Astro, Safari 1997-2005

Description & Operation


When wheel slip is detected during a brake application, the ABS enters antilock mode. During antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from slipping. A separate hydraulic line and specific solenoid valves are provided for each wheel. The ABS can decrease, hold, or increase hydraulic pressure to each wheel brake. The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.

During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the EBCM responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping noise may also be heard as the solenoid valves cycle rapidly. During antilock braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are considered normal during antilock operation.

Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides the shortest stopping distance while maintaining vehicle stability.

These vehicle are equipped with the Kelsey-Hayes EBC325 antilock braking system.

The vehicle is equipped with the following braking enhancement systems:

Antilock Brake System (ABS)
Dynamic Rear Proportioning (DRP)

The following components are involved in the operation of the above systems:

Electronic Brake Control Module (EBCM) - The EBCM controls the system functions and detects failures.

The EBCM contains the following components:

System Relay - The system relay is internal to the EBCM. The system relay is energized when the ignition is ON. The system relay supplies battery positive voltage to the solenoid valves and to the pump motor. This voltage is referred to as system voltage.
Solenoids - The solenoids are commanded by the EBCM to operate the appropriate valves in the brake pressure modulator valve.

Brake Pressure Modulator Valve (BPMV) - The BPMV uses a 3-circuit configuration to control the left front wheel, the right front wheel, and the combined rear wheels. The BPMV directs fluid to the left front and right front wheels independently. The BPMV directs fluid to the two rear wheels on a single hydraulic circuit.

The BPMV contains the following components:

Pump Motor
Three isolation valves (one for each hydraulic circuit)
Three dump valves (one for each hydraulic circuit)
A front low-pressure accumulator
A rear low-pressure accumulator

Wheel Speed Sensors (WSS) - As the front wheels spin, a toothed ring located at each wheel hub interrupts a magnetic field in the wheel speed sensors. This causes the wheel speed sensors to generate an AC signal. The EBCM uses this AC signal to calculate the wheel speed. The wheel speed sensors are replaceable only as part of the wheel hub and bearing assemblies. Any imperfections in the toothed ring, such as a missing or damaged tooth, can cause an inaccurate WSS signal.

Vehicle Speed Sensor (VSS) - The input signal for rear wheel speed originates with the VSS. The Powertrain Control Module (PCM) receives rear wheel speed information from the VSS and supplies this information to the EBCM via a serial data message.

ABS Indicator

The IPC illuminates the ABS indicator when the following occurs:

The Electronic Brake Control Module (EBCM) detects an ABS-disabling malfunction. The IPC receives a class 2 message from the EBCM requesting illumination.
The IPC performs the bulb check.
The IPC detects a loss of class 2 communications with the EBCM.

Brake Pressure Modulator Valve

The Brake Pressure Modulator Valve (BPMV) is part of the EHCU. The BPMV is the hydraulic portion of the EHCU. The EBCM controls the BPMV.

The BPMV is split into the following three hydraulic channels:

Left front
Right front

Each channel has an isolation valve and a dump valve. The front channels share a low pressure accumulator, attenuator, and a pump. The rear channel shares use of the pump, but uses it's own low pressure accumulator and attenuator.

Brake Warning Indicator

The Instrument Panel Cluster (IPC) illuminates the brake warning indicator when the following occurs:

The Body Control Module (BCM) detects that the park brake is engaged. The IPC receives a class 2 message from the BCM requesting illumination.
The EBCM detects a low brake fluid condition and sends a class 2 message to the IPC.
The IPC performs the bulb check.
An ABS-disabling malfunction also disables dynamic rear proportioning (DRP).

Dynamic Rear Proportioning (DRP)

The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system.

The DRP control system is part of the operation software in the EBCM. The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.

The red brake warning indicator is illuminated when the dynamic rear proportioning function is disabled.

