The thermal clutch responds to air temp, in the vicinity of a bi-metalic strip of metal, curled up in a coil, mounted to the front of the viscous clutch.One end is anchored to the housing, while the other end operates valve inside the unit.As the coil absorbs heat it , closes the valve and engages the clutch which locks the unit up in or near direct-drive,and spins the fan. When the air passing over the unit eventually cools it, the valve opens and the fan coasts. It never really shuts off, the viscous fluid is always driving, to some extent. It can't be much tho, cuz I've stuck a finger into it a time or two;no blood. I wouldn't do it on purpose tho!
-Rpm has no effect on the valve, it's just air-temp coming through the rad, or being pulled through the rad.But rpm does affect the fluid drive. As it rises, the viscous clutch drags the fan along too.And the viscosity of the fluid changes with heat, being much less able to drive the fan when it is cold, and much more able as it heats up.
-So if you leave the line with a hot engine and a hot clutch, engaged, anything can happen along the way to the traps. In could decouple the valve and freewheel at 60ft, 40mph, 80 mph, or not at all.But when it decouples, the viscous fluid will still drive it. And if the unit is then operating in slip mode, it will lag behind anything the crank is doing, which has just got to be kind to the belt. Mine has got many years on it, and many hard miles.
-And from what I have experienced in the shop, when the unit fails, it does so in full lock-up; meaning when you start the engine cold, Whoosh is instant.And the non-thermo's fail by losing their fluid and failing to drive the fan at all.
--I like the Mopar thermo units as they seem to be a lower profile than the Fords,But I have seen many more failed Mopar units,than Fords.
I chose the Ford pick-up unit, on account of the first time I ever heard one kick in. The truck was just idling on the pad, when suddenly;whoosh, the fan kicked in, and hot air began to pour out from underneath the vehicle. No Mopar I had ever come across, operated like that.In my experience,Mopes are a softer start. So after a couple of cycles,I popped the hood and had a look.I saw that it was a bit thicker than a Mope but looked like it would fit on my car. So I motored over to the Ford dealer with my fan and spacer in hand, and sure enough it was a direct bolt-on. I took it home straightaway.Now, some 12 years later, it is still going strong.