Express, Savana, G1500, G2500, G3500, 1998-2005

Description & Operation


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.

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.

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.

Wheel Speed Sensors 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.

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.

When wheel slip is detected during a brake application, an ABS event occurs. 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. The ABS does not, 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. The typical ABS activation sequence is as follows.

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 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 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.

These vehicles are equipped with an EBC 310 or an EBC 325EV ABS/DRP module.

This module provides the following vehicle performance 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 valve solenoids and to the ABS pump motor. This voltage is referred to as system voltage.
Solenoids - The solenoids are commanded ON and OFF by the EBCM to operate the appropriate valves in the brake pressure modulator valve (BPMV).

Brake Pressure Modulator Valve (BPMV) - The BPMV uses a 3-circuit configuration to control hydraulic pressure to each front wheel independently, and to the rear wheels as a pair. The BPMV contains the following components:

ABS pump motor and pump
Three isolation valves
Three dump valves
A front low-pressure accumulator
A rear low-pressure accumulator

Wheel Speed Sensors (WSS) - As the wheels spin, toothed rings interrupt magnetic fields in the wheel speed sensors. This causes each wheel speed sensor to generate an AC signal. The EBCM uses these AC signals to calculate the wheel speed. Any imperfections in the toothed ring, such as a missing or damaged tooth, can cause an inaccurate WSS signal.

Driver Information Indicators And Messages

The following indicators are used to inform the driver of several different factors.

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 serial data message from the BCM requesting illumination. The brake warning indicator flashes at a rate of approximately twice per second when the park brake is engaged
The EBCM detects a low brake fluid condition or a base brake pressure differential and sends a serial data message to the IPC requesting illumination.
The IPC performs the bulb check
The EBCM detects an ABS-disabling malfunction which also disables Dynamic Rear Proportioning (DRP) and sends a serial data message to the IPC requesting illumination

ABS Indicator

The IPC illuminates the ABS indicator when the following occurs:

The electronic brake control module (EBCM) detects an ABS-disabling malfunction and sends a serial data message to the IPC requesting illumination
The IPC performs the bulb check
The IPC detects a loss of serial data communication with the EBCM
A DTC is set during the previous ignition cycle which requires an ECE 13 response at the beginning of the current ignition cycle. The EBCM sends a serial data message to the IPC requesting illumination

Dynamic Rear Proportioning (DRP)

The dynamic rear proportioning (DRP) is a control system that enhances 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.

Ece 13 Response

The EBCM illuminates the ABS indicator when a malfunction which disables ABS is detected. Usually, the ABS indicator is turned OFF during the following ignition cycle unless the fault is detected during that ignition cycle. However, the setting of a most wheel speed sensor related DTC causes the ABS indicator to remain illuminated during the following ignition cycle until the vehicle is operated at a speed greater than 8mph (13 km/h) or, occasionally, 40 mph (64 km/h), depending on which DTC sets. This allows the EBCM to verify that no malfunction exists, before turning OFF the ABS indicator. It is important to verify that ECE 13 is not the cause of an ABS indicator which is illuminated when no DTCs are set, before attempting to diagnose other possible causes.

Power-Up Self-Test

The EBCM is able to detect many malfunctions whenever the ignition is ON . However, certain failures cannot be detected unless active diagnostic tests are performed on the components. Shorted solenoid coil or motor windings, for example, cannot be detected until the components are commanded ON by the EBCM. Therefore, a power-up self-test is required at the beginning of each ignition cycle to verify correct operation of components before the various control systems can be enabled. The EBCM performs the power-up self-test when the ignition is first turned ON . The system relay, solenoids and the ABS pump motor are commanded ON and OFF to verify proper operation and the EBCM verifies the ability to return the system to base braking in the event of a failure. The power-up self-test may be heard by the driver, depending on how soon the engine is cranked and started after turning ON the ignition.