6.7 ISB Odd Fan Belt Wear

[Dgreen1069]

Originally Posted by fagnaml

Dgreen --

I can't offer any help on the belt wear but I am very curious about why you replaced your fan clutch. What "symptoms" did you experience that made you decide to replace the fan clutch?

Reason for asking is while driving between Houston an Baton Rouge for LSU football weekends during hot, 95 F+ ambient temperatures my engine temp would constantly fluctuate between 187 F and 212 F. When the engine temp reached 212 F I could hear the fan go into high speed until the engine temp fell to 187 F at which point the fan returned to normal speed. I'm wondering if I have a thermostat or fan clutch or some other cooling problem.

Freightliner used a Horton Fan Clutch on many of their chassis's with Cummins engines. The clutch is fluid filled and is infinitely adjustable by the engine's computer. I believe the optimum temperature is 195 degrees. As the coolant temp reaches 212 degrees, the fan clutch should be locked up providing maximum cooling. When the clutch goes bad, the computer tells it to lock up but it is unable to do so. The fan still works but the clutch is slipping which provides less cooling than needed. Increasing RPM's NORMALLY aids in cooling by speeding up the fan, but as the fan clutch fails speeding up the engine just makes the clutch slip more and more.

On our rig, my cruising temps this summer were higher than they should have been. At 65-70mph, we were seeing 210 degrees regularly. On any mountain pass, temps would climb into the 220 degree range and often hit 224. 225 degrees is the max normal operating temperature but the alarm sounds at 224. We babied ourselves up and over several mountain passes as we traveled from Virginia to Montana.

Do a search on this forum for Horton Fan Clutch and you will see several posts about the issue. Ours failed at around 40k miles which seems to be the norm. Hopefully our new one has been redesigned and will last many years to come.

[Dgreen1069]

Regarding my fan belt, I took my rig to a Freightliner Service Center on Thursday. I hate paying for service I can normally do myself, but we were heading out of town Friday afternoon and it was pissing rain all day Thursday (and I had to work Thursday which made it impossible to diagnose or fix the rig myself).

First off, I was wrong about the pulleys on the 6.7 ISB. The top of the fan belt runs across the tensioner and water pump pulleys. I mistakenly thought the water pump pulley was an idler pulley.

Freightliner said the water pump pulley had some jagged rust on it that lined up with the wear on the belt. They sanded down the pulley and installed a new belt I had in my spares box.

Our camping trip was only 70 miles from home. When we arrived I looked at the belt and noticed it was wearing along the edge again. When we got home on Sunday, I read the replies and Nathaniel's had me thinking about the tensioner. I went outside and noticed the belt was hanging off the tensioner by about 1/8". I pulled the tensioner thinking I may have installed the wrong one, but it matched the original perfectly. What I found was that the mating service had some corrosion I had not noticed when I installed the new tensioner.


After sanding down the mating surfaces, the tensioner lined up perfectly.


Problem solved!