Rick's Free Auto Repair Advice

ABS — What are ABS brakes?

What are ABS brakes?

How do ABS brakes work?

Anti-lock brakes (ABS) prevent skidding on slick and dry pavement by preventing the vehicle’s wheel from locking up and skidding. A wheel creates the most friction with the road when it is slowing down due to braking. But the instant the wheel locks up, it loses most of that traction. So ABS brakes sense when the wheel is about to lock and then releases brake pressure on that wheel’s brake to allow it to rotate back up to speed. Then the ABS system applies the brake to that wheel to slow it again. This cycle repeats rapidly 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 componnets

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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