My 1999 sportsman 500 overheating light comes on while plowing. No leaks, clean outside radiator, goes off if I drive down road, but comes back on.

 

Sounds like it may not be getting enough air flow at low speeds.If the fan is cycling on and off,but still over heating at low speeds still sounds like the back of the radiator may be filled with dirt blocking air flow unless you've removed the fan shroud and the radiator itself and cleaned it back and front. Fans weren't that great to begin with also on these older ones. You can try first to raise the front wheels off the ground,leave the radiator cap off and see if you can possibly remove any trapped air pockets while it's idling. If it still gets hot I'd thoroughly clean the radiator and bleed the system again.If it still gets hot at low speeds,then the problem could be an impeller nut may have backed off,impeller not pushing coolant through,stuck thermostat,that you can check with boiling water,even the fan motor may be wearing out. You can run 12 volts directly and listen for any bearing noise that's usually a sign it needs to be replaced. The manual has trouble shooting steps and checks on the cooling system also that may help you.1999 Polaris Sportsman 500 RSE Service Manual PDF Download