408 Auto Electricity and Electronics
Copyright by Goodheart-Willcox Co., Inc.
Electronic control module—small ECM that uses sen-
sor inputs to control shock actuators.
Shock actuators—solenoid-operated valves for con-
trolling fl uid fl ow inside the shock absorbers.
If the car is being driven on curving country roads, the
driver might switch to a stiff setting with the mode switch.
The ECM would then energize the shock actuators to close or
restrict the shock valves to increase dampening action. This
would make the car ride more stiffl y but corner better.
If driving on rough highways, the mode switch might
be set to soft. The ECM would then energize the shock
actuators to open the valves more. This would soften the
ride by allowing easier shock movement.
Under hard braking, the brake sensor would send a
signal to the ECM. The ECM could then stiffen the shocks
to prevent the front of the car from diving.
With rapid turning or cornering, the steering sensor
could also signal the ECM. The ECM could then stiffen the
shocks to prevent excess body roll or lean in turns.
Figure 21-20 shows one type of shock actuator. Note
how it uses a solenoid and small DC motor to act on the
shock absorber piston rod. The shock piston rod can be
moved up or down to control fl uid fl ow resistance and
shock stiffness or dampening.
Some cars use air shocks or air bags instead of hydraulic
shock absorbers. The operation of this type system is similar.
One type of electronic shock absorber system uses a
sonar (sound wave) sensor to detect actual road conditions.
The sensor mounts at the front of the car.
The sonar sensor produces sound waves that bounce
off the road and are defl ected back into the sensor. The sen-
sor action detects the time needed for the waves to bounce
back into the sensor. If there is a dip in the road, the sen-
sor can signal a different distance and signal the shock or
suspension system ECM. The ECM can then adjust shock
action for the road surface.
Figure 21-20. This shock actuator uses a small electric motor
and solenoid to move a shock piston rod in and out. Piston rod
movement alters shock dampening action.
Stopper
Solenoid
Shock Absorber
Control Rod
Shock Absorber
Piston Rod
Sector Gear
Pinion Gear
Electric
Motor
Magnetic Fluid Shock Absorbers
Magnetic fl uid shock absorbers use tiny particles of
iron in their hydraulic fl uid and an electromagnet to alter
shock stiffness.
When the suspension system ECM does not send cur-
rent to the shock electromagnets, the hydraulic fl uid fl ows
through the shock valves normally for a soft ride. When
the ECM sends current to the electromagnets in the shock
absorbers, the magnetic fi eld attracts the iron particles in
the fl uid and the fl uid resists fl ow. The ECM makes the
shock stiffen for high-performance driving.
Electronic Air Shock Absorbers
Air bags or air shocks can also be under ECM control. As
shown in Figure 21-21, a plastic air supply line runs to each
shock absorber from a small air compressor. An ECM-controlled
solenoid valve in the shock can adjust air pressure in each air
bag or shock to regulate vehicle ride height and ride stiffness.
A ride height button in the dash allows the driver to raise
the ride height when snow is on the ground or when driving
on rough surfaces. Another button controls ride stiffness. One
setting is for a soft ride, and another is for a stiffer sport setting.
Active Suspension System
An active suspension system uses hydraulic rams instead
of conventional suspension system springs and shock absorb-
ers. The hydraulic rams support the weight of the car and also
react to the road surface and different driving conditions. An
active suspension system is similar to an electronic shock
absorber system but is more complex. See Figure 21-22.
The active suspension system can theoretically eliminate
most body movement as the car travels over small dips and
bumps in the road. It can prevent body roll or even tilt the car
Figure 21-21. Photo shows the top of an ECM-controlled air
shock absorber. Note the plastic line that carries air pressure into
the air shock or bag. Solenoid valve in the shock can increase or
decrease air pressure to alter ride height and shock stiffness.