The engine has to reach approximately 300 degrees F. before the fan will come on. If the engine is, in fact, reaching this temperature, then I would say your oil temp sensor is bad. You usually have to run them pretty hard to reach that temperature, though. Or running slowly in low gear for a while will usually make the fan kick on, too.
In the service manual, there's a procedure shown for testing the oil temp sensor. It involves submerging the oil temp sensor in oil and heating the oil to a couple different temperatures and then using an ohmmeter to measure resistance across the sensor terminal and the body of the sensor. I will find the procedure and post it for you.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.