Description & Operation: Notes

WARNING: Vehicles are equipped with air bag supplemental restraint system. Before attempting ANY repairs involving steering column, instrument panel or related components, see SERVICE PRECAUTIONS and DISABLING & ACTIVATING AIR BAG SYSTEM in appropriate AIR BAG RESTRAINT SYSTEMS article.

WARNING: The accumulator contains high pressure nitrogen gas, do not puncture, expose to flame, or attempt to disassemble the accumulator or it may explode; severe personal injury may result.

CAUTION: When battery is disconnected, vehicle computer and memory systems may lose memory data. Driveability problems may exist until computer systems have completed a relearn cycle. See COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION before disconnecting battery.

NOTE: For more information on brake system, see DISC article.

NOTE: Anti-lock Brake System (ABS) may also be referred to as Anti-Lock Brakes (ALB).

The Anti-lock Brake System (ABS) prevents locking of wheels as a result of heavy braking and, or poor traction. A vehicle with locked wheels cannot be steered. ABS controls application of brakes, allowing driver to maintain control of steering. Under anti-lock conditions, ABS modulates pressure of brake fluid to each brake caliper. This system is a four channel system. Pressure at each caliper is controlled independently of pressure at any other caliper. Whenever a wheel is likely to lock during braking, ABS modulates brake pressure at that wheel. When brake pressure is reduced to the point where there is no longer any possibility of wheel locking, system returns to conventional braking system mode of operation.

Battery voltage is applied at all times from fuse 32 and fuse 40 to ABS control unit. When ignition is in RUN position, battery voltage is supplied to control unit from fuse No. 4. Control unit is case grounded. Inputs are received from brake light switch, alternator, brake indicator light, individual speed sensors, located at each wheel and from ABS pressure switch. With charging and brake systems functioning correctly, control unit can activate the fail safe relays, modulator solenoids and ABS motor on demand. ABS control unit uses inputs from brake switch and wheel speed sensors to control modulator solenoid unit. Solenoids adjust hydraulic pressure applied to each caliper. ABS control unit has a self-diagnosis function. When self-diagnosis function detects a fault, control unit turns on ANTILOCK (1991-92), ABS (1993-96) or ABS icon (Canada) indicator and disables ABS. If fault is not in conventional braking system, brakes will continue to operate normally but without ABS feature.

In a conventional brake system, if brake pedal is depressed excessively, the wheels can lock before vehicle comes to a stop. In such a case, stability of vehicle is reduced if rear wheels are locked, and maneuverability of vehicle is reduced if front wheels are locked, creating an extremely unstable condition. Anti-lock Brake System modulates pressure of brake fluid applied to each caliper, thereby preventing locking of the wheels, whenever wheels are likely to be locked due to excessive braking. It then restores normal hydraulic pressure when there is no longer any possibility of wheel locking.

4-Channel anti-lock brake system (ABS) features:

Increased braking stability can be achieved regardless of changing driving conditions.
Maneuverability of vehicle is improved as the system prevents front wheels from locking.
When anti-lock brake system goes into action, kickback is felt on brake pedal.
Anti-lock brake system is equipped with a self-diagnosis function.
When an abnormality is detected, the system indicator light illuminates. The location of system's trouble can be diagnosed from the frequency of the system indicator light display blinks.
This system has individual control of front and rear wheels. Braking force is controlled on each of the four wheels independently, resulting in optimum braking effect on the four wheels. The system has a fail-safe function that allows normal braking if there's a problem with the anti-lock brake system.

In addition to conventional braking system, anti-lock brake system is composed of:

Gear pulsers attached to rotating part of individual wheels.
Speed sensors, which generate pulse signals in correspondence to the revolution of the gear pulsers.
Control unit, which controls the working of anti-lock brake system by performing calculations based on the signals from individual speed sensors and individual switches.
Modulator unit, which adjusts the hydraulic pressure applied to each caliper on the basis of the signals received from the control unit; an accumulator, in which high-pressure brake fluid is stored.
A pressure switch, which detects pressure in the accumulator and transmits signals to control unit.
A power unit, which supplies high-pressure working fluid to the accumulator by means of a pump.
A motor relay for driving power unit.
A fail-safe relay, which cuts off the solenoid valve ground circuit when fail-safe device is at work.
An anti-lock brake system indicator light.

Fig 1: Difference Between 3-Channel & 4-Channel Anti-Lock Brake System

Courtesy of AMERICAN HONDA MOTOR CO., INC.