Chapter 21 Chassis System Electronics 401
Copyright by Goodheart-Willcox Co., Inc.
Accumulator—chamber for storing fl uid under high
pressure.
Hydraulic pump and motor—high-pressure pump oper-
ated by a small electric motor; the hydraulic pump and
motor provide brake fl uid pressure for the system.
Pressure switch—monitors system pressure and con-
trols the operation of the electric motor for the hydrau-
lic pump.
Master cylinder-booster assembly—conventional mas-
ter cylinder with power assist for operating the brakes
under normal conditions.
Figure 21-6. ABS actuator assembly normally mounts in the
engine compartment.
Figure 21-7. Exploded view of a hydraulic actuator shows a valve
block that holds the solenoid valves. (Acura)
Fluid
Reservoir
Solenoid
Value
Strainer
5 mm Screw
Solenoid Head
Solenoid
Cover
Solenoid
Value
Solenoid
Plate
ABS systems and hydraulic actuators vary with the spe-
cifi c make and model car. Another type of hydraulic actuator
is illustrated in Figure 21-8. It also uses a small electric motor
to drive the high-pressure pump. Note that it only has three
solenoid valves. Two operate the front brakes, and the other
operates both rear brakes. The ECM energizes the pump and
valves using the relays mounted on the unit.
ABS Operation
Refer to Figure 21-9 as the operation of a typical ABS is
summarized. Under normal braking, the ABS system is not
used. The master cylinder reacts to brake pedal movement.
It sends fl uid pressure out to each wheel cylinder normally.
A proportioning valve is commonly used to reduce pressure
to the rear brakes.
If the brakes are applied in a panic stop and one wheel
begins to stop rotating, the ABS system activates. The wheel
sensor on the slowing wheel would instantly send a slower
AC pulsing signal to the ECM. The ECM would detect that
this wheel is slowing down more than the others and is
getting ready to slide or skid. The ECM would then send
an electrical current output to the correct solenoid on the
hydraulic actuator assembly.
When current is sent to the solenoid, the solenoid closes
a valve to limit the pressure to the brake unit with the slowing
wheel. The ECM quickly cycles the current to the solenoid
on and off to artifi cially “pump the brakes” to keep the tire
from skidding and losing traction. It does this several times a
second. When ABS takes over, the brake pedal usually rises
and vibrates slightly. This is due to the pressure entering the
system from the accumulator and from the cycling of the
solenoid valves. This is usually normal and will stop when
the ABS system is no longer functioning.
If hydraulic pressure in the system drops below a
specifi c point, the pressure switch closes and energizes the
electric motor. This drives the pump to rebuild pressure.
Once pressure is normalized, the pressure switch opens
and the motor and pump shut off.
If an ABS component malfunctions, the ECM will
detect an abnormal condition. It will then light an ABS
warning light in the dash and deactivate the ABS. The brake
system will still function normally, but without the anti-lock
feature. Figure 21-10 shows a block diagram for a modern
anti-lock brake system.
Electric Brake Calipers
An electric brake caliper uses a servo motor to force
the brake pads into the rotating brake disc to slow and stop
the vehicle without the use of hydraulics (fl uid under pres-
sure). Auto manufacturers have designed and tested electric
brakes, as shown in Figure 21-11.
Basically, when the driver presses the brake pedal,
a sensor on the brake pedal sends a signal to the braking
ECM. The ECM can use this signal to detect how hard the
driver is pressing on the brake pedal. The ECM can then
send control current to each electric brake caliper to turn