GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

System Operation


Certain components in the ABS are not intended to be serviced individually. Attempting to remove or disconnect certain system components may result in personal injury and/or improper system operation. Only those components with approved removal and installation procedures should be serviced.

The use of rubber hoses or parts other than those specified for the Antilock Brake System (ABS) may lead to functional problems requiring replacement of the hydraulic parts. Replace all components included in the repair kits used to service this system. Lubricate the rubber parts with clean, fresh brake fluid to ease assembly, Do not use lubricated shop air on brake parts as damage to rubber components may result. If any hydraulic component is removed or brake line disconnected, it may be necessary to bleed all or part of the brake system.

Use only DOT 3 brake fluid. The use of DOT 5 (silicone) brake fluid is not recommended. Reduced braking performance or durability may result.

Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded. Also, do not allow any brake fluid to encounter a painted surface; it will damage the paint.

Under normal operating conditions, the brake systems will operate using conventional braking by means of brake pedal force, the vacuum booster, and the compact master cylinder. Each front channel consists of a motor, solenoid ESB, ball screw piston, and check valve. Under normal operating conditions (base brakes), the piston is held in the upmost (or "home") position and the solenoid is open (not energized). This is accomplished by turning the ball screw via the motor to drive the nut upwards. Once at the upmost position, the piston is held by an Expansion Spring Brake (ESB). Two paths are available to transfer braking pressure to the wheel: (1) through the modulator, around the check valve, and out to the wheel, (2) through the modulator, past the normally open solenoid, and out to the wheel. The solenoids in the front circuits provides an alternate brake pressure path to the wheel. With this arrangement, if the ABS were to lose power or malfunction with the piston not in its home position, a redundant brake fluid path is available. The rear channels operate in a similar manner except that no solenoid exists.

See Figures 1 and 2

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Fig. Fig. 1: View of the Anti-lock Brake System (ABS) components

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Fig. Fig. 2: Some ABS components have electrical connectors equipped with a Connector Position Assurance (CPA) lock to make sure they fasten securely

ABS has been deigned to improve the controllability and steerability of a vehicle during braking. ABS accomplishes this objective by controlling the hydraulic pressure applied to each wheel brake. Antilock braking occurs only when the brake switch is closed and a microprocessor, located in the Electronic Brake Control Module (EBCM), determines one or more wheels is about to lose traction during braking. The ECBM will then allow the ABS hydraulic modulator to change the brake pressures several times each second to keep the wheel(s) from locking and provide the driver with maximum vehicle controllability. ABS does not necessarily decrease the stopping distance of the vehicle; it allows you to steer the vehicle with more control than you would have if the wheels were locked up.

ABS cannot increase the brake pressure above the master cylinder pressure applied by the driver, and cannot apply the brakes by itself.