The Antilock Braking System consists of the following components:
The conventional brake system includes the following components:
The ABS includes the following components:
The Brake Pressure Modulator Valve (BPMV) mounted on the left side of the engine compartment, provides brake fluid modulation for each of the 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.
The Brake Pressure Modulator Valve (BPMV) is a non-serviceable component. The BPMV should never be disassembled.
The BPMV consists of several other components which are described as follows:
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.
The ABS valves decrease or maintain brake fluid pressure at the individual wheel circuits. There are three Inlet, and three Outlet solenoid valves. The solenoid valves maintain, increase, or decrease brake fluid pressure to the individual wheel circuits. The EBCM commands the valves 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.
The Electronic Brake Control Module (EBCM) controls the ABS.
The inputs to the system include the following components:
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.
The EBCM 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:
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.
Terminal C11 of the EBCM harness connector is a vent tube that vents the cavity between the EBCM and the BPMV to the atmosphere.
The lamp driver module (LDM) contains electronic components that turn the ABS indicator ON unless the EBCM provides a ground to turn the indicator off . If the EBCM loses power or is disconnected, the LDM commands the ABS indicator ON.
When the ignition switch is turned to the ON position, the EBCM turns the ABS warning indicator ON for about 3 seconds during self test. Whenever a malfunction is detected, the EBCM stores a Diagnostic Trouble Code (DTC) and turns the ABS warning indicator ON.
The LDM is located on the back side of the instrument cluster assembly.
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 are used for this ABS application and are located at each rear brake assembly. They operate in the same manner as the front wheel speed sensors.
The sensors and the toothed rings are not repairable.
The air gap cannot be adjusted.
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 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.
Refer to Brake Warning System Description and Operation in Hydraulic Brakes.
The EBCM uses an accelerometer to sense vehicle motion.
The EBCM provides power (5 volts) and ground to the accelerometer. The accelerometer sends a voltage signal back to the EBCM relative to vehicle motion. This signal is about 2.4 to 2.6 volts at zero speed change (constant motion) and stationary, the voltage signal decreases when the vehicle is under acceleration, and the voltage signal increases when the vehicle is under deceleration.
The accelerometer is located inside the parking brake lever bezel.
The four-wheel drive switch is used to notify the EBCM when the vehicle is in four-wheel drive mode. Under certain road conditions, a vehicle in four-wheel drive mode requires special measures to support the reference vehicle speed by means of an accelerometer.
The four-wheel drive switch is mounted on the transfer case.
The EBCM uses the Powertrain Control Module (PCM) to increase engine idle speed.
When the transfer case is set in 4H or 4L, the EBCM may command the Powertrain Control Module (PCM) to increase the idle speed during an ABS event.
The reason to increase engine speed above base idle is to reduce engine-induced braking. Engine braking, especially on icy road conditions, tends to reduce ABS performance.
The EBCM logic determines when it is necessary to idle-up. However, the PCM will ignore any requests to idle-up from the EBCM until the vehicle speed has reached at least 20 km/h (12 mph). The reason for this default in vehicle speed is to avoid a fast idle at all times when the idle-up circuit has been shorted to ground due to an electrical problem.
The wiring harness is the mechanism which electrically connects the EBCM to the following components:
And on vehicles equipped with 4WD, the EBCM is also connected to the accelerometer, 4WD switch and PCM.
The wiring harness consists of the following components:
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 red BRAKE warning indicator alerts the driver that a base brake problem may exist.
The power distribution system of the vehicle consists of the fuses and the ignition switch.
The ABS fuses are:
Fuse Block 1 is located on the RH side of the engine compartment, in front of the battery.
Fuse Block 2 is located under the LH side of the I/P.
When wheel slip is noted during a brake application, the ABS will enter the antilock mode. During antilock braking, the hydraulic pressure in the individual wheel circuits is controlled in order to prevent any wheel from slipping.
The ABS can decrease, hold, or increase the hydraulic pressure to each wheel brake. However, the ABS cannot increase the hydraulic pressure above the amount that the master cylinder transmits during braking.
During antilock braking, a series of rapid pulsations will be felt in the brake pedal. The rapid changes in the position of the individual solenoid valves as they respond to the desired wheel speed causes the pulsations. This pedal pulsation is present during antilock braking and will stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping nose may also be heard as the solenoid valves rapidly cycle and the pump operates.
During antilock braking on dry pavement, the tires may make intermittent chirping noises as they approach slipping. These noises and pedal pulsations are normal during antilock operation. Brake pedal operation during normal braking should be no different than previous systems. Maintaining a constant force on the pedal provides the shortest stopping distance while maintaining vehicle stability.
When the EBCM senses wheel slip, it closes the inlet valve and keeps the outlet valve closed in the BPMV 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 brake. The inlet valve is left closed and the outlet valve is opened. The excess fluid is stored in the accumulator until the return pump can return the fluid to the master cylinder.
If during the pressure hold or the pressure decrease mode the EBCM senses that the wheel is not slipping, the EBCM will increase the pressure to the brake. The inlet is opened and the outlet valve is closed. The increased pressure comes from the master cylinder and is relative to the pressure applied to the brake pedal.
The EBCM performs self-diagnostics during initialization.
The EBCM also verifies correct operation of the following components during initialization:
The EBCM inspects the wheel speed sensor circuitry for the following conditions during initialization:
If the EBCM detects a malfunction in itself or in other ABS components, the EBCM will perform the following actions:
The following indicators will remain on for approximately three seconds with the key in the ON position:
The initialization performs during engine cranking and engine starting. A slight mechanical noise may be heard during the system initialization. This noise is normal.
During this process, when the vehicle attains a speed of 5 km/h (3 mph). The EBCM will verify motor, hydraulic modulator and solenoids by using specific control schemes to verify all components are functioning properly.
If brake pedal pressure interrupts the system initialization, a slight movement in the brake pedal may exist.
The standard brake system uses a single red BRAKE warning indicator, which is located in the instrument panel cluster.
The antilock brake system uses an amber ABS warning indicator
The following indicators will turn ON for approximately three seconds and then turn OFF when the ignition is turned to the ON position:
During engine cranking, the red BRAKE warning indicator and the amber ABS indicator should illuminate steadily.
Three seconds after the engine is started, the ABS indicator should be OFF. The red BRAKE warning indicator will go OFF immediately.
Using the compact spare supplied with the vehicle will not affect the operation of ABS. The EBCM software can compensate for this smaller tire. However, increased stopping distances may occur due to reduced tire tread depth on compact spare tires.
Tire size is important for proper performance of the ABS. Replacement tires should be the same size, load range, and construction as the original tires. Replace tires in axle sets and only with tires of the same Tire Performance Criteria (TPC) specification number. Use of any other tire size or type may seriously affect the ABS operation.
