REMOVAL & INSTALLATION
See Figure 1
When the ABS modulator cylinder pistons are in their uppermost position, each motor has prevailing torque due to the force necessary to ensure each piston is held firmly at the top of its travel. This torque results in gear tension, or force on each gear that makes motor pack separation difficult. To avoid injury, or damage to the gears, the Gear Tension Relief Sequence briefly reverses each motor to eliminate the prevailing torque. This procedure is one of the many functions of GM's Tech 1® scan tool. Use care when using a substitute. In general, make sure the ignition switch is in the OFF position. Install the Tech 1® or equivalent with the correct chassis cartridge. Turn the ignition switch to the ON position, leaving the engine OFF. Select the proper function. The Gear Tension Relief Sequence is F5 on the Tech 1® scan tool. Note that this same scan tool is needed to bleed the system after repairs to the hydraulic system.
To perform the Gear Tension Relief Sequence, a Tech 1® tool or equivalent scan tool must be used prior to removal of the ABS modulator/master cylinder assembly.
- Perform gear tension release procedure using the Tech 1® scan tool or equivalent.
- Remove the battery.
- Remove the air cleaner assembly.
- Detach the electrical connector from fluid level switch.
- Detach the electrical connectors from the ABS solenoids.
- Detach the 3-pin and 6-pin motor pack electrical connectors from the ABS hydraulic modulator.
- Remove the 4 brake pipe tube nuts from master cylinder and modulator assembly. If necessary, tag for identification and placement. Place shop cloths on top of the motor pack to catch any dripping fluid. Take care not to allow brake fluid to enter the bottom of the motor pack or the electrical connectors.
When disconnecting the brake lines from the master cylinder and removing the unit, use care not to spill brake fluid on any painted surfaces or electrical connectors. Once the brake lines are disconnected from the master cylinder, plug the lines to prevent excess brake fluid loss and contamination.
- Remove the master cylinder mounting nuts. It may be necessary to remove the vacuum check valve from the vacuum booster to access the nut closest to the check valve.
- Remove the master cylinder and modulator as an assembly.
Separate the modulator assembly from the master cylinder assembly. Use the following procedure.
- Turn the assembly upside down so the flat gear cover is facing up.
- Remove the 6 gear cover attaching screws. This should allow access to the motor pack screws.
- Remove the 4 motor pack screws and separate the motor pack from the ABS modulator assembly.
- Remove the 2 Torx head through bolts that retain the master cylinder to the motor pack assembly. Use care not to loose or damage the 2 small transfer tubes that connect between the lower part of the master cylinder assembly and the hydraulic modulator assembly. Watch for O-rings seals at both the transfer tubes and the Torx head attaching bolt openings. The transfer tubes and all O-rings must be replaced with new parts whenever the master cylinder and modulator are separated.
- Clean all parts well. Use new O-rings and transfer tubes to assemble the master cylinder to the modulator. If the hydraulic modulator is to be replaced, install the 3 gears in the same location on the replacement modulator. Note that no repair of the hydraulic modulator is authorized.
- Make sure 2 O-rings are properly installed on each transfer tube. Lubricate the O-rings with clean brake fluid. Install the transfer tube assemblies in the ports in the hydraulic modulator and push in by hand until they bottom.
- Lubricate with clean brake fluid, new O-rings for the 2 Torx head through bolts and install in both the master cylinder openings and the modulator. Assemble the modulator to the master cylinder. It may be helpful to clamp just the mounting flange of the master cylinder in a soft-jaw vise. Hold the modulator and rock it into position on the master cylinder, inserting the transfer tube assemblies into the master cylinder ports. Install the 2 Torx head through bolts and torque to 18 ft. lbs. (24 Nm).
- With the hydraulic modulator upside down and the gears facing up, rotate each gear counterclockwise until movement stops. This procedure will position the piston very close to the top of the hydraulic modulator bore.
- Assemble the motor pack to the hydraulic modulator. Install the gear cover.
- Install the master cylinder and modulator assembly to the power booster assembly and tighten the mounting nuts to 20 ft. lbs. (27 Nm).
- Connect the brake lines to the master cylinder and tighten them to 17 ft. lbs. (23 Nm).
- Attach the electrical connector to the fluid level switch.
- Connect the electrical connectors for both of the solenoids.
- Attach the 3-pin and the 6-pin motor pack electrical connectors.
- Fill the master cylinder to the proper level. The proper level is the MAX level indicator on the reservoir.
- Bleed the hydraulic brake system.
- Road test vehicle and verify proper operation.
- Secure the master cylinder in a bench vise using soft jaws.
- Remove the master cylinder reservoir cap.
- Manufacture or purchase bleeding tubes and install them on the master cylinder.
- Fill the master cylinder reservoir with clean, fresh brake fluid until the level is within 0.25 in. of the reservoir top.
Ensure the bleeding tubes are below the level of the brake fluid, otherwise air may get into the system making your bleeding efforts ineffective.
- Use a blunt tipped rod (a long socket extension works well) to slowly depress the master cylinder piston. Make sure the piston travels full its full stroke.
- As the piston is depressed, bubbles will come out of the bleeding tubes. Continue depressing and releasing the piston until all bubbles cease.
- Refill the master cylinder with fluid.
- Remove the bleeding tubes.
- Install the master cylinder reservoir cap.
- Install the master cylinder on the vehicle.