Control Trac

Control Trac


ControlTrac four-wheel-drive is the brand name of a selectable part-time/full-time four-wheel-drive system offered by Ford Motor Company. The four-wheel-drive system was designed and developed at BorgWarner in the mid 1980s. BorgWarner calls the system torque-on-demand (TOD). ControlTrac was the first automatic part-time/full-time system to use software control and no planetary or bevel-geared center differential.[1] Instead of a planetary or bevel-geared center differential, the system uses an infinitely variable electromagnetic multi-plate clutch pack-based center differential.[2]


ControlTrac made its first début in late 1994 (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 the1997 model year) on the first generation Ford Expedition full-size sport utility vehicle.

Vehicles available with ControlTrac 4-wheel-drive
Vehicle Model Year Transfer case
Ford Explorer MY1995–2010 two-speed
Ford Expedition MY1997–present two-speed
Ford Expedition EL/Max MY2007–present two-speed
Lincoln Navigator MY1998–2006 two-speed
Lincoln Navigator MY2007–present single-speed*
Lincoln Navigator L MY2007–present single-speed*

*Does not have 4 Low mode with low range off road reduction gearing.

The fifth generation 2011 Ford Explorer (U502) will replace ControlTrac 4-wheel-drive with Intelligent 4-wheel-drive and Terrain Management.[3]

Design & development

The idea for a electronically controlled four-wheel-drive system emerged at BorgWarner in 1985. BorgWarner’s original design called for using both a software controlled infinitely variable electromagnetic multi-plate clutch pack system and 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 infinitely variable multi-plate 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 fixed 50/50 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 infinitely variable electromagnetic multi-plate 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. Essentiality the new software algorithms had turned the clutch pack itself into a mechanical center differential, making a geared center differential, redundant. Thus, the planetary or bevel geared center differential was abandoned and therefore, never included on the final production version of the four-wheel-drive system.

"We asked ourselves: Why do we need a differential? There was all of that iron, all that weight, all that cost.

And here, we saw that we could control the clutch pack very precisely without it." - Ronald A. Schoenbach

The design team gained confidence in their concept in 1989, when they made a breakthrough in the multi-plate clutch's control system. Development of closed-loop control smoothed its operation. Using it, the multi-plate 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-plate 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.[1]

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.

However, heat proved to be a persistent issue. Excessive heat buildup would burn out the new clutch material. To solve the problem, engineers studied the clutch's torque. It was determined that excessive heat buildup was caused by inadequate clutch torque. Engineers increased the system's capacity and equipped a fleet of test vehicles, which they took to the Anza-Borrego desert in southern California. They made a total of 11 trips, subjecting the prototypes to heat, mountain driving, and deep sand with the clutches being called upon for almost continuous delivery of torque. At first, the environment quickly burned out the clutches. But as engineers gained knowledge, the clutches improved. Eventually, the engineers were able to improve the clutch packs enough that the test vehicles could be virtually destroyed from hard off road use, but the clutch packs would still look good.[1]

The Modes

ControlTrac has different modes as well as different operational behavior for those modes depending on which vehicle it is used in. For example, the Ford Expedition when introduced in 1996 used a new auto-lock feature in Auto mode.[4] Auto mode with auto-lock was not available on the Explorer at that time.

  • 2 High mode

Predominantly, Explorer was not equipped with 2 High mode except in the MY1995-MY1996, where no 4 High was available. Only Auto, 4 High and 4 Low modes were offered. 2 High mode was unique to the Expedition, though it was discontinued between the 1999–2002 model years. 2 High mode was reintroduced on the redesigned second generation Expedition (U222) for the 2003 model year and featured a new full front axle disconnect system with vacuum operated front hubs that would disconnect the front axle, front differential, and front drive shaft when not needed for quieter, more fuel-efficient operation on pavement. Ford claimed that the improved 2 High mode could help increase fuel economy up to half-a-mile per gallon of gasoline.[5] In 2 High mode, torque is routed to the rear drive wheels only, imitating rear-wheel-drive.

  • Auto mode

Auto mode was featured on both Explorer and Expedition an allows for all-weather, all-wheel-drive capability on pavement as needed. In Auto mode, the engine’s torque is normally routed to the rear drive wheels. A misconception about the system is that it continuously shifts into and out of four-wheel-drive as needed. This is not true, for when Auto mode is selected, the front axle hubs are permanently engaged, locking them to the front axle shafts, front differential and front drive shaft. This is so the front drive shaft always rotates (turns) when the vehicle is being driven at any speed. The entire driveline is always engaged. The computer control system needs the front drive shaft to turn, so that it can monitor and compare the rotational speed of both the front and rear drive shafts. If the rear drive shaft starts to turn faster than the front, the system interprets that (along with input from other sensors) as traction loss. When traction loss is detected, up to 100 percent of the engine's torque can be sent forward, to the front differential, via the center multi-plate clutch. As it does so, control software varies the center multi-plate clutch to behave like a geard center differential, such that driveline binding and torque windup do not occur. The transfer of torque is done in secrecy, undetectable by the occupants of the vehicle. Newer versions of Auto mode include updated software logic that allow the system to predict traction loss before it happens, so that torque can be transferred before it’s needed.[6] This improvement in software allowed the system to operate more like other "always-on" full-time four-wheel-drive systems where torque is permanently supplied to all four wheels, all the time.

  • 4 High and 4 Low modes

4 High and 4 Low modes were also featured on both Explorer and Expedition. 4 High locks up the infinity variable center multi-plate clutch, forcing the front and rear drive shafts to turn at the same speed. 4 Low mode is similar to 4 High, but additional low range off road reduction gearing is utilized to reduce the vehicle’s speed to a manageable crawl and to increase (multiply) the supplied torque coming from the engine. Thus the drive wheels have ample torque to move the vehicle at low speeds. It is also used to control downward speeds while descending steep gradients and to improve the vehicle’s off road crawl ratio.

4 High and 4 Low modes cannot be used on pavement as driveline binding and torque windup can occur, causing damage to the four-wheel-drive system.


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.

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.


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