The ABS (Antilock Braking System) consists of the following components:
| • | A conventional hydraulic brake system |
| • | Antilock components |
The conventional brake system includes the following components:
| • | A vacuum power booster |
| • | A master cylinder |
| • | Front disc brakes |
| • | Rear drum or disc brakes (if equipped) |
| • | Interconnecting hydraulic brake lines |
| • | A low fluid sensor |
| • | A red BRAKE warning indicator |
The ABS includes the following components:
| • | An ABS brake modulator/brake motor pack assembly |
| • | An electronic brake control module (EBCM) |
| • | An electronic brake control relay |
| • | Four wheel speed sensors |
| • | An amber ABS warning indicator |
| • | ABS Low Traction active indicator (LOW TRAC) |
ABS Brake Modulator/Brake Motor Pack Assembly
The ABS Brake Modulator/Brake Motor Pack Assembly controls hydraulic pressure to the following areas:
| • | The front calipers |
| • | The rear calipers (if equipped) |
The ABS Brake Modulator/Brake Motor Pack Assembly controls hydraulic pressure by modulating the hydraulic pressure. This action prevents wheel lock-up.
The basic ABS brake modulator configuration consists of the following components:
| • | The gear subassemblies |
| • | The ball screws |
| • | The nuts |
| • | The pistons |
| • | The hydraulic check valves |
The ABS brake motor pack consists of the following components:
| • | Three motors |
| • | Three drive gears |
| • | Three ESBs |
Electronic Brake Control Module (EBCM)
Notice: There is no serviceable or removable PROM. The EBCM must be replaced as an assembly.
The Electronic Brake Control Module (EBCM) controls the ABS VI system. A microprocessor controls the EBCM.
The inputs to the system include the following components:
| • | The four wheel speed sensors |
| • | The brake switch |
| • | The ignition switch |
| • | The unswitched battery voltage |
| • | The serial data communications with the PCM |
The outputs include the following components:
| • | Three bi-directional motor controls |
| • | Four indicator controls |
| • | Two solenoid controls (ABS) |
| • | A system enable relay |
A serial data line is provided for service diagnostic tools and assembly plant testing. The serial data line is located in terminal 9 of the Data Link Connector (DLC).
The EBCM monitors the speed of each wheel.
The EBCM controls the motors and the solenoids in order to reduce brake pressure to a particular wheel when the following conditions exist:
| • | The wheel is approaching lock-up. |
| • | The brake switch is on. |
When the wheel regains traction the brake pressure increases until the wheel approaches lock-up again.
This cycle will repeat until one of the following conditions exists:
| • | The vehicle comes to a stop. |
| • | The brake releases. |
| • | The wheel no longer approaches lock-up. |
The EBCM monitors the following components for proper operation:
| • | Itself |
| • | Each of the inputs (except for the serial data line) |
| • | Each of the outputs |
If a system malfunction is detected the EBTCM will store a DTC in nonvolatile memory (i.e., the DTC will not disappear if the battery is disconnected.)
Refer to Diagnostic System Check - ABS for more information.
Front Wheel Speed Sensors
Each of the front wheel speed sensors consists of a variable reluctance sensor.
The sensor and the toothed ring are part of the sealed hub and bearing unit. Being part of the sealed hub and bearing unit allows improved environmental protection and easier assembly.
When teeth pass by the sensor, the sensor generates an AC voltage that has a frequency proportional to the speed of the wheel.
The sensor is not repairable.
The air gap is not adjustable.
The sensor and the toothed ring cannot be serviced separately from the hub and bearing assembly.
The entire integral bearing/sensor unit must be replaced if a front wheel speed sensor fails.
Refer to Front Wheel Drive Shaft Bearing Replacement .
The front hub/bearing/speed sensor is service as an assembly.
Rear Wheel Speed Sensors
The rear wheel speed sensors operate in the same manner as the front wheel speed sensors.
When teeth pass by the sensor, the sensor generates an AC voltage that has a frequency proportional to the speed of the wheel.
The rear wheel speed sensors and the toothed ring are contained within the dust cap of the integral rear wheel bearing. This allows approved environmental protection and easier assembly.
The sensor and the toothed ring are not repairable.