Electro-Hydraulic Control Unit

The Electro-Hydraulic Control Unit (EHCU) is located in the left hand side of the engine compartment. The EHCU assembly includes the Electronic Brake Control module (EBCM) and the Brake Pressure Modulator Valve (BPMV). The EHCU regulates hydraulic pressure in the brake system during an antilock stop.

Electronic Brake Control Module

The Electronic Brake Control Module (EBCM) is part of the EHCU. The EBCM is the electronic portion of the EHCU. The major function of the EBCM is to control the BPMV.

Inputs to the EBCM include the following items:

Three wheel speed sensors
Stop Lamp Switch
Differential pressure
Ignition switch voltage
Unswitched battery voltage

Outputs of the EBCM include the following items:

Three isolation solenoids (internal to the EHCU)
Three dump solenoids (internal to the EHCU)
The amber ABS indicator lamp
The red BRAKE warning lamp
The pump motor

A diagnostic serial data line (ABS only) is also used for diagnostic service tools and assembly plant testing.

The EBCM monitors the speed of each wheel. If any wheel approaches lockup, the EBCM controls the solenoids (isolation solenoid and dump solenoid) in order to reduce brake pressure to the wheel approaching lockup. Once the wheel regains traction, brake pressure is increased until the wheel again approaches lockup. This cycle repeats until either the vehicle comes to a stop, the brake is released, or the wheel is no longer approaching lockup. The EBCM also runs self diagnostics in order to check for any system malfunctions. Refer to Self Diagnostics. If the EBCM detects a malfunction with the system, the ECBM will illuminate the amber ABS indicator in order to alert the driver of a malfunction.

Initialization Sequence

The EBCM performs one initialization test each ignition cycle. The initialization of the EBCM occurs when the following conditions are met:

The ignition is ON .
The bulb check has been completed.
Vehicle speed is greater than 4 mph (6 km/h).

The initialization sequence briefly cycles each solenoid and the pump motor to verify proper operation of the components. The EBCM sets one or more DTCs in accordance with any malfunction that is detected.

The EBCM defines a drive cycle as the completion of the initialization sequence.

Pressure Decrease

If a pressure hold does not correct the wheel slip condition, a pressure decrease occurs. The EBCM decreases the pressure to individual wheels during deceleration when wheel slip occurs. The isolation valve is closed and the dump valve is opened. The excess fluid is stored in the accumulator until the pump can return the fluid to the master cylinder or fluid reservoir.

Pressure Hold

The EBCM closes the isolation valve and keeps the dump valve closed in order to isolate the slipping wheel when wheel slip occurs. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.

Pressure Increase

After the wheel slip is corrected, a pressure increase occurs. The EBCM increases the pressure to individual wheels during deceleration in order to reduce the speed of the wheel. The isolation valve is opened and the dump valve is closed. The increased pressure is delivered from the master cylinder.

Tire Size Calibration

The EBCM accepts wheel speed signals from several different sizes of tire and wheel combinations. All vehicles are pre-programmed from the factory with the proper tire size calibration. Whenever you replace the EBCM or change the tire size, you must reset the tire size calibration in the EBCM using the scan tool. Once programmed, this calibration will remain, even if the battery is disconnected or if the EBCM is removed from the vehicle.

Wheel Speed Sensor Tone Wheels

Each Wheel Speed Sensor uses a tone wheel in order to produce an AC voltage signal. Tone wheels are metal rings with teeth on the outside diameter. The AC voltage is produced as the teeth pass through the magnetic field of the WSS pole piece. The tone wheels are either attached to the rotor, or inside the hub/bearing assembly. Any imperfections in the tone rings, such as a broken tooth or a missing tooth, can cause an inaccurate wheel speed signal.

Wheel Speed Sensors

The front Wheel Speed Sensors (WSS) are a magnetic coil/pickup type. Each WSS produces an AC voltage signal which is transmitted to the EBCM in order to indicate how fast the wheel is turning. The speed of the wheel is directly proportional to the frequency and amplitude of the wheel speed signal.