1991 Audi 100 Quattro 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.

Any brake system component failure should be repaired right away. The brakes are by far the most effective safety component of any vehicle, and are required to function properly to be considered safe to drive. Yes, the ABS system is an added safety feature, and the service brakes will likely not be affected by the failure of the ABS system, but the vehicles brakes will not perform to the degree to which the driver is accustomed. Also, failures of the system are exacerbated by the cancellation of traction control capabilities, and hard acceleration will likely result in wheel-spin instead of acceleration.

ABS system failures are common, especially when speaking of wheel speed sensors. They are mounted in a high-heat area, and very exposed to elements. The wiring harness, or pigtail, for the ABS wheel speed sensors are exposed to road debris as well, so damage to the pigtails is also common. In general, the ABS warning light is nearly expected to be illuminated some-time in a vehicle's service life, and this may be more commonly due to failure of a wheel bearing and hub assembly. Inspection of the anti-lock brake system should be performed as a portion of the factory scheduled maintenance for every vehicle, and this will include the service brakes. This being the case, an inspection should occur as specified in the owner's manual for each vehicle, but having a failure within 100,000 miles is also very likely.

ABS systems will show the average consumer a code when scanned with a generic code reader, but professional diagnostic software is much more in depth, and is able to access enhanced on-board diagnostic (OBD) trouble codes. With e-OBD codes, the ABS system will generally describe its own failures, but the system has to be tested hands on to determine if the error is due to component failure, circuit failure, or control module failure. In many instances, replacing the part that says it is having trouble will result in a vehicle with a new part and the old fault.

During the inspection and scan testing, the system may return that it is working properly, as will normally be the case. However, if the ABS light is on, or pending trouble codes are stored, the technician will conduct diagnostics as described, and the faulty component will be replaced. In most cases a wheel speed sensor will be replaced by removing it from the hub assembly and installing the new sensor. However, if the control module or modulator valve has failed, the repair will require removal of at least six brake lines from the modulator valve, disconnecting the battery and control unit, and installing a replacement in reverse order. Finally, the entire brake system will be flushed, filled, and bled to complete the repair. These are just examples, and several other faults can be found when inspecting the ABS system. Repair of the specified fault will vary by make, model, and fault type.

In order to maintain the ABS system and prevent untimely failure, we recommend following the manufacturer's specifications for brake system maintenance. Remember that the service brakes are tied into the ABS system, and, for maintenance purposes, these two systems should be considered synonymous. Proper maintenance of the brake system can reduce the risk of contamination of the modulator valve, which is a very costly replacement in most cases.

Mixing brake fluid can lead to brake system failure due to congealing of the fluid within the brake lines. The different types of brake fluid, i.e. DOT3, DOT4, or DOT5 are meant for higher and higher braking demand, and will not always result in better braking performance. In fact, except for the most strenuous braking conditions, upgrading brake fluid will likely have no benefit.

Diagnosing the anti lock brake system is not difficult if the proper tools are on hand. Generic code readers inspire confidence for the average user, and may allow the user to diagnose the system properly. However, this is not normally the case, and the codes that will be displayed are general and nearly worthless as there are many duplicate components on the ABS system. A computerized scan tool is usually the best choice, and will allow for much more in-depth analysis and accuracy in diagnosis. If the solution is not obvious, trusting a certified technician can save time, frustration, and cost.