Special Functions Procedures

Some of the following service procedures for the ABS BRAKE CONTROL ASSEMBLY require that the ABS motors be run up and/or down using the portable diagnostic tool (PDT) or its equivalent.

The ABS modulator pistons are normally at their top most or HOME position when the antilock brake system is not in operation. In the HOME position, the rear displacement pistons open check valves at the top of the modulator allowing unrestricted brake fluid flow from the master cylinder to the rear brakes. The pistons must be in this position to bleed the rear brakes.

When the pistons are in the HOME position, the drive gears are under spring tension from expansion spring brakes (EBS) which prevent the motors from back-driving. Removing a drive gear or the motor pack with the pistons in this position will result in a sudden release of tension and rotation of the gears as they loose mesh. This sudden and quick rotation of the gears could cause injury if done without care; therefore, it is recommended that the tension be released prior to performing service to the gears, motor pack, or modulator.

To release tension, use the PDT or equivalent to perform SPECIAL TEST, RUN ABS MOTORS, PISTONS, DOWN-REL (TENSION RELEASE).