" a thermostatically-controlled hydraulically-driven fan that operates only when the engine cooling is needed is used as standard on trucks and as an optional on coach engines" pg 92 Detroit Diesel In-Line Series 71 maintenance manual. That's what I mean by "hydro lock thingy" also I guess this was really a 6-71 specific question, I'll try to modify the title. The reason I'm asking is a few here have had this hub fall apart, and I thought it would be cool (too cool) if I didn't have one. Also my bus takes forever to warm up which sounds like maybe the fan is always on. Finally, on cars when there is a thermo couple on the fan you can stop the fan from moving on a cold engine, how do you tell with this system?