Control Trac
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Control Trac is the brand name of a four wheel drive system marketed by Ford Motor Company for their sport-utility vehicles. The system was developed by BorgWarner. The Control-Trac system has three driver selectable modes, "Auto", "4H", and "4L". The "Auto" mode was revolutionary at the time, working transparently to provide four wheel drive when needed and if even more traction was required, a twist of a knob or push of a button provides four wheel drive high or low. Control Trac is used on the Ford Explorer and the Ford Expedition.
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[edit] The Modes
The "Auto" mode normally routes all engine power to the rear differential until wheel slippage is detected by electronic sensors. When slippage is detected, an electromagnetic clutch activates in the transfer case, progressively transferring power to the front differential. This mode should be used all the time on the road and in nearly all weather conditions. The "4H" mode locks the center clutch, forcing the front and rear drive shafts to turn at the same speed. This mode should only be used in relatively extreme situations such as off-roading or deep snow or sand. Since the center clutch is locked, turning can cause binding in the drivetrain which can cause damage. "4L" is similar to 4H, but additional low gearing is utilized to maximize torque, such as towing a boat out of water. Due to the low gearing, the fastest one can go in 4L is roughly 20 mph (32 km/h).
[edit] AdvanceTrac
Control Trac can be coupled with the stability and traction control system known as AdvanceTrac. The combination of these two systems is innovative due to a vehicle equipped as such, can continue to move forward with only one wheel having traction. AdvanceTrac transfers torque side to side while Control Trac transfers torque front and rear.
[edit] Control Trac II
Control Trac II was a Ford all wheel drive system based on a viscous coupling unit. The viscous coupling replaced the typical center differential used in four-wheel drive applications. The system worked normally in front wheel drive but in addition to the front wheels being driven, a shaft is powered that runs to the viscous coupling in the rear. If the front wheels slip, the viscous coupling progressively releases torque to the rear wheels. Additionally, the Control Trac II system provided a mechanical lock of four-wheel drive though there was no low range provided by the system.
Control Trac II has since been replaced by the "Intelligent 4WD" system, a similar system with the viscous coupling replaced by a computer controlled clutch. The Intelligent 4WD system functionally acts similar to the Control Trac II system, with the front wheels being driven predominately with torque being sent to the rear wheels only as conditions dictate. The Intelligent 4WD system also lacks the mechanical lock provided by Control Trac II.
