Chapter 21 Chassis System Electronics 407
Copyright by Goodheart-Willcox Co., Inc.
When the car body is at a normal or riding height,
the electronic height control system is off. Air pressure in
the shocks is adequate to keep the car body the correct
distance from the road surface. The height sensor does not
feed current to the compressor or solenoid.
If the trunk is loaded with heavy luggage for example,
the added weight will compress the rear air shocks. This
will make the ride height too low. When the car is started,
the sensor link “mechanically informs” the height sensor of
the low ride height condition by closing the sensor switch
contacts. The compressor motor turns on and pumps more
air pressure into the rear shock absorbers. This extends the
shocks and raises the car body.
When the specifi c ride height is reached, the height sen-
sor switch opens to turn off the compressor. This restores the
previous ride height, even with extra weight in the trunk.
When the weight is removed from the trunk, the car
body would tend to rise. The height sensor switch is then
moved in the other direction by the link. This closes another
set of contacts in the switch and energizes a pressure release
solenoid valve. Air pressure is then expelled from the rear
shocks until the body drops down to the correct ride height.
Electronic Shock Absorber Systems
An electronic shock absorber system uses various vehi-
cle sensors, an ECM, and shock absorber actuators to control
ride stiffness. It is designed to increase comfort and safety by
matching suspension system action to driving conditions. See
Figure 21-19.
Although exact designs vary, the major components of
a typical electronic shock absorber system are:
Steering sensor—detects steering wheel rotational
direction and speed to feed data about the vehicle
direction to the ECM.
Brake sensor—usually a brake light switch that reports
when brakes are applied.
Acceleration sensor—usually a throttle position sen-
sor that detects when the car is accelerating rapidly.
Mode switch—dash switch that allows the driver to
input the desired shock action or ride stiffness.
Figure 21-18. Infrared sensor sends out invisible light that
bounces off of objects behind or in front of bumpers. The time
it takes for the light to refl ect off of objects is used by the park-
ing ECM to avoid hitting objects while automatically parking the
vehicle. (Bosch)
Parking Assistance
(Parkpilot)
Figure 21-19. Block diagram shows how various sensors feed electrical data to the ECM. ECM can then energize the shock actuators
to control ride stiffness and shock action. (Toyota)
Sensors
Selector Switch
Speed Sensor
Steering Sensor
Stop Lamp Switch
Neutral Start Switch
(A/T Vehicle)
Throttle Position Sensor
ECU
Shock Absorber
Control
Computer
Shock Actuator
Shock Actuator
Shock Actuator
Shock Actuator
TEMS Indicator