The air gap cannot be adjusted.
The entire integral bearing and speed sensor unit must be replaced if a rear wheel speed sensor fails.
Refer to Rear Wheel Bearing and Hub Replacement . The rear hub/bearing/speed sensor is service as an assembly.
Electronic Brake Control Relay
The Electronic Brake Control Relay is a normally-open contact type.
The Electronic Brake Control Relay has special contact material to handle the high currents required for the ABS operation.
The relay allows battery voltage and current to be supplied to the EBCM. The EBCM supplies power to the motors and the solenoids.
Brake Fluid Level Indicator Sensor
Refer to Brake Fluid Level Indicator Switch Replacement in Hydraulic Brakes.
Wiring Harness
The wiring harness is the mechanism which electrical/mechanical interface from each wire to a system component that connects to the EBCM through the following components:
| • | The power and ground connections |
| • | The wheel speed sensors |
| • | The motors |
| • | The solenoids |
| • | The fuses |
| • | The switches |
| • | The indicators |
| • | The serial communications port |
Wheel Speed Sensor Jumper Harness
A jumper harness located between each wheel speed sensor and the main wiring harness.
Each jumper harness is made of highly flexible twisted pair wiring.
The wiring exists because the main harness must connect to the suspension of the vehicle. Therefore, the wiring in this area is subjected to the same motion as a spring or a shock absorber. Any repair to this section of wiring will result in stiffening. Stiffening will eventually result in failure due to wire fatigue.
The wheel speed sensor jumper harnesses are not repairable and must be replaced. Do not attempt to solder, splice, or crimp the harnesses. Eventual failure will occur.
Refer to Wheel Speed Sensor Jumper Harness Replacement ..
Warning Indicators
Caution: The braking system may not be working properly if the ABS warning indicator is flashing. Driving with a flashing ABS warning indicator can lead to an accident. If the ABS warning indicator is flashing, pull safely off the road and have the vehicle towed for service.
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 either flash or light continuously (solid indicator) in order to alert the driver of the problem.
The amber ABS warning indicator will flash if the EBCM attempts to turn on following indicators at the same time, but is unable to do so:
| • | The red BRAKE warning indicator |
| • | The amber ABS warning indicator |
The amber ABS warning indicator flashes to warn the driver of an existing ABS malfunction which may have degraded the base brake performance. In this situation, the vehicle must be serviced immediately in order to regain full base and ABS operation.
A solid amber ABS warning indicator indicates that a problem has been detected. The problem is one that affects the operation of the ABS. Normal (non-antilock) brake performance will still be available.
The ABS must be serviced in order to regain ABS braking ability.
If the EBCM detects a malfunction with the ABS, the amber Low-Trac indicator will illuminate in order to alert the driver of the malfunction. . The base braking and the ABS will still be available.
The following indicators will illuminate any time when the EBCM suspects that the ABS operation is degraded because of a system malfunction:
| • | The amber ABS warning indicator |
| • | The amber LOW-TRAC indicator |
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 bulb test switch section of the ignition switch is closed |
| • | 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.
The amber LOW-TRAC indicator illuminates in order to alert the driver that the ABS is active. Whenever the EBCM determines that the vehicle has entered a braking event that requires the ABS, the amber LOW-TRAC indicator will turn on. The amber LOW -TRAC indicator will remain on for approximately 3 to 4 seconds after the ABS event is completed.
When the EBCM receives power (the ignition is placed in the ON or the START position), the EBCM will command the LOW -TRAC indicator to turn on for three seconds. This action is a bulb check procedure.
Fuses and Fusible Links
The power distribution system of the vehicle consists of the following components:
| • | The fuses |
| • | The fusible links |
| • | The circuit breakers |
| • | The ignition switch |
Fusible links are short pieces of wire that are several sizes smaller than the circuit wire that supply power. They are covered with special high temperature insulation.
When the fusible links are conducting an improperly high current flow, they will melt and stop the current flow.
The fusible links are designed to protect the vehicle's electrical system from the electrical shorts in the circuits that are not protected by the circuit breakers or fuses.
Refer to Power Distribution Schematics in Electrical Diagnosis for fuse and fusible link locations.
