Help! Fan keeps coming on

My fan is acting up. yesterday while cruising 65m.p.h. the fan just kicked on for about 30 seconds, then off for about 2 minutes, then on for about 30 seconds. drove another 80 miles to unload & then 120 miles back home. fan never came back on again. today, drove 20 miles to load, then drove 130 miles to unload, but 1/2 way back empty, the fan came on again. approx 25- 30 times. on for 30 seconds, then off for about 2 min & repeats. then the last 15 miles home, it did not come on. I also noticed that it came on when starting on a pull up a hill. every hill it would come on & then go off after the hill was crest. though it did not come on on the hills when I was loaded, that was strange to me. I got truck home & moved wires all around selenoid, could not get fan to come on or see any sparks from wires. fan wire to fuse is good, manual fan switch is new. going to buy a new selenoid, tomorrow. if I am not mistaken the wires from the selenoid go to the sensor at the thermostat housing? I replaced that sensor 2 years ago. would that sensor if bad cause this? what got me stumped, it the fan seems to have no pattern of when it come on. temp gauge in dash, seems to work just fine. I don't want to throw parts at it. I replaced the oil temp sensor about 1 month ago. . would that sensor or it's wires if loose, bad e.t.c cause my fan to come on? shop says to bring in &they will hook up to comp. these shops are getting stupid on me. it is not throwing any codes, the comp cannot tell if there is a wire short, or other things. folks want to rely on what the comp tells them & if it tells them nothing, they want to throw parts at it. I want to DIAGNOSE first. if possible. 95'ser 60 detroit. thanks folks.

 

I forgot to mention. the a/c is not hooked up. no belt on. no freon in sys.

 

where would the sensor be? I have 2 wires from compressor. 1 is ground, 1 must be the hot. I unplugged both wires from wire clips, but did not look for sensor. would unplugging these wire have the same affect as un plugging at the sensor itself?

 

I don't know why, but my problems seem to get worse before getting better. I guess it's one way how I learn. I just remembered I had a old solenoid in my side pocket, as far as I remember, there was nothing wrong with it, but changed out cause it was several years old. anyway I put it on, & the fan immd came on. sooo I put the previous one back on. the FAN came on immd & now will not go off. before now, the fan would do as stated in original post.

fan comes on here & there, now & then. take off. put back on & now fan stays on??? now somehow I made it worse. I was not as concerned as I am now. at least before I may go 200 miles before fan came on. not it comes on & stays on. going to truckpro to get new solenoid this morn, just to rule that out, though I somehow don't think that is the problem, but is is cheaper for me to throw parts at it than to pay others to do it. I unplugged the oil temp, both water temp, sensors, wiggled wires to sensors, no difference. fan still on. any more advice ? will get it eventually, will post back after new solenoid is put on.

 

I put new fan solenoid on fri morning. did not fix the problem. the fan comes on as soon as the engine starts. I cannot get it to shut off. checked fan fuse, it is good. checked manual switch, it is new, but by passed anyway to be sure. checked wires to oil temp sensor, & 2 water temp, sensors. dropped off at local shop just a couple miles from me, though the owner say's he cannot think of anything he could check, & I already did everything that he knows to check. but left it with him & maybe one of the shop guy's will think of something.

 

the air dryer? my dryer is on the bottom of truck right in front of fuel tank, & my a.c. sys is on the top front of engine. there is no sensor on top of air dryer. ????