The Electro-Hydraulic Control Unit (EHCU) is located on the left frame rail behind the cab. 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.

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:

Outputs of the EBCM include the following items:

A diagnostic serial data link (UART, ABS only) is also used for diagnostic service tools and assembly plant testing. A serial data circuit (Class 2) is used for transmitting a "rough road" signal to the PCM.

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.

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 four hydraulic channels:

Each channel has an isolation valve and a dump valve.

The 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.

Each Wheel Speed Sensor (WSS) uses a tone wheel in order to produce an alternating current (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 attached to the rotor or the drum. Any imperfections in the tone rings, such as a broken tooth or a missing tooth, can cause an inaccurate wheel speed signal.

The EBCM accepts wheel speed signals from several different sizes of tire and wheel combinations. All vehicles are preprogrammed 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 Scan Tool

The ABS can operate with different brake types and different wheel bases from several different vehicle configurations. All vehicles are pre-programmed from the factory with the proper trim level calibration. This trim level calibration corresponds to the different wheel bases and axles/brake types available on each vehicle. Whenever you replace the EBCM or change the wheel base of the vehicle, you must recalibrate the trim level within the EBCM.

Use the Scan Tool . You must first obtain the wheel base and axle type using RPO codes in order to select the correct trim level on the Scan Tool . Refer to RPO Code List in General Information. Once programmed, this calibration will remain, even if the battery is disconnected or if the EBCM is removed from the vehicle. Refer to the following table for trim level calibration selection.