414 Auto Electricity and Electronics
Copyright by Goodheart-Willcox Co., Inc.
When set on cruise using the power and control
switches, the vehicle speed sensor feeds an AC signal
into the ECM. The ECM circuits use this signal to move
the throttle actuator back and forth to maintain the same
vehicle speed or sensor frequency.
For example, if the car starts to go up a hill, the vehicle
speed will start to drop. The ECM will detect a slower fre-
quency signal from the speed sensor. It can then move the
throttle actuator for more engine power to keep the car
traveling at the preset speed. The opposite occurs if the car
starts down a hill.
Electronic Transmission/
Transaxle Control Systems
In many late-model vehicles, the transmission or
transaxle is controlled electronically. An electronically-
controlled transmission (and transaxle) uses a control
module, sensors, and solenoids to control shift points and
torque converter (fl uid clutch) lockup. Electronic control
provides more effi cient transmission operation, improving
performance and increasing fuel economy.
A diagram of an electronically-controlled transaxle is
shown in Figure 21-29. Note how various sensors feed data
to the transaxle control unit. The transaxle control unit can
then activate the shift solenoids and torque converter lockup
solenoid as necessary. As the solenoids are turned on and off,
they open and close fl uid passages to operate the transaxle.
Electronic Differential Control
Electronic differential control uses large electromag-
nets or oil-control solenoid valves and the drive train ECM
to control the amount of torque going to each drive axle
and wheel.
One type of electronic differential control is shown in
Figure 21-30. It can disengage or fully engage either driving
axle to avoid wheel spin and improve handling when cor-
nering. It uses large electromagnets to engage or disengage
the clutch packs that operate each drive axle going out to
the drive wheels and tires.
Summary
Anti-lock brake systems use wheel speed sensors as
the main input devices.
Wheel speed sensors produce a lower frequency signal
if a wheel begins to slow down and skid. The ECM can
use this slower signal to activate a hydraulic actuator.
A hydraulic actuator can cycle a valve to reduce pres-
sure to that brake assembly. This keeps the tire and
wheel from locking up and skidding.
An electronic height control system uses a height sensor,
which is linked to the suspension system. It signals the
air compressor if weight is added to the rear of the car.
The air compressor turns on to raise the rear of the car.
The height sensor activates a solenoid valve to release
pressure and lower the car when weight is removed.
An electronic shock absorber system uses a steer-
ing sensor, brake sensor, acceleration sensor, and
sometimes other sensors to feed data to an ECM. It
also reacts to rapid cornering, braking, and accelera-
tion by automatically stiffening shock action. It helps
prevent squat, sway, and dive.
Active suspension uses hydraulic rams in place of
shock absorbers and suspension system springs. A
hydraulic pump provides pressure for ram operation.
Electronic climate control systems use a small ECM to
monitor and control system operation. A vacuum actua-
tor in an electronic climate control system serves as an
interface between the ECM and vacuum diaphragms.
The vacuum diaphragms are used to move the control
doors for feeding airfl ow through the vents.
Electronic cruise control uses sensors, an ECM,
switches, and a throttle actuator to maintain a preset
vehicle speed. Brake, clutch, and neutral switches are
commonly used to disable the system.
In an electronic cruise control system, the speed sensor
produces a signal frequency that corresponds to vehicle
speed. If the speed or frequency rises or falls with the
angle of the road surface, the ECM activates the throttle
actuator, which moves the engine throttle valve to
increase or decrease engine power and cruising speed.
Electronically-controlled transmissions or transaxles
use sensors, an electronic control module, and sole-
noids to improve shifting effi ciency.
Electronic four-wheel steering systems use steering
gearboxes that are driven by electric motors to turn all
four wheels.
Figure 21-28. Note the basic parts of an electronic cruise con-
trol system.
Throttle
Actuator
Throttle
Actuator
Vehicle Speed Sensor
Control
Switch
Parking Break
Switch
Neutral Start
Safety Switch
Brake Light Switch
Cruise Control
ECM
Power Switch
with Indicator
Clutch Switch