The anti-lock brake system on your car or light truck allows you to maintain directional control during panic stops on slippery roads. Without this system, wheel lock-up will occur. and you'll lose directional control and the ability to steer around an obstacle. Accurate troubleshooting strategies are needed to diagnose problems in this system, and a scan tool is required.
Wheel Speed Sensors
The quickest, most accurate way to diagnose wheel speed sensor problems is with a scan tool. Select a scan tool that has ABS capabilities, and data-stream functions. A good example of this type of tool is found at http://www.diy-auto-repair.com/computerdiagnostic.html and is reasonably priced. With the tool installed as directed by the manufacturer, test-drive the vehicle while observing the wheel speed sensor readings in the data stream. They should all read very close to the same. A sensor that drops the signal, or shows a large difference between the sensors, is suspect. Check for worn bearings and contaminated sensors before replacing the sensor, as these are common causes of inaccurate readings.
Valve Assemblies
Bi-directional control of the solenoid valves in the ABS system is the most accurate way to test the modulator valve assemble, and the control module. Unfortunately, the equipment needed to do this is expensive, and usually beyond the price range of the home mechanic. To run the test, select "Special Functions Tests" and then "Rehome". This cycles all the valves and runs a self test using the onboard computer. If this equipment is not available, the local auto shop will be happy to run it for you for a small fee.
Repair Information Systems
A tool that has recently become available to the home mechanic is professional level information systems. These are the same systems of repair information used by the professional mechanic, except that they are now available in single-car versions for a subscription available to the do-it-yourselfer. A good example of this type of online repair manual is found at http://www.diy-auto-repair.com/alldata.html
