Anti-lock braking (ABS) systems are designed to prevent locked-wheel skidding during hard braking or during braking on slippery surfaces. The front wheels of a vehicle cannot apply steering force if they are locked and sliding; the vehicle will continue in its previous direction of travel. The four wheel anti-lock brake systems found on Acura automobiles hold the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways.
The system is designed to prevent wheel lock-up during hard or emergency braking. By preventing wheel lock-up, maximum braking effort is maintained while preventing loss of directional control. Additionally, some steering capability is maintained during the stop. The ABS system will operate regardless of road surface conditions.
There are conditions for which the ABS system provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. This renders the vehicle totally uncontrollable until road contact is regained. Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking. For this reason, the system is named anti-lock rather than anti-skid. Wheel spin during acceleration on slippery surfaces may also fool the system into entering its fail-safe mode.
The acronyms ALB and ABS are used interchangeably in this section. They both denote Anti-lock Brake System.
Under normal braking conditions, the ABS system functions in the same manner as a standard brake system. The system is a combination of electrical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary.
The ABS Control Unit is the electronic brain of the system, receiving and interpreting speed signals from the speed sensors. The unit will enter anti-lock mode when it senses impending wheel lock at any wheel and immediately controls the brake line pressure(s) to the affected wheel(s). The modulator assembly is separate from the master cylinder and booster. It contains the wheel circuit solenoid valves used to control the brake fluid pressure to each wheel circuit.
During anti-lock braking, line pressures are controlled or modulated by the rapid cycling of electronic valves within the modulator. These valves can allow pressures within the system to increase, remain constant or decrease depending on the needs of the moment as registered by the control unit. The front wheels are controlled individually. The rear wheel circuits receive the same pressure control, based on the rear wheel with the greatest locking tendency.
The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during anti-lock operation. The sounds are due to normal operation and are not indicative of a system problem. Under most conditions, the sounds are only faintly audible. When ABS is engaged, the operator may notice some pulsation in the brake pedal and/or body of the vehicle during a hard stop; this is also normal operation but can be surprising to an operator who engages ABS for the first time.
Although the ABS system prevents wheel lock-up under hard braking, as brake pressure increases, wheel slip is allowed to increase as well. This slip will result in some tire chirp during ABS operation. The sound should not be interpreted as lock-up but rather than as indication of the system holding the wheel(s) just outside the point of lock-up. Additionally, the final few feet of an ABS-engaged stop may be completed with the wheels locked; the electronic controls do not operate at very low speeds.
When the ignition is ON and the vehicle's speed is over 6 mph, the control unit monitors the function of the system. Should a fault be noted, such as loss of signal from a speed sensor or excessive pump running time, the ABS system is immediately de-energized. Normal brake function remains but the anti-lock function is disabled. One or more diagnostic codes are assigned to the fault(s) and the dash warning lamp is illuminated. The diagnostic codes are stored within the control unit's memory for future retrieval.
Wheel Speed Sensors
See Figure 1
The speed signal for each wheel is sent to the control unit through the wheel speed sensor. A small sensor unit is fixed in position over a toothed ring or pulser wheel which rotates with the axle or road wheel.
The wheel speed sensors generate an AC voltage as the toothed ring passes. The frequency of this voltage-which increases with wheel speed-is used by the control unit to determine wheel speed. By comparing wheel speeds during braking, the control unit determines impending wheel lock.Control Unit
See Figure 2
The control unit performs the main function of controlling the modulator solenoid valves based on electrical signals from the wheel sensors. Additional or sub-functions of the control unit include self-diagnosis, pump motor control and fail-safe (disable) functions.
Additionally, the ABS control unit receives signals from the parking brake switch, the charging system, the ignition switch, the brake lamp switch, the pump motor relay and the fail-safe relays. Each of the signals is used to aid the control unit in viewing system operation. A fault in any of the related inputs can disable the ABS system.Hydraulic Modulator
See Figure 3
This assembly contains the electric solenoid valves for each brake circuit. The control unit controls each solenoid individually (one for each front wheel, one for both rear wheels) based on the signals from the wheel speed sensors.
The modulator receives extremely high pressure brake fluid from the accumulator. This high pressure fluid is admitted into the system through a solenoid valve and used to increase brake line pressure if necessary. Separate channels allow fluid to escape if pressure must be reduced. Additionally, the assembly contains 4 modulators or spring-loaded pistons, which serve to dampen the high pressure pulses within the system.
Under some conditions system pressure may be over 3000 psi. For this reason, extreme caution must be used when working on the modulator or its related lines and fittings. Always properly relieve the system pressure before any work is performed.Accumulator and Pressure Switch
See Figure 4
The accumulator stores brake fluid from the pump under high pressure, making it available to the system when necessary. When anti-lock control is called for by the control unit, pressurized fluid is released into the control fluid chamber of the modulator through the inlet solenoid valve. The ball-shaped accumulator is mounted separate from the modulator unit.
The pressure switch, mounted with the accumulator, monitors the pressure within the accumulator and transmits a signal to the control unit. Should the pressure drop below minimum requirements, usually from anti-lock engagement, the control unit signals the pump to engage and rebuild pressure. If correct pressure is not achieved after the pump has run 120 seconds, the control unit disables the ABS system and triggers the dashboard warning lamp.Pump Motor (Power Unit)
See Figure 5
The pump and motor for the ABS system is not part of the modulator; it is mounted separately and found adjacent to the accumulator or as a unit with the accumulator. The power unit consists of a motor and a pump. The unit creates high-pressure brake fluid and sends it to the accumulator for storage. The control unit will cycle the pump each time the vehicle is started and speed exceeds 6 mph. The pump maintains accumulator pressure at a maximum of 3,271 psi or 230 kg/cm. The pump motor is protected by a fuse in the fuse or relay box.Motor and Fail-Safe Relays
See Figures 6 through 13
The three system relays are controlled by the control unit. Each relay connector is color-coded; the motor relay connector is green, the front fail-safe relay uses a yellow connector and the fail-safe relay for the rear circuit is in a pink connector. Care must be taken not to confuse the yellow connector for the front fail-safe circuit with any of the yellow connectors used in the air bag circuits. The motor relay is protected by a fuse in the interior fuse box.
The fail-safe relays energize the output control solenoids within the modulator. If the control unit detects a fault within the ABS system, the relays are de-energized. The solenoids default to the full-open position, providing no pressure control. The fail-safe relays are also protected by a fuse.Dash Warning Lamp
The amber dashboard warning lamp signals the driver that an ABS fault has been detected and that anti-lock function has been disabled. On Canadian vehicles, the light reads ABS; U.S. vehicles use ANTILOCK as a warning. The dash warning lamp will illuminate: