Antilock General Description
This vehicle is equipped with a four wheel antilock brake system. Vehicle speed information is obtained using a wheel speed sensor (WSS) at each front wheel and the vehicle speed sensor (VSS) for rear wheel speed information. The ABS reduces the occurrence of wheel lockup during severe brake applications. The system regulates hydraulic pressure to all four wheels. The pressure is regulated by the brake pressure modulator valve (BPMV).ABBREVIATIONS/DEFINITIONS
BPMV Brake Pressure Modulator Valve
CKT Circuit
DLC Data Link Connector
DTC Diagnostic Trouble Code
DUMP Dump Valve (Cartridge)
DMM Digital Multi-Meter
EBCM Electronic Brake Control Module
EHCU Electro-Hydraulic Control Unit
4WAL Four Wheel Antilock
ISO Isolation Valve (Cartridge)
LPA Low Pressure Accumulator
VSS Vehicle Speed Sensor
WSS Wheel Speed Sensor
The EHCU refers to the entire ABS assembly, including the BPMV, EBCM and combination valve. The BPMV is defined as the lower, hydraulic control portion of the EHCU. The BPMV includes the internal control valves, electric motor and pump. It does not include the EBCM or combination valve. The EBCM is the electronic control portion of the EHCU. It mounts to the top of the BPMV and is housed in aluminum, with a black plastic top.
ANTILOCK BRAKE SYSTEM INTRODUCTION
ABS is designed to provide the driver with:
- Optimal steering control and stability by enabling the vehicle to move in a driver controlled direction during braking.
- Optimal braking performance by making the most of the available traction (on most road surfaces).
Wheel Slip
How well a vehicle can stop is related to how well the tire contact patch grips the road surface. At 0% slip, the tire rotates freely. At 100% slip the tire and wheel are locked and the momentum of the vehicle pushes the locked tires along. Stopping distance increases and steering control lessens.
If the tires grip well (10%-20% slip), vehicle stopping distance will be as short as possible and steering control will be at its best. Some slip is necessary to stop the wheel and achieve maximum braking. When Antilock Brake System (ABS) operation occurs, the driver of the vehicle should always continue to push hard on the brake pedal. NEVER PUMP ABS BRAKES, the Antilock Brake System will automatically moderate the brakes to keep the tires in contact with the road.
Steering Control
Steering control, like braking, also depends on tire traction: a locked tire in a 100% slip condition delivers less than optimum braking and directional control. Thus some tire rotation is desirable for steering control. The tires must regain traction before steering control is restored to the vehicle
BASIC KNOWLEDGE REQUIRED
It is important that you have a basic knowledge of the following items. Without this basic knowledge, it will be difficult to use the diagnostic procedures.
Electrical Circuits
- You should Understand the basic theory of electricity and know the meaning of voltage (volts), current (amperes) and resistance (ohms).
- You should Understand what happens in a circuit with an open or shorted wire.
- You should Be able to read and understand a wiring diagram.
Use Of Circuit Testing Tools
- You should be familiar with the high impedance Digital Multimeter (DMM) J 39200 (or equivalent) and be familiar with the meter controls and how to use them correctly.
- You should be able to measure voltage, resistance end current.
- You should also know how to use jumper wires to bypass components to test circuits.
TIRES
Correct tire size, proper inflation, accurate alignment and even wear are needed for good brake performance. These items are essential for proper antilock performance.
Spare Tire
Using the spare tire supplied with the vehicle will not affect the performance of the Four Wheel Antilock (4WAL) system.
Replacement Tires
Replacement tires should be the same size, load range, and construction on all four wheels. Failure to comply with this can affect the performance of the Four Wheel Antilock (4WAL) system.