The ABS system consists of a conventional hydraulic brake system plus antilock components. The conventional brake system includes a vacuum booster, master cylinder, front disc brakes, rear drum brakes, interconnecting hydraulic brake pipes and hoses, brake fluid level sensor and the BRAKE indicator.
The ABS components include a hydraulic unit, an electronic brake control module (EBCM), 2 system fuses, 4 wheel speed sensors, 1 at each wheel, interconnecting wiring, the ABS indicator, and the rear drum brake, For the general layout of the ABS system, refer to ABS Component Views.
Caution: Refer to Brake Fluid Irritant Caution in the Preface section.
Caution: Refer to Brake Fluid Caution in the Preface section.
Caution: To help avoid personal injury due to poor braking, DO NOT tap into the vehicle's brake hydraulic system to operate a trailer brake system.
Notice: Refer to Brake Fluid Effects on Paint and Electrical Components Notice in the Preface section.
Notice: Refer to Fastener Notice in the Preface section.
Take care to avoid EBCM circuit overloading. In testing for opens or shorts, do not ground or apply voltage to any circuit unless instructed to do so by the diagnostic procedure. Test circuits only with a high-impedance multimeter. Never remove or apply power to any control module with the ignition switch in the ON position. Always turn the ignition to the OFF position before removing or connecting battery cables, fuses, or connectors.
Caution: Refer to ABS Component Handling Caution in the Preface section.
Disconnect the EBCM connector before performing any vehicle welding work using a electric arc welder.
Do not attempt to disassemble any component designated as non-serviceable. The hydraulic modulator and the EBCM can be separated from each other and replaced separately but cannot be serviced. They have no replaceable parts, and there is no access to the components they contain.
The purpose of the anti lock brake system (ABS) modulator is to minimize wheel slip during heavy braking. The brake modulator performs this function by monitoring the speed of each wheel and controlling the brake fluid pressure to each wheel independently during a braking event. This allows the driver to retain directional stability and better steering capability.
Before using this section, 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 contained in this section.
The ABS brake modulator system consists of a conventional hydraulic brake system plus antilock components. The conventional brake system includes a vacuum booster, master cylinder, front disc brakes, rear drum brakes, rear drum brakes, interconnecting hydraulic brake pipes and hoses, brake fluid level sensor, and the brake warning lamp indicator.
The ABS components include a hydraulic unit, an Electronic Brake Control Module (EBCM), system fuse, 4 wheel speed sensor at each wheel, interconnecting wiring, ABS indicator, The de-coupled dynamic rear proportioning (DDRP) indicator, which is connected to the parking lamp, and the rear drum brakes. See Antilock Brake System Component Views for the general layout of this system.
The hydraulic unit with the attached EBCM is located between the surge tank and the fire wall on the left side of the vehicle.
The basic hydraulic unit configuration consists of the hydraulic check valves, 2 solenoid valves for each wheel, a hydraulic pump, and 2 accumulators. The hydraulic unit controls the hydraulic pressure to the front calipers and the rear wheel cylinders by modulating hydraulic pressure to prevent wheel lockup.
The baseline braking mode of the ABS modulator used in this vehicle is a diagonal split system. In this system, 1 master cylinder circuit supplies pressure to the right front and the left rear brakes. The other circuit supplies pressure to the left front and the right rear brakes. All valves in the hydraulic modulator are in their normal, non-energized positions as shown in the drawings found in the hydraulic diagram.
If during the pressure hold or pressure decrease mode the EBCM senses that the wheel slip has reduced, the EBCM will increase the pressure to the affected wheels by applying master cylinder pressure. The inlet valve is opened and the outlet valve is closed, now base brake master cylinder pressure can be applied to the wheel.
When the EBCM senses the wheel slip, the EBCM closes the inlet valve and keeps the outlet valve closed in the brake pressure modulator valve in order to isolate the system. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
If during the pressure hold mode the EBCM still senses wheel slip, the EBCM will decrease the pressure to the affected wheels. The inlet valve is left closed and the outlet valve is opened. The excess fluid/pressure is temporarily stored into an accumulator within the brake pressure modulator valve, until the pump can return the fluid to the master cylinder reservoir.
De-Coupled Dynamic Rear Proportioning (DDRP) is a proportioning system to maintain vehicle stability during braking. In normal braking conditions, equal wheel speed should be maintained for efficient and balanced braking. In hard braking conditions, a vehicle requires relatively less brake force at the rear wheel due to the vehicle's weight transfer to the front. DDRP maintains desired brake pressure to the rear wheel by using the ABS rear inlet and outlet valve in order to provide efficient braking and vehicle stability. In the DDRP system, power to the rear hold valve solenoid is provided from the ignition. If the following fault conditions are existing, the red brake warning lamp will be illuminated.
