Fan clutch 101:
Viscous fan clutches are a fluid coupling, same basic idea as a torque converter. 2 rotors adjacent to each other with a fluid between them so viscosity of the fluid transfers power. There's a valve and pumping setup so that the fluid is kept generally out of the rotors when not engaged and ported to the rotors when needed. On the old mechanical system this is controlled by the bi-metallic spring on the from of the clutch. In theory there's a degree of portoptioning, ie greater oil flow for greater engagement as temp goes up, but in reality I'm not sure how true that is - it seems much more of a on/off actuation in reality.
This is a simple, and for the most part highly reliable setup. The down side is that when the oil goes away so does the ability to run the fan, and there's no good way to check the oil level or see how well it's going to work before you need it. Also, the actuation temp is based on the spring, which is not controllable or readily adjustable.
The newer systems replace the bi-metallic spring with an electric actuator. This is the electro-viscous setup discussed here. The mechanical side is the same, but what you add is the ability to control the clutch electronically, so it can be varied as needed. This is done primarily for emissions/fuel economy reasons, **not** because it's really any better. Yes, you get more theoretical control, but at the cost of more stuff to break/mess up. All of the potential failure points and downsides of the old clutch are still there but now with more things to go wrong. This is especially true for something like this when you're using a customized control setup.
Another point to make is that a viscous clutch is not ever going to 100% lock - there's always some amount of slip. Same idea as a non-lockup toque converter in an automatic. You get 90%, but never 100%.
The Horton type clutch is a full 100% lockup clutch, and it's simple to boot. Also relatively easy to override and manually lock on if you needed in an emergency.
