No, no transition involved. It is what it was made as. All of those type clutches are viscous. Some have thermal assist and some do not. Most of the heavy duty units have the thermal spring. As the radiator heats up, the hot air caused the thermal spring to coil and turn the control valve inside the clutch and cause the clutch lock up even more. They can lock completely up if enough heat is generated. "Viscous" only refers to the viscous fluid (usually a silicone compound) inside the clutch. So both styles are viscous, but one has the addition of a thermal spring that aids in additional lockup as heat increases. The fan in the picture above is a viscous clutch fan with thermal control. There are standard viscous clutch fans that do not have the thermal control, but both are viscous clutch fans.
