Antilock Brakes / Traction Control Systems: Service and Repair

Running ABS Motors


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).






The PDT Run ABS Motors Special Test will allow you to command the Anti-Lock Brake modulator pistons to their TOP (home) or Down (tension released) positions.

CAUTION: DO NOT DRIVE OR OPERATE THE VEHICLE WITH THE ABS BRAKE CONTROL ASSEMBLY IN THE TENSION RELEASED POSITION. EXCESSIVE BRAKE PEDAL TRAVEL AND REDUCED BRAKE EFFECTIVENESS COULD OCCUR WITH THE TENSION RELEASED. ALWAYS PERFORM THE PDT SPECIAL TEST, RUN ABS MOTORS, PISTONS UP - HOME, BEFORE DRIVING THE VEHICLE.

Select Run ABS Motors from the PDT Special Test Menu. The PDT will display two options:
^ PISTONS Up - HOME
^ PISTONS Down - RELEASE

Select PISTONS UP-HOME with ignition on to bleed rear brakes, and after any motor repair.

Select PISTONS DOWN-RELEASE to remove a drive gear or the motor pack.

Follow the procedures provided for ABS repair.

With the tension released and the pistons free to travel down the ball screws, applying the brake pedal will require fluid from the master cylinder to first displace the modulator pistons and fill the modulator piston bores. This will cause excessive brake pedal travel and severely reduced brake effectiveness on initial application. Pumping the brake pedal will restore some braking force. Once the vehicle reaches approximately 13 km/h (8 mph), the antilock brake system will initialize and force the pistons back to their upmost or HOME position.