Chapter 21 Chassis System Electronics 403
Copyright by Goodheart-Willcox Co., Inc.
on the servo motor actuators. When the servo motors rotate,
they force steel wedges and steel rollers into each to push
the brake pads into the rotor to cause friction.
When the driver releases the brake pedal, the ECM
reverses servo motor rotation to allow the brake pads to
move away from the rotors.
The ECM can apply different amounts of braking force
to each wheel to improve how much friction is applied at
each wheel independently of the others. Total electronic
control of the friction brakes is ideal for ABS, stability
control, collision avoidance, and other systems discussed
shortly. See Figure 21-12.
Electric Parking Brake
An electric parking brake uses a dash switch and elec-
tric servo actuator to mechanically apply the rear brakes.
Also called electric emergency brake, it does not use a
large parking brake lever and long steel cable to engage the
emergency brakes. Look at Figure 21-13.
When the parking brake servo motor is energized, it
rotates a worm gear that acts on a lever mechanism that
pulls on a mechanical arm and cable. The cable mecha-
nism forces the brake pads or shoes into the rotors or drums
Figure 21-10. Block diagram shows the three modes of ABS hydraulic control: pressure intensifying, pressure reducing, and pressure
retaining. (Honda Motor Co.)
Figure 21-11 Photo shows high-speed test of electric wedge
brakes. Instead of using brake fl uid pressure, servo motors apply
brake pads against the spinning rotor to stop the vehicle. Brake
rotor is so hot, it is glowing red. Electric brakes do not suffer from
problem of boiling fl uid during high-speed stops as do hydraulic
brakes. (Siemens)
Figure 21-12. Phantom view shows how servo motors can turn
worm gears to force steel wedges and rollers sideways. This
action pushes the brake pad against the spinning rotor. Electric
brakes can react more quickly than hydraulic brakes because
electricity moves faster than fl uid. (Siemens)