Purpose
The throttle actuator control (TAC) system delivers improved throttle response
and greater reliability and eliminates the need for mechanical cable. The TAC system
performs the following functions:
| • | Accelerator pedal position sensing |
| • | Throttle positioning to meet driver and engine demands |
| • | Throttle position sensing |
| • | Cruise control functions |
| • | Manage TAC electrical power consumption |
The TAC system includes the following components:
| • | The accelerator pedal position (APP) sensors |
| • | The throttle body assembly |
| • | The engine control module (ECM) |
Accelerator Pedal Position (APP) Sensor
The accelerator pedal contains 2 individual APP sensors within the assembly.
The APP sensors 1 and 2 are potentiometer type sensors each with 3 circuits:
| • | A 5-volt reference circuit |
| • | A low reference circuit |
The APP sensors are used to determine the pedal angle. The ECM provides each
APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors
provide the ECM with signal voltage proportional to the pedal movement. The APP sensor 1
signal voltage at rest position is less than 1 volt and increases to above
2.5 volts as the pedal is actuated. The APP sensor 2 signal voltage
at rest position is near the 4 volts and decreases to less than 1 volt
as the pedal is actuated.
Throttle Body Assembly
The throttle assembly contains the following components:
| • | The throttle actuator motor |
| • | The throttle position (TP) sensor 1 and 2 |
The throttle body functions similar to a conventional throttle body with the
following exceptions:
| • | An electric motor opens and closes the throttle valve. |
| • | The throttle blade is spring loaded in both directions and the default
position is slightly open. |
| • | There are 2 individual TP sensors within the throttle body assembly. |
The TP sensors are used to determine the throttle plate angle. The TP sensors
provide the ECM with a signal voltage proportional to throttle plate movement. The
TP sensor 1 signal voltage at closed throttle is above 4 volts and decreases
as the throttle plate is opened. The TP sensor 2 signal voltage at closed throttle
is below 1 volt and increases as the throttle plate is opened.
Engine Control Module
The ECM is the control center for the TAC system. The ECM determines the drivers
intent and then calculates the appropriate throttle response. The ECM achieves throttle
positioning by providing a pulse width modulated voltage to the TAC motor.
Modes of Operation
Normal Mode
During the operation of the TAC system, several modes or functions are considered
normal. The following modes may be entered during normal operation:
| • | Minimum pedal value--At key-up the ECM updates the learned minimum
pedal value. |
| • | Minimum TP values--At key-up the ECM updates the learned minimum
TP value. In order to learn the minimum TP value, the throttle blade is moved to the
closed position. |
| • | Ice break mode--If the throttle is not able to reach a predetermined
minimum throttle position, the ice break mode is entered. During the ice break mode,
the ECM commands the maximum pulse width several times to the throttle actuator motor
in the closing direction. |
| • | Battery saver mode--After a predetermined time without engine RPM,
the ECM commands the battery saver mode. During the battery saver mode, the TAC module
removes the voltage from the motor control circuits, which removes the current draw
used to maintain the idle position and allows the throttle to return to the spring
loaded default position. |
Reduced Engine Power Mode
When the ECM detects a condition with the TAC system, the ECM may enter a reduced
engine power mode. Reduced engine power may cause one or more of the following conditions:
| • | Acceleration limiting--The ECM will continue to use the accelerator
pedal for throttle control; however, the vehicle acceleration is limited. |
| • | Limited throttle mode--The ECM will continue to use the accelerator
pedal for throttle control; however, the maximum throttle opening is limited. |
| • | Throttle default mode--The ECM will turn off the throttle actuator
motor and the throttle will return to the spring loaded default position. |
| • | Forced idle mode--The ECM will perform the following actions: |
| - | Limit engine speed to idle by positioning the throttle position, or by
controlling the fuel and spark if the throttle is turned off. |
| - | Ignore the accelerator pedal input. |
| • | Engine shutdown mode--The ECM will disable fuel and de-energize the
throttle actuator. |
