Anti-Lock Brake System: Description
The ABS-VI Anti-Lock Brake System (ABS) uses conventional hydraulic brake system components, plus anti-lock components. ABS components include a hydraulic modulator assembly, Electronic Brake Control Module (EBCM), enable relay, 4 wheel speed sensors and an ABS warning light. See Fig 1.
The hydraulic modulator assembly consists of ball screws, nuts, pistons, hydraulic check valves, drive gear subassemblies and motor pack. The EBCM is the controlling component in the ABS system. Inputs to EBCM include 4 wheel speed sensors, brake switch, ignition switch and unswitched battery voltage. Outputs include 3 bidirectional motor controls, 2 solenoid controls and an enable relay. A serial data link (terminal "M" of ALDL connector) is provided for service diagnostic tools and assembly plant testing. If a system malfunction is detected, EBCM will store a trouble code in nonvolatile memory (memory is not erased when battery is disconnected).
Front and rear wheel speed sensors are integral with hub and wheel bearing assemblies. When a toothed ring passes wheel speed sensor, an AC voltage with a frequency proportional to wheel speed is produced. This AC voltage signal is used by EBCM to detect wheel lock-up.
Enable relay is a normally open relay. This relay has special contact material to handle high currents required for ABS-VI operation. Enable relay supplies battery voltage to EBCM, which supplies battery power to motors and solenoids.
ABS-VI contains sophisticated on-board diagnostics which can only be accessed with a bidirectional scan tester. These on-board diagnostics are designed to identify the source of a system fault as specifically as possible, including whether a fault code is intermittent or history. There are 58 diagnostic codes available for the diagnostician to use.