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Some background - my 2017 crf250L gets plenty hot and the fan is not turning on. I have 6,000 mi on it and I'm very familiar with when it usually turns on. However on a near 100 deg. day on a long ride, I noticed it was not turning on. I called for help to get it home without further heating. More testing at home and fan still will not turn on.

I disconnected the fan and gave it power - fan comes on.

I'm thinking that the ECT sensor (basically a temp. sensor switch) is the next most likely failure, so I disconnect the 3 pin connector and plan to cross two terminals and see if the fan comes on. I'm thinking that this would tell me if the ECT was bad. If it came on while hot wired, and if it did not come on to keep searching.

Here is where I'm lost - With the key on of course (engine not on), No fan while connected (normal, because not hot). When I disconnect the wiring plug to the ECT the fan comes on. The thought the ECT was basically a temp switch that should interupt current when not to temp right? Why does the fan come on when disconnected?? What does that tell you? I'm confused.

Welcome to the site !!

I am going to say it sounds like the "thermostat" is bad and stuck open , when it is disconnected , it is allowing the fan to ground by possibly a second ground in the system , I am guessing it is acting as a ground switch , so when it goes thru the thermostat , it completes the circuit to the already there/secondary ground , when you bypass the malfunctioning thermostat , it is now going directly to ground .... that would be my guess

What I would do is trace the wires (if you do not have a manual) , and follow the wires to their locations , I am thinking you will find one of the 3 goes to ground , as long as the fan has power and is grounded , it will run .... follow ??

I would also look behind the left side panel and check the fuses and make sure they are all good , its possible that just a fuse is blown

The fan should have a 10 amp fuse , look at position #4 in the fuse panel

I found this on Google , its a 2018 but it should be the same .... Hope that helps ...

Thanks. I failed to mention that first thing I did was check fuses and tested fan relay. I did see that wiring diagram and the path of wires to the fan is very complex in my opinion...... not just power and ground going through the ECT. I agree that the ECT is probably the source of the problem (to the point that I'm going to purchase one today and see) but I was very surprised that the fan comes on when unpluged from the ECT. I also found a way to test the ECT in hot water to check the output voltage, but I think I will skip that test since I'm not sure I could maintain an exact temp of water on my stove.
I figured the ECT somehow reduces voltage when the water is cold and increases voltage when the water gets hot, but that is not what I originally expected.
What I learned....... If stuck in the hills with a hot bike and a bad ECT....... Unplug it!

Well Tim that sounds like an important lesson for others in the same situation. Be interested to see if replacement cures the problem.

Hi Tim did you put in the new ECT sensor and was that the problem? I'm having the same problem with my 2016 CRF 250l rough start keeps cutting off fan doesn't come on unless the the ECT sensor switch is out thanks any help is appreciated