Can someone expain how a fan activates? I figure it must have a fluid in it that gets thicker with heat but that goes against my limited common logic.

 

The fan clutch locks up once the coil spring on the front heats up and expands, moving the clutch body into a lock up with the center shaft via some form of clutch dogs. At least that is my guess.

 

Thanks Walt, that sounds better than the fluid idea. I have noticed that it will be engaged upon start up after shutting down following a long pull in hot weather. The fan was not running when shut down at 300 turbo.

 

RAFTEN, Normaly the fan should have smooth resistance while turning it by hand . Somtimes they loose that resistance to a degree if they go bad (so Iv been told). I have read of people getting the fan to lock up at a lower engine temp by grinding the coil spring stop back about 1/16th and filling in the slop behind the spring tab with JB weld. Just not sure which side of the spring to grind to bring the lock up in earlier. If you grind the wrong side, it will lock up at a higher than normal temp. Cleaned all the crap of my rad a few days ago, Man,you should have seen all the oily grime from 100K of fumes from the oil vent bottle. I had about 15 percent complete blockage to the left/center bottom of the front of the rad. Take a good look at yours with a strong flash light down from the top. If you see some fuz down their, you will be pluged also. To any one planing on cleaning their rad, get the Vac Pump seal kit from cummins and do your leaky vac pump once you have the rad out. This way you separate the power stearing pump from the vac pump ON THE TRUCK. Making it a snap to do with alot of room to R+R the V pump. Check My Gallery for a photo of the nastiness stuck on the front of my rad before the cleaning.