the fan blade wheel is not designed to spin up to high rpm, the visco clutch is there for 2 jobs,

one to freewheel when the clutch is cold and then to freewheel at high rpm even when the clutch is hot. You do not want it blowing apart at high rpm and take out the radiator etc.

So do not lock it. If you are on a budget proceed to a hobby store and get some RC diff fluid about 8000 wt and fill about 5 or 8 cc and test again.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)