Engine fan troubles

I have a 2000 Kenworth W 900 L engine fan will not automatically come on with air conditioner or water temp. Engine fan however works just fine when I throw the switch to manual, and occasionally the fan will cycle automatically on it's on sporadically sometimes the water temp reads 185° And fan cycles rolling down the highway other times it I'll get clear up almost to the high temp alarm and I have to lock it in manually. I have change the sensor underneath the alternator for the water temp and I have also trace all the wires and can't find any problem with the wires or the selenoid. Any help would be great thank you

 

Sorry forgot to mention it has a Catapillar 3406E