ControlTrac

ControlTrac four-wheel drive is the brand name of a selectable automatic full-time four-wheel drive system offered by Ford Motor Company. The four-wheel drive system was designed and developed at BorgWarner under its TorqTransfer Systems division in the mid 1980s. BorgWarner calls the system Torque-On-Demand (TOD). ControlTrac was the first automatic system to use software control and no planetary or bevel geared center differential. Instead of a planetary or bevel geared center differential, the system uses a variable intelligent locking center multi-disc differential.
Availability.
ControlTrac made its debut on January 12, 1995, for the 1995 model year on the second generation Ford Explorer mid-size sport utility vehicle. A heavy-duty version of ControlTrac was introduced in 1996 for the 1997 model year on the first generation Ford Expedition full-size sport utility vehicle.
The fifth generation 2011 Ford Explorer (U502) replaced automatic ControlTrac four-wheel drive with automatic Intelligent four-wheel drive and Terrain Management.
Design & development.
The idea for an electronically controlled four-wheel drive system emerged at BorgWarner in 1985. BorgWarner's original design called for using both a software controlled electromagnetic multi-disc (also called multi-plate) clutch pack and a planetary or bevel geared center differential together. The first prototype system was crude, and its original clutch pack controller was a rheostat.
The geared center differential would be used to allow the front and rear drive shafts to turn at different rotational speeds so as to eliminate any "drivetrain binding" or "torque windup" while the system was being used on pavement. The intelligent multi-disc clutch would be used to progressively transfer torque back-to-front and front-to-back between the front and rear drive shafts when needed and would also lockup providing a permanently locked front 50:50 rear torque distribution.
Within a year, the controller had become larger and more complex. A breadboard electronic version complete with sensor amplifiers and large control modules consumed the entire back end of a station wagon. Input data came from variable reluctance sensors installed at the front drive shaft, rear drive shaft, throttle, brakes, and steering. The goal was to control clutch actuation by controlling current.
However, as the development process continued, engineers at BorgWarner discovered that, with clever software programming, they could control the variable electromagnetic multi-disc clutch to a point where it would allow the front and rear drive shafts to turn at different rotational speeds on its own, without the aid of a planetary or bevel geared center differential. In Auto mode the new software algorithms allowed the multi-disc clutch pack's friction and clutch discs to slip as needed, simulating a planetary or bevel geared center differential, making a geared differential redundant. Thus, the geared differential was abandoned and therefore never included on the final production version of the four-wheel drive system.