Rick's Free Auto Repair Advice

ABS — What is ABS?

What is ABS?

ABS — Anti-lock brakes

Anti-lock brakes (ABS) prevent skidding on slick and dry pavement. ABS works by preventing the vehicle’s wheels from locking up and skidding.

ABS increases friction by preventing wheel lock up

A tire creates the most friction with the road when it is slowing down. The instant the wheel locks up and the tire begins to skid across the pavement, it loses most of its traction with the road. In effect, there’s high initial friction that generates enough heat to melt the rubber tire. From that point on, the tire has almost no traction as it is skidding on a film of liquefied rubber.

ABS prevents skidding/tire melting by sensing when the wheel is slowing down and about to lock up. At that instant, the ABS valve module releases brake hydraulic pressure to that wheel, allowing it to regain speed. Once it regains speed, the ABS immediately applies braking pressure again to slow that wheel down. The brake, release, reapply cycle repeats itself rapidly (up to 15 times per second) to gain maximum stopping power from the wheel.

How does an ABS system know when the wheel is about to lock up?

ABS equipped vehicles have a wheel rotation

what are ABS brakes

Wheel speed sensor

speed sensor and at each front wheel and either a single sensor for both rear wheels (on a rear wheel drive vehicle) or an individual sensor on each rear wheel. The wheel speed sensor contains a magnet that detects the movement of a teeth on a rotating “tone ring.” The sensor

How ABS brakes work

ABS wheel speed sensor and tone ring

creates a voltage as the tooth approaches and the voltage drops as the tooth passes past the sensor. The ABS brake module compares the rotation speed from each wheel speed sensor to determine which wheel is about to lock up.

How does the ABS system release and apply the brake at each wheel

The ABS system consist of a computerized

what are ABS brakes

ABS Hydraulic Control Unit

controller, pump, and release and apply valves. The pump computer and valves are usually built into a single unit called the Hydraulic Control Unit (HCU). The fluid circuit to each wheel has a “dump valve” to release brake fluid pressure and an “apply valve” to re-apply brake fluid pressure. The valves are opened and closed by an electric solenoid. The pump supplements the driver’s brake pedal pressure during periods of ABS operation.

What goes wrong with ABS brakes?

Since each wheel speed sensor is mounted directly to the wheel spindle, it is subject to harsh conditions like rain, snow and road debris. In addition, the sensor is connected to the control unit by a wiring harness that must flex up and down and turn left and right with the wheel movement. So sensor and wiring harness failures cause the most frequent problems with a vehicle’s ABS system.

what are ABS brakes

ABS components

The wheel speed sensor must also be set away from the tone ring by a specified gap. As a tone ring rusts, the gap changes and that can cause a fault code to be set in the ABS controller, which turn on the ABS warning light. The gap can also be affected by debris buildup on the magnetic wheel speed sensor.

Pump and valve failures are less common, but far more costly to repair. However, these units can be rebuilt for a fraction of the price of a new part. If your HCU fails, search for an ABS rebuilder online and send yours off to be rebuilt. The difference in price between new and rebuilt is staggering; $200 versus $2,200. Modulemasters.com is one ABS HCU rebuilder.

What does an ABS light on mean? Read this post for more information

rebuilt ABS HCU

Rebuilt ABS HCU

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Posted on by Rick Muscoplat

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