GM Century/Lumina/Grand Prix/Intrigue 1997-2000

ABS Brake Modulator/Master Cylinder Assembly


The ABS brake modulator/brake motor pack assembly controls the hydraulic pressure to the front calipers and rear calipers (or rear wheel cylinders, as equipped) by modulating the hydraulic pressure to reduce the tendency of wheel lock-up.

The basic ABS brake modulator configuration consists of gear subassemblies, ball screws, nuts, pistons and hydraulic check valves. The ABS brake motor pack consists of three motors, three drive gears and three Expansion Spring Brakes (ESBs). The ESBs are used to hold the pistons in the upmost (or home) position. An ESB is a spring that is retained in a housing at a close tolerance. One end of the spring is in contact with the motor drive dog and the other end is in contact with the pinion drive dog. In normal braking, brake pressure is present on the top of the piston, applying a downward force. The force applies a counterclockwise torque to the motor pinion which tries to rotate the spring counterclockwise. The counterclockwise torque expands the spring outward within the housing and prevent gear rotation


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Fig. View of the ABS hydraulic modulator/master cylinder assembly

To avoid personal injury, due to a retained spring load on the brake modulator, the gear tension relief function of a scan tool must be performed prior to removal of the ABS Brake Modulator/Master Cylinder assembly.

Proper operation of any ABS system depends on clean DOT 3 brake fluid and a system free of moisture and any dirt or foreign material. Clean all of the brake line connections well and use great care to keep the any contamination out of the system.

  1. Using a Tech 1® or equivalent scan tool, perform the Gear Tension Relief Sequence.
  3. Disconnect the negative battery cable.
  5. Detach the two brake solenoid valve electrical connectors.
  7. Detach the brake fluid level indicator sensor electrical connector.
  9. Detach the ABS brake motor pack 6-way electrical connector.
  11. Using a brake line wrench, disconnect the four brake lines. Cap the disconnected lines to prevent the loss of fluid and the entry of moisture and contaminants.
  13. Remove the two nuts holding the ABS brake modulator/master cylinder assembly to the vacuum booster and remove the assembly from the vehicle.
  15. Installation is the reverse of the removal process. Torque the nuts holding the ABS brake modulator/master cylinder assembly to the vacuum booster to 20 ft. lbs. (27 Nm) and the brake line tube nuts to 18 ft. lbs. (24 Nm). Verify that all electrical connectors are secure.
  17. Properly bleed the system, as outlined later in this information.
  19. Connect the negative battery cable.