How anti-lock brake systems work

Anti-lock Braking System (ABS) problems?

By Jim Smart

Guide to understanding, troubleshooting and diagnosing your ABS

Evolution of ABS

Anti-lock braking technology has been around for nearly a century. Since World War II, aircraft have had anti-skid braking systems, and the earliest anti-lock braking systems on automobiles date back to the 1920s. Anti-skid or anti-lock braking first became more commonplace on cars and trucks in the 1990s as a positive step toward improved safety and vehicle control during hard braking in slippery conditions.

Although ABS seems complicated, it’s actually quite simple in function. If you encounter a skid while braking, the ABS control module senses a slowdown or pause in wheel rotation, modulating brake application to help you steer out of trouble. In a conventional skid, steering control is lost and the vehicle continues to travel in the direction of the skid. Then, anti-lock braking pulses the brakes, which results in an improved measure of control out of the skid.

A typical ABS consists of four wheel sensors (sometimes two or three), an anti-lock electronic control module and a hydraulic control unit. Under normal conditions, this system applies master cylinder hydraulic pressure to all four brakes, and pulsing pressure to each brake when a skid is detected.

ABS function

Early anti-lock braking systems were non-electrical, hydromechanical models. They were mechanically controlled to modulate brake application. Contemporary anti-lock braking systems are computer- controlled, electrohydromechanical brake hydraulic systems. The ABS electronic module or controller can be integral with the hydromechanical braking controller or it can be separate. There can also be electrical relays that fire when the system is called to duty.

Anti-lock brake sensors are typically magnetically triggered. As the reluctor’s teeth pass the sensor, the normal pulsing rhythm of wheel motion indicates normal operation. It is when the reluctor speed across the sensor changes dramatically (wheels slow down or stop) that the ABS will pulse brake application. When the ABS pulses, it pumps hydraulic pressure to the brakes in rapid-fire succession, sometimes as rapidly as 15 times a second depending on the system. This function produces intermittent braking and some level of steering control.

ABS types

There are three basic types of anti-lock braking systems: four-channel/four-sensor, three-channel/three-sensor and one-channel/one-sensor. The best option is the four-channel system because it can micromanage brake action in a skid by pulsing only the affected wheel or wheels. A three-channel system has two ABS sensors in front and one in the rear. The rear ABS sensor is located in the axle housing and affects both rear brakes. The one-channel system is rear anti-lock brake only with a single ABS sensor in the rear axle housing. Generally, one-channel systems are common on trucks with rear anti-lock brakes.

The proper way to use anti-lock brakes is to never pump the brake pedal during an abrupt stop. Instead, apply a solid, steady pedal and let the anti-lock braking system do what it was designed to do.


ABS problems

A vehicle’s ABS control module is designed to notify the driver with a warning light if there is a malfunction in the system. Rarely is a malfunction the module or ABS itself. It is often one or more sensors, or the wiring to the sensors. The most common ABS problems occur when sensors become contaminated with debris or metal shavings.

Malfunctions also occur when sensor wiring becomes damaged, resulting in intermittent or no continuity. In more corrosive environments or serious brake system neglect, brake fluid can become contaminated and the hydraulic control unit fails to function.

If you have a malfunction in the ABS, physically check all wiring and the brake sensors first. When checking the brake sensors, look for metal shavings and other debris that could cause false feedback to the electronic ABS controller. False feedback causes the ABS to trigger when it shouldn’t, or not function when it should.

You may not have an ABS scan tool at home, but any reputable repair shop will have one, and the scan tool is invaluable if you can’t find a physical reason for your ABS issues. After the scan tool produces a fault code, you can proceed with your ABS troubleshooting. Whatever the problem, the repair is much easier once a fault is established.