate around 275-280.
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.

 

Welcome to the forums.

The fan seemed like it never came on when I had my 2000 boss. First time it did, I wasn't sure what the sound was. Even running it here in the Texas summer heat, it never gave me any problems. And this thing took a beating from just about everyone in the family.

One thing you may want to look at is the rear shock bushings. I'd bet if they haven't been replaced, they need it.

Later on, if you want to give it some extra zip, call Rick Ritter and see about a stage 1 kit. Really liked what it did to mine. Always said I'd put the 414 kit in it when the motor blew. Sold it to a nephew and it's still running strong.

 

Thanks for the info, I'll take a look at those shock bushings tonight. As for a power up, my 10 and 12 year old will be using this quad as much, if not more, than I do...I think I'll skip the power increases for now.

 

Only bad thing I can say about it, it was very cold natured. But once it got warmed up, it ran like a champ.

 

Since the 325 trail Boss is a air cooled motor all the fan is for to "Assist" the oil cooler. The oil cooler will still do its job even without a fan ,just by rerouting the oil the long way around allowing cooling the oil off before entering back in the motor.

My 350 Yamaya 4x4 Big Bear had a oil cooler without fan assist and never over heated,and my 250 Moto4 did not even have a oil cooler period.

 

Under 95% of all riding, the oil cooler / air cooled head should be fine. The problem, as I see it, is that oil temps above 260 Degrees F ARE PROVEN DAMAGING! Now I know, that synthetic oil CAN handle temporary jumps in temp, even to 400 + degrees, but it still breaks down the oil. I imagine this is why Polaris states to use 0w-40 (or now 2w-50, which doesn't really exist, btw...and is a genius Polaris marketing trick), because these ARE synthetic.
So basically, leave your fan/thermister alone...but if your fan turns on, consider IT your high heat light! If my oil reached 260, I'd change it! Because remember, that is 260 degrees AT THE RADIATOR OUTPUT! (Which is probably closer to 300 at the engine).
As a comparison, most truck oil coolers have 180-200 degree thermostats...but then most trucks don't spec "synthetic only" oil.
Further more, there is absolutely no reason I can see, to intentionally have high oil temps, at least higher than necessary. As I said before, a temp above 212 will boil off water collected in the oil...anything higher than that is a complete waste. The perfect oil temp, if there is such a thing, would be 213.

 

My wifes 2000 Trail Boss's fan comes on with the key and switch and stays on all the time. I assume the PO had simular problems to what your finding and decided to wire it hot to the switches.

Any one see a problem with leaving it on always?

CW

 

I would assume the reason Polaris made it come on at 260 degrees was that it is more of a heat fan, than a cooling fan. What I mean is the fan is more for dangerous heat than normal cooling. I would also have to assume this was done for amp draw reasons. Leaving the fan on all the time, in addition to making your oil too cool, would probably over tax the electrical system.
In later years (2003+ I think), the ECM is programed to run the fan if the circuit becomes open to the thermister, but in 2000, an open circuit means the fan never runs....so I would assume your problem is a closed circuit. Check for crossed bare wires, a bad connector, or most likely a shorted thermister. Unfortunately, new thermisters cost about $70!
It would be easy to check: just try unpluging the thermister (it is the little silver plug with 2 wires coming out of the top, on top of the radiator. 11mm wrench will fit it). If the fan shuts off, then it is the problem.

 

the only problem with that would be it running sumerged on deeper water or mud hole crossing,permature fan ware,there only so many hour of life in a fan and the noise of the fan all the time.