what causes the fan to keep going on and off on a c15 cat.

When the solenoid for the clutch went out it randomly kicked on and off for a couple days prior to it completely failing on my 387 with a c15

 

Our 97 T300 had something like this happen, and it turns out that the rubber cowling that "seals" the frame to the radiator housing was missing, so the fan was not pulling as much air through the radiator, therefore not cooling enough, and the fan kept coming on. Some new rubber, and rivets, and now the fan only kicks on when it is under hot/heavy load.

 

I know this is an old thread, but I have a 2ws 3406E that is doing this. It will run just fine on manual but when on automatic it will kick on and off when it hits the temperature for the fan to kick on. It will not stay on until the off temperature. Are these programmable temperatures or is it just a set temp in the ecu?