How to Diagnose ABS Brakes

Anti-lock brakes (ABS) are common systems in many cars on the road today. Late-model vehicles come equipped with sophisticated systems that require special and expensive scanning tools to diagnose and pinpoint a system's trouble source. Still, whether you own a late or older vehicle model, there are some common problem spots you can check to diagnose possible sources of trouble on your car's ABS. You need only a minimum of tools to perform this diagnosis at home.

Instructions

Make a visual inspection of the brake system components under the hood for obvious signs of trouble, such as fluid leaks, loose connectors, bolts and sensors around the brake master cylinder and connected lines. The master cylinder is attached to the brake booster, a drum-like component in the engine compartment on the driver's side.

Inspect the brake fluid in the reservoir, the plastic bottle on top of the brake master cylinder. Make sure the fluid level in the reservoir is correct. If you find yourself frequently adding fluid to the reservoir, check for leaks around the hydraulic modulator, master cylinder, calipers and connecting lines. Since anti-lock brake system designs vary from one vehicle model and year to another, consult the vehicle service manual for your particular model to locate and identify these components.

Raise the front wheel/tire assemblies off the ground using a floor jack and support the vehicle on jack stands. Block the wheels on the ground with wooden blocks.

Set the transmission to neutral and manually rotate the wheel assemblies to make sure they rotate freely. Otherwise, check for damage to the caliper or a mechanical bind preventing the wheel hub from rotating freely.

Listen for noises as you rotate the wheel/tire assembly. If the wheel bearing is worn out or damaged, it will prevent the wheel speed sensor and ABS system from operating properly.

Inspect the thread depth on each of the front tires using a tread wear indicator. Both front tires should have the same wear. Also, make sure these tires are of equal size. Uneven tire wear or size will affect the operation of the ABS system.

Remove the wheel assembly using a lug wrench.

Check the brake pads, rotors and caliper for loose mounting bolts. Also check for a leaking or damaged brake hose and lines connecting to the brake caliper. Also, look for signs of damage and wet spots around the brake pads and rotors that might indicate damage to the brake lines and caliper piston seals.

Inspect the wheel speed sensor for damage. Check the sensor body and follow the electrical wire to its connector. Make sure the wire is in good condition and there is a good connection. Then check the sensor-mounting bolt. Using a wrench, make sure the bolt is tight. When ready, lower your front wheel/tires off the jack stands.

Check the rear wheel/tire assemblies this time, following Steps 3 through 9.