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.
*Does not have Four Low mode with low range off road reduction gearing.
The fifth generation 2011 Ford Explorer (U502) replaced automatic ControlTrac four-wheel drive with automatic Intelligent four-wheel drive and Terrain Management.
Design and 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.
The design team gained confidence in their concept in 1989, when they made a breakthrough in the multi-disc clutch's control system. Development of closed-loop control smoothed its operation. Using it, the multi-disc clutch made smaller adjustments, but did it more frequently. The microprocessor would review input from sensors every 20 milliseconds and decide if the front axle needed more torque. Using this technique, the unit's computer watched for drive wheel slip. If it sensed as little as half an rpm difference between the front and rear axles, it sent a power signal to the multi-disc clutch. The clutch engaged, diverting torque to the front axle in 10 percent increments, until it alleviated the drive wheel slip. As a result, the system could control runaway drive wheel speed in as little as a third of a wheel revolution.
Off road testing
Early in the development, BorgWarner had employed a sintered bronze clutch material that exhibited an operating condition commonly known as "stick slip." The slippage had inspired engineers to replace it with a paper-based material mounted atop metal. The paper offered a better coefficient of friction and solved the slippage problem. BorgWarner's automatic transmission components division is credited for the solution.