I seem to remember reading somewhere that the Power Wagon uses the heavy duty snowplow fan clutch due to the likelihood of the truck being used for slow speed, technical, off road driving which lacks good natural air flow through the radiator like highway driving has. Brief morning roar for a minute or less is normal, constant roaring indicates either increased radiator temperature or a stuck fan clutch.
From the 2008 factory service manual:
A thermostatic bimetallic spring coil is located on the
front face of the viscous fan drive unit (a typical viscous
unit is shown). This spring coil reacts to the
temperature of the radiator discharge air. It engages
the viscous fan drive for higher fan speed if the air
temperature from the radiator rises above a certain
point. Until additional engine cooling is necessary, the
fan will remain at a reduced rpm regardless of engine
. Only when sufficient heat is present, will the viscous
fan drive engage. This is when the air flowing through
the radiator core causes a reaction to the bimetallic
coil. It then increases fan speed to provide the necessary
additional engine cooling.
Once the engine has cooled, the radiator discharge
temperature will drop. The bimetallic coil again reacts
and the fan speed is reduced to the previous disengaged
Fan drive engagement should start to occur at/between:
• 5.7L Automatic Standard Cool - 94° C - 99°C (201° F - 211 0 F)
• 5.7L Manual - 93° C - 100°C (199° F - 212° F)
• 5.7L Heavy Duty Snowplow - 76° C - 83°C (169° F - 181° F)
• Engagement is distinguishable by a definite increase in fan flow noise (roaring).
Fan drive disengagement should start to
occur at or between:
• 5.7L Automatic Standard Cool - 69° C - 74°C (156° F - 166° F)
• 5.7L Manual - 71 0 C - 76°C (160° F - 170° F)
• S.7L Heavy Duty Snowplow - 55° C - 60°C (168° F - 178° F) A definite decrease of fan flow noise (roaring)
should be noticed. If not, replace the defective viscous fan drive unit.
Have fun in Moab!