Repair manual for polaris 2002 550 fan cooled motor

Normally cooling fan starts after temperature rises above set limit by vehicle manufacturer, till engine coolents cools down. As you say thermostat & relay(coil) is ok. Now you have to check thermostat contact point To relay point connectivity. When temparature rises relay switches to start cooling fan, either a contact failure or wire rusted at one of parts.

Check the attached links,Manuals and guides, Good luck with your ATV
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ATV Service and Repair Manuals
ATV Motorcycle Repair Manuals Repair Manuals Online
How to Diagnose Motorcycle Electrical Problems Dennis Kirk Inc
http://www.atv-forum.com/general-atv-discussion/706-atv-troubleshooting-guide.html

that sounds like the fan relay is bad. if you call your local dealer and ask for maintenance they should be able to offer a little more insight

This sounds like the thermal sensor on the head/block is faulty or in some cases the wires have shorted to the chassis somewhere causing the fan relay to stay on, clould be a faulty relay also.

Is your atv air or water cooled if water cooled is the fan working and radiator clear of mud and horizantal ribs inflat radiator flat. If swollen it will overheat also.

unplug and test the leads with a 12v battery and regular copper wire always test the motor first before going into the electrical relays and sensors

That 550 fan is not a good motor from Polaris. Many engine failures from what should be a very simple and straightforwad fan cooled 550.

Check that the electric fan motor comes on ,and check the radiator for mud and debris u may also need new oil and filter change,All of these things could make it run hot 1 more thing roll it in neutral 2 make sure breaks arent hanging up.

check your tempeture sensor check to see if your faan motor is working

There is a sensor on the lower right hand side of the radiator. It is about $75 to replace.