Difference between four wheel ABS and rear wheel ABS

Four wheel ABS is used on passenger cars and some light trucks and vans. Rear wheel ABS is used only on trucks. Rear wheel antilock systems are typically used on applications where rear wheel traction is affected by vehicle loading. Rear wheel ABS systems are simpler and less costly
than their four wheel counterparts.

On a four wheel application, the ABS system keeps track of wheel deceleration rates with wheel speed sensors. Some have one speed sensor at each wheel while others use a common sensor in the differential or transmission for both rear wheels.

With rear wheel ABS, only a single wheel speed sensor in the differential or transmission is used for both rear wheels.

Four wheel ABS systems include those made by Bendix, Bosch, Delco Moraine, and Teves. Most rear wheel ABS systems are made by Kelsey-Hayes, though Kelsey-Hayes also makes some four wheel systems.

Kelsey-Hayes rear wheel ABS systems have been in use since 1987 on Ford F series trucks, as well as later model Ranger, Bronco, Bronco II and Explorer trucks and Aerostar vans. Ford calls their version the Rear-wheel Antilock Brake System or RABS system.

On General Motors applications, it is called the Rear Wheel Antilock or RWAL system. It is on ’88 and later Chevrolet "C" and "K" series pickups, ’89 "M" series (Astro) minivans and "S" and "T" series pickups, some "S" series Blazers, and ’90 to ’92 "R" and "V" series light trucks and "G" series vans. Dodge has used the RWAL system since 1989 on its "D" and "W" 150/350, Dakota and Ram Charger pickups.

Kelsey-Hayes RABS and RWAL systems are nonintegral rear wheel only antilock brake systems. The conventional master brake cylinder and power booster supply brake pressure to a dual solenoid control valve for the rear brakes.

The ABS control module receives a speed signal from a single vehicle speed sensor. On Ford and Dodge applications, the sensor is in the differential. On GM, it is located in the transmission
tail shaft.