Electro-Hydraulic Control Unit
The Electro-Hydraulic Control Unit (EHCU) is located inside the the
left hand frame rail under the driver door. 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:
| • | Two wheel speed sensors (front RH and LH) |
| • | One vehicle speed sensor (rear) |
| • | Brake fluid level switch |
| • | Ignition switch voltage |
| • | Unswitched battery voltage |
| • | Front axle actuator/switch input |
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 (via a class 2 message) |
| • | The red BRAKE warning lamp (via a class 2 message) |
A class 2 serial data line 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:
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 frequency and
amplitude of the voltage signal is directly proportional to the
speed of the wheel.
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 located in each front wheel hub/bearing
assembly. Any imperfections in the tone wheels, 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 Scan Tool
. Once
programmed, this calibration will remain, even if the battery is disconnected
or if the EBCM is removed from the vehicle. Refer to
Scan Tool Diagnostics
for setting or changing the tire size
calibration.
