my sportsman 800 has 940 miles on it, well kept. But last weekend my gf took it out for 10 mins and it overheated and boiled the antifreeze out, through the overflow. So i flushed the remaining antifreeze out and filled it back up, compleatly primed the line so ik it was full, ran it around the yard and warmed it back up, shut it off and then turned the key on, fan came on....ok good.... few days later the same thing happend, now the fan wont come on at all, i wired it directly to the battery and the fan works fine.....? sensor? ecu? termostat? please help...
Thanks

 

It can be the thermistor sensor(located on the left side of the engine) that has shorted out or can't complete the circuit, you can pull it out and check the ohms resistance while it's cold then when hot using a cup of hot water or hair dryer. Don't have the specs,but the ohms resistance values should change. This could be the original reason the fan may have quit kicking on. Plus if you find the problem leave the radiator cap off to purge any trapped air in the system when the fan does cycle on and off or it can overheat again.. OPT

 

hooked up the fan to a 12 volt battery and it works fine, took the sensor out and tested that and the ohms do change when heat is applied....also i did leave the cap off to get all the air out, the local dealer said that the fan wont kick on unless the coolant is full....so shes full

 

Then you might have to trace the thermistor harness into the area around the fuse panel and check for any wiring shorts in the fuse panel,etc. Plus don't hold me to it to it,but I think on the 07 there is a thermal circuit breaker taped to the top of the wiring harness just behind the fuse block. We did have some problems with these but I can't remember which model year 800?? It does show on the wiring diagram there is one. Check your model out here and see if it gives you any help.http://parts.polarisind.com/Browse/Browse.asp
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don't know about the 05' model year but i'm pretty sure my 06 and 08' if you unhook the sensor with the key on(bike not running) then the fan should come as a back up to having a bad sensor. it is a way of testing the system i think i read that in the service manaul some where. also make sure the radiator is clean front and back, through and through as the twins run hot and it doesnt take alot of blockage to over heat them.