Hi, I have 1 1991 E150 4.9L econoline. I am trying to finf out some information on how the cooling system works. If anyone can help me it would be great.

I need to know what tells/signals the fan to turn faster as the engine gets warmer. I thought it was the collant temperature sensor or temperature sender? But I just spoke with someone who knows these vans well and he says that there is a Viscous unit where as the lequid in this unit gets warmer it expands and makes the fan turn faster. I have looked on the parts webstites and do not see any viscous unit for my van. I do see Fan clutch units for it.

Can anyone tell me what system is in my van and how they work?

Thanks a lot.