Subaru Coupes/Sedans/Wagons 1985-1996 Repair Guide

General Description


See Figures 1, 2 and 3

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Fig. Fig. 1: ABS operational schematic

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Fig. Fig. 2: Component schematic-Nippon ABS system

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Fig. Fig. 3: Component schematic-Bosch ABS system

Anti-lock braking systems (ABS) 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 Subaru vehicles hold the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways under braking.

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.

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 anti-lock brake system electronic control unit (ECU) is the electronic brain of the system, receiving and interpreting speed signals from the speed sensors. The ECU will enter anti-lock mode when it senses impending wheel lock at any wheel and immediately control the brake line pressure(s) to the affected wheel(s). The hydraulic actuator assembly is separate from the master cylinder and booster. It contains the wheel circuit valves used to control the brake fluid pressure to each wheel circuit. If the ABS becomes inoperative for any reason, the fail-safe system insures that the normal braking system is operative. The dashboard warning lamp is activated to show that the ABS is disabled.


The Subaru Legacy anti-lock brake system uses a 4-sensor, 4- channel system. A speed signal for each wheel is generated by a speed sensor at the wheel. The hydraulic actuator contains 4 control solenoids, one for each wheel brake line. The system is capable of controlling brake line fluid pressure to any or all of the wheels as the situation demands.

When the ECU receives signals showing one or more wheels about to lock, it sends an electrical signal to the solenoid valve(s) within the actuator to release the brake pressure in the line. The solenoid moves to a position which holds the present line pressure without allowing it to increase. If wheel deceleration is still outside the pre-programmed values, the solenoid is momentarily moved to a position which releases pressure from the line. As the wheel unlocks or rolls faster, the ECU senses the increase and signals the solenoid to open, allowing the brake pedal to increase line pressure.

This cycling occurs several times per second when ABS is engaged. In this fashion, the wheels are kept just below the point of lock-up and control is maintained. When the hard braking ends, the ECU resets the solenoids to its normal or build mode. Brake line fluid pressures are then increased or modulated directly by pressure on the brake pedal. Fluid released to the ABS reservoirs is returned to the master cylinder by the pump and motor within the actuator.

The front and rear wheels are controlled individually, although the logic system in the ECU reacts only to the lowest rear wheel speed signal. This method is called Select Low and serves to prevent the rear wheels from getting greatly dissimilar signals which could upset directional stability.

The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during normal 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. If ABS is engaged, the operator may notice some pulsation in the body of the vehicle during a hard stop; this is generally due to suspension shudder as the brake pressures are altered rapidly and the forces transfer to the vehicle.

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 below about 3 mph.


See Figures 4, 5, and 6

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Fig. Fig. 4: Location of ABS system components

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Fig. Fig. 5: Front ABS components

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Fig. Fig. 6: Rear ABS components

Wheel Speed Sensors

See Figure 7

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Fig. Fig. 7: The mounted speed sensor's magnetic flux changes with variations in the tone wheel speed

The speed of the front and rear wheels is monitored be the sensor. A toothed tone wheel rotates in front of the sensor, generating a small AC voltage which is transmitted to the ECU. The controller compares the signals and reacts to rapid loss of wheel speed at a particular wheel by engaging the ABS system. Each speed sensor is individually removable. In most cases, the toothed wheels may be replaced if damaged, but disassembly of other components such as hub and knuckle, constant velocity joints, or axles may be required.

ABS Electronic Control Unit (ECU)

See Figure 8

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Fig. Fig. 8: An example of an ABS control unit-Legacy

The solid-state control unit computes the rotating speed of the wheels by the signal current sent from each sensor. When impending lock-up is detected, the ECU signals the actuator solenoids to move to predetermined positions to control brake fluid pressure to the wheels. The control unit also controls the on-off operation of the solenoid valve relay and the pump relay.

The ECU constantly monitors the function of components within the system. If any electrically detectable fault occurs, the control unit will illuminate the dashboard warning light to alert the operator. When the dash warning lamp is lit, the ABS system is disabled. The vehicle retains its normal braking capabilities without the benefit of anti-lock.

The ECU will assign and store a diagnostic or fault code. The code may be read and used for system diagnosis. If more than one fault occurs, the system will only display the first code noted. Repairs must be made based on the first code, after which the vehicle must be road tested to expose any subsequent faults.

The ECU is located under the right front seat.

ABS Hydraulic Unit

See Figure 9

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Fig. Fig. 9: An example of a hydraulic control unit-Nippon-type mounted in a Legacy

The actuator contains the solenoid control valves, pump and motor, reservoirs for temporary collection of brake fluid released from the lines as well as check and relief valves. The actuator is located at the right front of the engine compartment.

The relays and solenoids are controlled by the ECU. Under normal braking conditions, the solenoids are in the open or pressure-build position, allowing brake fluid to pass proportional to pedal pressure. During anti-lock function, the solenoids are commanded into positions to either hold or release brake fluid line pressures as required. When anti-lock function is no longer needed, the solenoids reset to the normal position. Additionally, if the ECU detects a system fault, the solenoids are immediately set to the normal or default position.

The control relays for the pump motor and solenoid valves are located externally on the actuator case. These relays are the only components on or in the actuator which may be replaced. Any failure within the actuator requires the unit to be replaced.

ABS Warning Lamp

The ABS dashboard warning lamp is controlled by the ABS controller. The lamp will illuminate briefly when the ignition switch is turned ON as a bulb check. The lamp should then extinguish and remain out during vehicle operation. If only the ABS warning lamp illuminates while driving, the controller has noted a fault within the ABS system. ABS function is halted, but normal braking is maintained.

BRAKE Warning Lamp

The red BRAKE warning lamp on the dashboard functions in the usual manner, warning of a fault within the hydraulic system. It is possible that a hydraulic fault within the ABS system will also trigger the BRAKE lamp. If both the ANTI-LOCK and BRAKE warning lamps are illuminated, great care must be taken in operating the vehicle; the function of the conventional brake system may be impaired.

During diagnosis of apparent ABS problems, make certain that the problem is not rooted in the normal brake system.


See Figure 10

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Fig. Fig. 10: G-sensor mounted to the front right fender apron

Found only on 4WD vehicles with manual transmissions, the G-sensor transmits a deceleration signal to the ECU. It is located on the right front wheel well.