Part
Hypothesized Failure
De-Coupled DDRP
ABS Warning Lamp
Brake Warning Lamp
DDRP Status
Sensor RF
Short or Open
ON
--
Degraded
Sensor LF
Sensor RR
Sensor LR
Two Sensor, Same Axle
Disabled
One Front and One Rear Sensor
Motor
Short to Ground - LOW Side
Short to Ground -- HI Side
Short to Battery - LOW Side
Short to Battery - HI Side
None
Motor Circuit Open
Motor Stalled
Front Apply Solenoids
Front Release Solenoids
-
Rear Apply Solenoids
Rear Release Solenoids
System Relay
Open - Unable to turn ON
Shorted ON -- Unable to turn OFF
Battery 2 Motor
Short to Ground
Open
Low Voltage
Ground - Motor
Open or Short to Battery
Enabled
Battery 1 - ECU, Solenoids
Open or Short to Ground
Ground - ECU, Solenoids
Ignition
Brake Switch
Not Applicable
Serial Communication
Open or Short
The EBCM performs the following primary functions in order to provide efficient braking and vehicle stability. In the DDRP system, power to the rear hold valve solenoid is provided from ignition. If the following fault conditions are existing, the red brake warning lamp will be illuminated.
The EBCM continuously checks the speed of each wheel to determine if any wheel is beginning to slip. If any wheel slip tendency is detected, the EBCM commands appropriate valve positions to modulate brake fluid pressure in some or all of the hydraulic circuits to prevent wheel slip and provide optimum braking. The EBCM continues to control pressure in individual circuits until a slipping tendency is no longer present. Also the EBCM continuously monitors the ABS for proper operation. If the EBCM detects an error, it can disable the ABS function and turn ON the ABS warning lamp in the instrument cluster. The EBCM also controls the display of the ABS diagnostic trouble codes (DTCs) while in diagnostic mode.
The solenoid relay provides power to the pump motor and solenoids. The switch in the relay is normally open, but is commanded to close during initialization. The relay switch will remain closed for the remainder of the drive cycle as long as no DTCs set which required the switch to open. If a DTC sets which requires the relay to be commanded OFF, battery voltage will be removed from the pump motor and solenoids for the remainder of the current drive cycle and the ABS cannot function. The relay is an integral part of the EBCM and cannot be serviced separately.
A wheel speed sensor is present at each wheel. The sensors transmit wheel speed information to the EBCM by means of a small A/C voltage. This voltage is generated by magnetic induction caused by passing a toothed sensor ring past a stationary sensor. The magnitude and frequency of the A/C voltage are proportional to the speed of the wheel and both will increase with increasing speed. The signal is transmitted to the EBCM through interface that can cause false or noisy wheel speed sensor input to the EBCM. Two different types of wheel speed sensors are used for the ABS brake modulator system.
Front wheel speed sensors are installed to the front knuckle.
Rear wheel speed sensors are installed to the backing plate.
Important: Avoid prying or contacting wheel speed sensor rings. Excessive contact may cause damage to one or more teeth.
Wheel speed sensors are not serviceable. The air cap is not adjusted. The front wheel speed sensor ring is pressed onto the drive axle shaft. Rear wheel speed sensor rings are incorporated into the hub drum. The front ring contains 46 equally spaced teeth, and the rear ring contains 34 equally spaced teeth.
The ABS warning lamp is located in the instrument cluster and will illuminate if a malfunction in the ABS is detected by the EBCM. The ABS warning lamp informs the driver that a condition exists which results in turning OFF the antilock brake function. if only the ABS warning lamp is ON, normal braking with full power assist is available.
Conditions for the ABS warning lamp to turn ON are as follows.
The red brake warning lamp in the instrument cluster will illuminate to warn the driver of a condition in the brake system, which may result in reduced braking ability. The lamp will illuminate when the parking brake is applied or not fully released, or if the brake fluid level switch is closed. Closed is when the brake fluid is low in the master cylinder reservoir. When the brake fluid level switch is closed, low condition, a brake warning lamp will stay illuminated until the condition has been repaired. Also some failure modes in the ABS brake modulator system will illuminate the lamp to let the drivers know the De-coupled Dynamic Rear Proportioning (DDRP) is disabled.
