GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

General Information


The Anti-lock Braking System (ABS) was first introduced on N-body cars in 1991. ABS provides the driver with 3 important benefits over standard braking systems: increased vehicle stability, improved vehicle steerability, and potentially reduced stopping distances during braking. It should be noted that although the ABS-VI system offers definite advantages, the system cannot increase brake pressure above master cylinder pressure applied by the driver and cannot apply the brakes itself.

The ABS-VI Anti-lock Braking System consist of a conventional braking system with vacuum power booster, compact master cylinder, front disc brakes, rear drum brakes and interconnecting hydraulic brake lines augmented with the ABS components. The ABS-VI system consists of a hydraulic modulator assembly, Electronic Control Unit (ECU)/Electronic Brake Control Module (EBCM), a system relay, 4 wheel speed sensors, interconnecting wiring and an amber ABS warning light.

The ECU/EBCM monitors inputs from the individual wheel speed sensors and determines when a wheel or wheels is/are about to lock. The ECU/EBCM controls the motors on the hydraulic modulator assembly to reduce brake pressure to the wheel about to lock. When the wheel regains traction, the brake pressure is increased until the wheel again begins to lock. The cycle repeats until either the vehicle comes to a stop, or the brake pedal is released. The ECU/EBCM also has the ability to monitor itself and can store diagnostic codes in a non-volatile (will not be erased if the battery is disconnected) memory. The ECU/EBCM is serviced as an assembly.

The ABS-VI braking system employs 2 modes: base (conventional) braking and anti-lock braking. Under normal braking, the conventional part of the system stops the vehicle. When in the ABS mode, the Electromagnetic Brakes (EMB) action of the ABS system controls the two front wheels individually and the rear wheels together. If the one rear wheel is about to lock up, the hydraulic pressure to both wheels is reduced, controlling both wheels together. Since the vast majority of the braking is controlled by the front wheels, there is no adverse effect on vehicle control during hard braking.