ABS (anti-lock brake) systems are a significant advancement in terms of vehicle safety. For the do-it-yourself mechanic, these advanced systems can seem a little intimidating. However, the job is not as complex as you might suspect. In this episode, Weekend Mechanic Bruce Bonebrake diagnoses problems on a car that has an ABS indicator-lamp that's lit. He then services the mechanical portion of the braking system. The problem: Our project car, a 1991 Buick Riata has an ABS indicator light that is illuminated and staying on. The car has also begun to exhibit some shudder -- perceptible in the steering wheel -- when the car is braking.
DIY Difficulty Rating for Repair: This job is not as difficult or complex as many people might think. We gave it a difficulty rating of 2 on a 5-point intermediate-level scale. The work isn't difficult, but tracking down the source of the problem may take a little time. If you're going to properly service your braking system, allow yourself about a day and a half to finish the job. It took us about 9 hours to finish this project over two days.
Important: Included below are the steps in this automotive procedure as shown in the episode. These steps are general guidelines that are applicable to most vehicles. With any particular vehicle, there may be procedures, specifications, settings, tolerances, components, etc. that are specific to that vehicle. There are also variations according to the type and brand of repair-kit you select. Always consult your vehicle's service manual when undertaking significant automotive repairs, and read and follow the manufacturer's directions and precautions that come with your kit or replacement parts. Safety Alert: Always wear eye protection, and follow proper safety precautions, when working with power tools. In addition to standard auto-mechanic's tools, some of the specialty tools that will be needed for this project include an ABS code key for your model car (figure A), a volt-ohm meter, fluid bleeder kitand a caliper piston tool (figure B) .
Inspection and Electronic Diagnosis
- How ABS Works: With ABS brakes, each drive axle has a toothed sensor ring and a fixed speed sensor (figure C). As the teeth pass the sensor, signals are sent to the electronic brake-control module (EBCM). If a wheel starts to lock up, the signal from that wheel varies from that of the others. If the EBCM detects a variation, it uses a pressure modulator to "pulse" brake fluid to the brakes in a rapid "on-off-on-off" cycle. This results in an automatic pumping of the brakes until the wheel is under control. It's based on the same principle as the sage advice to "pump your brakes" during a skid instead of applying steady, abrupt pressure. The ABS system just does it automatically and responds more quickly.
- Since 1980, automobiles have been equipped with electronic control modules -- or computers -- to control various functions. There are computer systems for the engine, transmission and anti-lock brakes. These systems typically have built-in self-diagnostics and can turn on warning lights to alert drivers when there's a problem with a given system. The systems also store in memory the source of the problem. In order to retrieve that stored information, there are a number of diagnostic tools that you can use. These may be jumper wires, access keys or plug-in computer modules and even PC-based software. In our case, the "trouble codes" can be retrieved by using an access key in conjunction with a diagnostic terminal called an ALDL connector underneath the dash (figure D). ALDL stands for "assembly line data link." Other car manufacturers call them by other names. (Ford calls them "self-test connectors.")
- By placing the key in the terminal (figure E), making the proper connections according to the service manual, the diagnostic codes can be read on the dash.
- With the key in the connector, and the ignition switch turned to the "on" position, the ABS warning light (figure F) will flash in a given series to indicate the source of the problem. In our case, a repeated series of 3 flashes followed by two flashes indicates a "code 32." According to the service manual for this vehicle, that code indicates a faulty sensor on the left-front wheel.
In the segment that follows, the sensor circuit is checked and work begins on servicing the brakes.
RESOURCES :
The Complete Idiot's Guide to Trouble-Free Car Care
Author: Dan Ramsey
ISBN: 0028635833
Alpha Books
Auto Repair for Dummies
Author: Deanna Sclar
ISBN: 0764550896
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