Chapter Four ABS Training
4.1 What is ABS?
Anti-Lock Brake System (ABS) is a combination of vehicle hardware and software that work together to maintain steering control and vehicle stability during hard braking. Initially developed as a special option for topof-the-line models, today’s ABS units are compact, easy to service, inexpensive and widely available. Now every major automotive manufacturer offers some form of ABS vehicle control that provides safe, maximum braking under all weather conditions and road surfaces. "FULL"
Integral. INTEGRAL systems incorporate the power unit (pump and accumulator), master cylinder, and control valve mechanism into one hydraulic unit. An external microprocessor and individual wheel sensors complete the system. The Teves Mark II system (Fig. 1) is an example of an integral unit.
INTEGRAL AND NON-INTEGRAL
There are basically two types of Anti-Lock (or AntiSkid) Brake Systems–Integral and NonBOOSTER PUMP AND MOTOR
The other type of ABS is referred to as NONINTEGRAL or ADD-ON. The non-integral system incorporates ABS components into the standard brake system. Added to the power brake unit, master cylinder, and calipers are a microprocessor (computer module), modulator valve, and speed sensor(s). A lateral accelerator BOOST PRESSURE SWITCH (IN MODULATOR) PROPORTIONING VALVE DIFFERENTIAL SWITCH PRESS MODULATOR
MASTER CYLINDER/ POWER BOOSTER
ECU ACCUMULATOR AND PRESSURE SWITCH LEFT FRONT SENSOR REAR WHEEL SENSOR SENSOR CONNECTORS/ WIRES RIGHT FRONT SENSOR
REAR BRAKE CHANNEL
CONTROL VALVE ASSEMBLY
switch, measuring side “G” forces, can also be a part of this system. Because of its lower cost, vehicle manufacturers are designing add-on systems for the majority of the future vehicle population. The typical components used in a nonintegral system are shown in Fig. 2.
speed of both rear wheels. This type of setup saves the cost of an additional sensor and reduces the complexity of the system by allowing both rear wheels to be controlled simultaneously. The last non-integral variation is the single channel or rear-wheel only ABS system. Used on many rear-wheel drive pickups and vans, Ford’s version is called Rear Anti-Lock Brakes (RABS) while GM and Chrysler call theirs Rear Wheel Anti-Lock (RWAL). A typical installation of a single channel system is shown in Fig. 3. In this system the front wheels have no speed sensors. Only a single speed sensor, mounted in the differential or transmission, monitors both rear wheels. Rear-wheel anti-lock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it’s used on pickup trucks and vans.
FOUR, THREE, OR SINGLE CHANNEL
Non-integral ABS systems are grouped by the number of wheel sensors used in the system. On some applications, each wheel is equipped with its own speed sensor. This type of system is referred to as a four wheel or four channel system. A variation of this system has a separate wheel speed sensor for each front wheel but uses a common speed sensor for both rear wheels. Known as a three channel system, the rear axle speed sensor is mounted in the differential or transmission and reads the combined or average
4.2 How ABS Works
ABS improves vehicle control through a blending of computer technology, modern electronics and standard brake components. Sensors constantly monitor the rotating speed of each wheel or the rear axle. These sensors are connected to the ABS control module. The signals sent from each sensor are constantly compared to stored parameters by the control module. In a hard braking situation, the control module can detect a slowing or lock-up of each sensor equipped wheel or axle and instantly pulse the braking force up to 15 times a second. This process “pumps” the brakes, avoiding wheel lockup while allowing the wheels to continue to turn. All the driver does is...
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