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Anti Lock Braking Sensors

An anti-lock braking system (ABS) is a safety system on motor vehicles which prevents the wheels from locking while braking.

A rotating road wheel allows the driver to maintain steering control under heavy braking, by preventing a locked wheel or skid, and allowing the wheel to continue to forward roll and create lateral control, as directed by driver steering inputs. Disadvantages of the system include increased braking distances under some limited circumstances (snow, gravel, "soft" surfaces), and the creation of a "false sense of security" among drivers who do not understand the operation, and limitations of ABS.



Theory

When the car brakes (normally), the momentum of the car must be reduced, so a backwards force needs to be transmitted to the car. This is achieved by the wheels exerting a forward force on the street which lies below the threshold of maximum static sliding friction. The wheels keep sticking to the road because of this friction.

If the driver brakes very hard (or accelerates extremely) it can occur that the maximum static friction is surpassed and the wheels lose their grip and begin sliding (or spinning). In this case the dynamic sliding friction (which is less than the maximum static friction) takes over.

The amount of traction which can be obtained for an auto tire is determined by the coefficient of static friction between the tire and the road. If the wheel is locked and sliding, the force of friction is determined by the coefficient of kinetic friction and is usually significantly less. A tire that is just on the verge of slipping (10 to 20% slippage) produces more friction with respect to the road than one which is locked and skidding (100% slippage). Once traction is lost, friction is reduced, the tire skids and the vehicle takes longer to stop. So locked wheels are less effective in stopping on a road.

But in gravel, sand and deep snow, locked wheels dig in and stop the vehicle more quickly. A locked tire allows a small wedge of snow to build up ahead of it which allows it to stop in a somewhat shorter distance than a rolling tire. That is why some vehicles have an on/off switch for deactivating the antilock system when driving on snow.

Components

Wheel Speed Sensors

The wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor ring (sometimes called a "tone" ring). The sensor(s) may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tail shaft or differential housing. On some applications, the sensor is an integral part of the wheel bearing and hub assembly. The sensor ring(s) may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, inside the brake drum, on the transmission tail shaft or inside the differential on the pinion shaft.

The wheel speed sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup magnetic field. This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the pickup windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up.

Hydraulic Modulator

The hydraulic modulator or actuator unit contains the ABS solenoid valves for each brake circuit. The exact number of valves per circuit depends on the ABS system and application. Some have a pair of on-off solenoid valves for each brake circuit while others use a single valve that can operate in more than one position. On Delco VI ABS systems, small electric motors are used in place of solenoids to drive pistons up and down to modulate brake pressure.

On some systems, the individual ABS solenoids can be replaced if defective, but on most applications the modulator is considered a sealed assembly and must be replaced as a unit if defective.

Hydraulic modulator has a hydraulic modulator block including a reservoir and a damper; and an electronic control block detachably attached to the surface of the hydraulic modulator block. The reservoir and the damper are overlapped with each other such that a surface of the hydraulic modulator block becomes entirely substantially flat. With this, the hydraulic modulator becomes simple in construction. The hydraulic modulator has a solenoid valve; an electronic control circuit board; and an electric wiring pattern prepared by pressing a metal plate having a first major surface formed with a first tin layer and a nickel layer and a second major surface formed with a second tin layer, into a three-dimensional shape such that the electric wiring pattern is formed with (1) a first terminal having a surface that is formed with the first or second tin layer electrically connected with the solenoid valve, (2) a connector terminal having first and second surfaces respectively formed with the first and second tin layers, and (3) a second terminal having a surface that is formed with the nickel layer electrically connected with the electronic control circuit board. Thus, each terminal has a secure electrical connection with another member.

Anti Lock Braking Sensors

 

One-channel one-sensor ABS

This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle.

This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness.

This system is easy to identify. Usually there will be one brake line going through a T-fitting to both rear wheels. You can locate the speed sensor by looking for an electrical connection near the differential on the rear-axle housing.

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