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J&W Auto Repair

AUTOMOTIVE: Anti-lock brake system

By Wyatt Nettles, J&W Repair

Last time we talked about brakes.  Now we are going to get into the anti-lock system.  The anti-lock system has no effect on having brakes or not having brakes.  This system only controls wheels from locking up.  When we use our brakes, there are times when one wheel or more could lock up because of road conditions.  When this happens, the anti-lock system sees the difference in wheel speed and releases brake pressure to unlock these wheels.  This will keep the vehicle straight through the stop.

A little about sliding wheels.  I was always taught sliding wheels pass rolling wheels.  Because the front of most vehicles weigh more than the rear, the rear wheels can lock under hard braking.  This is what will cause the rear of the vehicle to come around.  The first anti-lock brake systems were rear anti-lock brake systems.  Then they went to the four wheel anti-lock brake system.

These systems are controlled by using computers, modules, and wheel speed sensors.  On rear anti-lock vehicles, the sensor was in the rear differential.  When the computer sensed the wheel speed sensor quit or slow too much, it would send a signal to the module to release pressure on rear brakes.  Thus stopping the wheels from sliding.  Four wheel ABS uses sensors on all wheels.  As one or more wheel sensor senses wheel speed less than others, it will release pressure on that wheel to release brakes.  On vehicles with ABS, we older drivers have to learn not to pump brakes but apply steady brake pressure and let the system do the rest.