2004 Subaru Legacy Anti-lock Brake System Inspect Cost

Since the late 1980's, anti lock brakes have become more and more common, and are now a system that is expected to be included on most new automobiles. This important safety feature prevents the wheels from locking up under heavy braking or slippery road conditions. Since the wheels are not permitted to stop while the vehicle is in motion, the vehicle is much less likely to slide, especially in poor road conditions. While preventing sliding is the main concern, a nice byproduct of the anti lock brake system is that stopping distance in slippery conditions is greatly reduced.

The anti-lock brake system is made up of several components. At a minimum, the system will include wheel speed sensors, the anti-lock brake control module, the modulator valve, which is a hydraulic control unit, and the complete service brake system all function together in order to achieve the functions listed above. The control module needs information from the wheel speed sensors to understand the wheel speeds as compared to the other wheels, and it uses this information to effect change in braking force for each wheel via the modulator valve. The modulator valve receives pressure from the brake master cylinder when the brakes are applied. I the modulator valve needs to reduce hydraulic pressure from one wheel, the adjoining pump will recreate the pressure needed to reapply the brakes for that wheel position upon the control module's requests. The brake fluid is pushed under great pressure through the service brake lines, just as it would be for traditional brakes, except in the event of slippage, the brake fluid will be sent in pulses to prevent wheel lock-up.

For normal driving, the only symptom may be the anti-lock brake system warning lamp illuminated on the driver instrument cluster. This signifies that the anti-lock brake system (ABS) has an error, but it must be identified through scanning or diagnostic testing. Most times, the traction control warning lamp will illuminate as well, because these systems are reciprocal in their operation, and dependent on many the same components. In some situations, the vehicle may hesitate to accelerate, and driving may feel jerky, especially when braking. This is because a faulty wheel speed sensor will signal that a wheel is slipping, though it is not. Finally, skidding and sliding are nearly certain results of ABS failure, as drivers may rely on ABS in slippery conditions without realizing it.