What is the 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 tailshaft.
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