At what coolant temp should the fan clutch lock up and I hear the fan?

 

I always hear the fan! Sometimes just louder than others. As for it locking up I have no idea. Sorry brother.

 

When you crank the truck cold, the silicone fluid is pooled to one side of the viscous clutch. It will "roar" as it is coupled at maximum percentage (that is not 100%, many designs juist couple at 70-75%). As the fluid is circulated and warmed up, the fan noise drops away as truck warms up. Clutch is coupling at 20% or so. Good MPG. Still enough power to slice off fingers, giving you the nickname "Stubby".

As face of fan clutch gets warmed up by air flow from radiator the coupling increases back to the higher number and the roar comes back. Some clutch designs make this a gradual smooth change, like a variable speed drill.

If the radiator is sludged heavily, that center area may not pass enough air to warm up the clutch. At about 170F air temp (not coolant temp) the clutch moves to the higher coupling. That would normally be about 30F below coolant temp.

If engine is warmed up, and a spin/release of the fan clutch results in free wheeling, then time for a new clutch.

 

Hmmm, okay. I think the radiator may be sludged then. What's the easiest way to desludge? Something cheap and easy like vinegar maybe? Just looking for a quick fix.

 

Rod it out. 40 bucks for me. You can try pulling the radiator and letting CLR or something like that soak in it for a while. Id really try to find a shop to rod it out though.

 

I tried white vinegar, then removed rad and laid flat and used muriatic acid (makes brass bright), both did a little, as did the off the shelf flush. But calcium and scale was so strong I had to take to an indy rad shop. They unsoldered a side tank, and rodded out the calcium, and hot flushed with citric acid. $75, cools great.

You can tell if it is sludged, check temp top to bottom on the fins. More than 10F difference means lowewr rows are blocked.

 

Lye at 180F, most hardware/lumber yards have it. Protect yourself and paint.

 

I think the big diesel truck places use cascade.

 

calcium and scale was cooked on there at 200F+ and how many years of driving? Any flush chemicals will do a partial job of the heavy stuff. Rod out is best plan if you can find a shop to do it. They will also hot flush with citric acid.