The Antilock Braking System consists of the following components:
| • | A conventional hydraulic brake system |
The conventional brake system includes the following components:
| • | Interconnecting hydraulic brake pipes |
| • | A red BRAKE warning indicator |
The ABS includes the following components:
| • | A Brake Pressure Modulator Valve (BPMV) |
| • | An Electronic Brake Control Module (EBCM) |
| • | Four wheel speed sensors |
| • | The interconnecting wiring |
| • | An amber ABS warning indicator |
EBCM/BPMV Assembly
Legend
- Electronic Brake Control Module (EBCM)
- Brake Pressure Modulator Valve (BPMV)
Brake Pressure Modulator Valve (BPMV)
The Brake Pressure Modulator Valve (BPMV), mounted on the left side
of the engine compartment, provides brake fluid modulation for each of the
individual wheel circuits as required during Antilock braking. During the
Antilock mode, the BPMV can maintain or reduce brake fluid pressure independent
of the pressure generated in the master cylinder. The BPMV does not provide
more pressure than is applied by the master cylinder during braking.
With the exception of the EBCM, the Brake Pressure Modulator Valve (BPMV)
is an integral, non-serviceable component. The BPMV should never be disassembled.
The BPMV consists of several other components which are described as
follows:
Pump Motor
The BPMV contains a motor driven recirculation pump. The pump reduces
pressure during ABS events. It transfers fluid from the brake calipers back
to the master cylinder circuit. The pump and motor are located within the
BPMV and are not serviced separately.
ABS Valves
The ABS valves decrease or maintain brake fluid pressure at the individual
wheel circuits. There are four Inlet, and four Outlet solenoid valves. The
solenoid valves maintain, increase, or decrease brake fluid pressure to
the individual wheel circuits. The EBCM commands the valves to their correct
position during an antilock event. During antilock mode, the pressure in
each hydraulic circuit can be held or released by activating the appropriate
valves. The normal state of the inlet valves is open, while the normal state
of the Outlet valves is closed. This allows direct master cylinder pressure
to the brakes during normal braking. The ABS valves are located within the
BPMV and are not serviced separately.
Electronic Brake Control Module (EBCM)
The Electronic Brake Control Module (EBCM) is a microprocessor that
controls the ABS.
The inputs to the EBCM include the following components:
| • | The wheel speed sensors |
| • | The unswitched battery voltage |
The outputs include the following components:
A serial data line is provided for service diagnostic tools and assembly
plant testing.
The EBCM monitors the speed of each wheel and controls the solenoids
and pump motor in order to reduce brake pressure to a particular wheel when
the wheel is approaching slip.
This cycle will repeat until one of the following conditions exists:
| • | The vehicle comes to a stop. |
| • | The wheel no longer approaches slip. |
If any system malfunction is detected, the EBCM will store a Diagnostic
Trouble Code (DTC) in nonvolatile memory (DTCs will not disappear if the battery
is disconnected).
The EBCM is bolted directly to the BPMV located in the engine compartment.
Vent Tube
Terminal C11 of the EBCM harness connector is a vent tube that vents
the cavity between the EBCM and the BPMV to the atmosphere.
Front Wheel Speed Sensors and Rings
The front wheel speed sensors are of a variable reluctance type. Each
sensor is attached to the knuckle assembly in close proximity to a toothed
ring. An AC voltage is generated when the toothed ring passes by the sensor.
The magnitude and frequency are proportional to the speed of the wheel and
both will increase with increasing speed. The sensor is not repairable, nor
is the air gap adjustable.
Rear Wheel Speed Sensors
The rear wheel speed sensors are mounted in the bearing assembly and
are non-adjustable. Sensor gap is set at the time of assembly. This integral
hub/bearing/wheel speed sensor assembly is serviced as a complete assembly
only.
EBCM Relay
The EBCM relay is a normally-open contact type.
The EBCM relay has special contact material to handle the high currents
required for ABS operation. The relay allows battery voltage and current to
be supplied to the solenoids and the pump motor.
The relay is an integral part of the EBCM and cannot be serviced separately.
Wiring Harness
The wiring harness is the mechanism which electrically connects the
EBCM to the following components:
| • | The power and the ground |
| • | The wheel speed sensors |
| • | The serial communications port |
The wiring harness consists of the following components:
| • | The wires that provide electrical interconnection. |
| • | The connectors/terminals that provide an electrical/mechanical
interface from each wire to a system component. |
Warning Indicators
The Electronic Brake Control Module (EBCM) continuously monitors itself
and other ABS components.
If the EBCM detects a problem with the system, the amber ABS warning
indicator will light continuously (solid indicator) in order to alert the
driver of the problem.
Normal (non-antilock) braking will remain. In order to regain ABS braking
ability, the ABS must be serviced.
The amber ABS warning indicator will illuminate any time when the EBCM
suspects that the ABS operation is degraded because of a system malfunction.
The red BRAKE warning indicator illuminates when the following conditions
exist:
| • | The fluid level in the master cylinder is low |
| • | The parking brake switch is closed |
| • | The ignition switch is in START |
| • | Certain ABS DTCs are set |
| | (This is controlled by the EBCM.) |
The red BRAKE warning indicator alerts the driver that a base brake
problem may exist.
Refer to Brake Warning Indicator System Check in Hydraulic Brakes for
further diagnosis of the red BRAKE warning indicator.
Fuses
The power distribution system of the vehicle consists of the fuses and
the ignition switch.
The system fuses are:
