Such systems employ wheel-speed sensors used for antilock brakes to detect wheelspin, clamping the brakes to stop the offending wheel in a fraction of a rotation. "Of course, when you're driving off-road, the amount of wheelslip is one of the things you notice the most," says Mike Gabriel, manager of Jeep vehicle development.
Electronic systems have a lighter touch for more precise control than mechanical limited-slip differentials, according to Robert Beaver, manager of the system integration group at technology supplier Continental Teves. "Some of those have fairly low breakaway torques in situations like climbing a hill," when it's too easy for them to spin the wheel with less grip, he explains. Another benefit is that electronic systems can be cheaper and lighter than mechanical limited-slip differentials.
Many of today's most sophisticated transfer cases use an electronically controlled center differential (taking inputs from several different sensors from around the SUV) to control how much traction is sent to the front driveshaft. This center diff clutch pack is from a Grand Cherokee.
"The electronic limited-slip in the Grand Cherokee is a good example of that," says Gabriel. "Quadra-Trac II was a good system, but the differentials were controlled by wheelspin to drive the pump. Now [with electronic sensors, and not hydraulic pressure] we can detect lower levels of wheelspin and use electronics to drive the pump to transfer the power to the other wheels."
Ford trucks are gaining a smart transfer case to split power 50/50 between the front and rear axles, as well as send all of the available power to either end of the truck if needed, according to Alan Dona, driveline engineering manager for SUV body-on-frame at Ford. The computer-controlled transfer case, which was developed in partnership with Borg-Warner, was introduced on the Expedition and made its first pickup-truck appearance on the new Harley-Davidson F-150. It should move into other versions of the truck over time.
Alternately, the Hummer H2 SUV and SUT use an open center differential, using electronic controls at each wheel to divvy up power when only one wheel has traction, says Bill Knapp, program engineering manager for Hummer H2. The rear differential also can be electronically locked.