Control Trac - The Modes

The Modes

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

• Two High mode

Predominantly, Explorer was not equipped with Two High mode except in the 1995-1996 model years, where no Four High was available. Only Auto, Four High and Four Low modes were offered. Two High mode was unique to the Expedition, though it was discontinued between the 1999–2002 model years. Two 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 locking 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 Two High mode could help increase fuel economy up to half-a-mile per gallon of gasoline. In Two 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, full-time four-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 speed. 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, torque is sent forward to the front differential in 10 percent increments, via the center multi-disc clutch. As it does so, intelligent control software allows the center multi-disc 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, virtually undetectable by the occupants of the vehicle.

In the early 2000’s, ControlTrac was updated and introduced in 2002 with more advanced software programming, building on the system’s artificial intelligence. The four-wheel drive system's updated artificial intelligence allowed the system to predict traction loss before it happened, so that torque can be transferred before it was needed. This improvement meant the system could operate more like other "always-on" full-time four-wheel drive systems as it no longer had to "wait" for traction loss to take action. Another improvement was "torque biasing" capability in Auto mode. ControlTrac's intelligent multi-disc differential could now send all 100 percent of the engine’s torque forward, biasing it to the front differential if severe traction loss was anticipated, predicted, or detected.

• Four High and Four Low modes

Four High and Four Low modes were also featured on both Explorer and Expedition. Four High mode tells the intelligent locking multi-disc differential to lock, providing a permanently locked 50:50 torque distribution. The front and rear drive shafts are fully locked, forcing them to rotate at the same speed regardless of tractive conditions. Four Low mode also tells the intelligent locking multi-disc differential to lock, however it instructs the BorgWarner transfer case to select low range off road reduction gearing. 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.

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