Special Function Tests

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.