Here is the setup that Jon Haas (Pantera Electronics) has engineered. It uses one sensor on the outlet. The controller (box where relays are usually mounted) begins turning on the fans at 140 degrees.
I met Jon yesterday at Wilkinson's shop and he explained the setup to me. After understanding it I bought one. Here is what it does.
> Efficient proportional fan speed for radiator temperature control, fans consume less power, less load on alternator and battery.
> Utilizes power semiconductors to control fans ... NO contacts arcing during turn on or turn off.
> Fan operation is delayed 12 seconds to allow starting engine without the fans loading the battery.
> 7 Indicators, "READY", 50% SPEED, 60% SPEED, 70% SPEED, 80% SPEED, 90% SPEED, 100% SPEED.
> Activates amber light in speedometer when fans are ON, flashes amber indicator when the fan fuse is open.
> Ramped fan speed to minimize voltage drops due to high current in-rush.
> Only (1) temperature sensor is utilized, electrically insulated from coolant and radiator, no galvanic action.
> Control terminal to enable fans at maximum speed for forward mounted air conditioner condenser.
> Simple wiring and NO additional diodes needed, diodes reduce fan motor voltage 10% and decrease air flow