ABS Description

ELECTRO-HYDRAULIC CONTROL UNIT
The Electro-Hydraulic Control Unit (EHCU) is located inside the left hand side frame rail. 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 Electro-Hydraulic Control Unit (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. 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 Electro-Hydraulic Control Unit (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
^ Rear

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

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 Electronic Brake Control Module (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 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 Wheel Speed Sensor (WSS) pole piece. The tone wheels are attached to the rotor. 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 Electronic Brake Control Module (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 it the battery is disconnected or if the EBCM is removed from the vehicle.