Page 283 - Understanding Automotive Electronics

2735 | CH 8 Page 270 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

operate either from manifold vacuum or with a stepper motor. The stepper
motor implementation operates similarly to the idle speed control actuator
described in Chapter 7. The throttle opening is either increased or decreased by
the stepper motor in response to the sequences of pulses sent to the two
windings depending on the relative phase of the two sets of pulses.
The throttle actuator that is operated by manifold vacuum through a
solenoid valve is similar to that used for the EGR valve described in Chapter 7.
During cruise control operation the throttle position is set automatically by
the throttle actuator in response to the actuator signal generated in the control
system. This type of manifold-vacuum-operated actuator is illustrated in
Figure 8.7.
Throttle actuators use A pneumatic piston arrangement is driven from the intake manifold
manifold vacuum to pull vacuum. The piston-connecting rod assembly is attached to the throttle lever.
a piston that is mechani- There is also a spring attached to the lever. If there is no force applied by the
cally linked to the throt- piston, the spring pulls the throttle closed. When an actuator input signal
tle. The amount of energizes the electromagnet in the control solenoid, the pressure control valve is
vacuum provided is con- pulled down and changes the actuator cylinder pressure by providing a path to
trolled by a solenoid manifold pressure. Manifold pressure is lower than atmospheric pressure, so the
valve that is turned on actuator cylinder pressure quickly drops, causing the piston to pull against the
and off rapidly. throttle lever to open the throttle.

Figure 8.7
Vacuum-Operated
Throttle Actuator

FPO

270 UNDERSTANDING AUTOMOTIVE ELECTRONICS