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 powertrain control module (PCM) |
Accelerator Pedal Position (APP) Sensor
The accelerator pedal contains two 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 PCM provides each
APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors
provide the PCM with signal voltage proportional to the pedal movement. Both APP sensor
signal voltages are low at rest position and increase as the pedal is applied.
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 TAC module is located within the throttle body assembly. |
| • | 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 PCM with a signal voltage proportional to throttle plate movement. Both
TP sensor signal voltages are low at closed throttle and increase as the throttle
opens.
Throttle Actuator Control Module
The TAC module is the control center for the TAC system. The TAC system uses
Electronically Erasable Programmable Read Only Memory and is self-diagnosing. The
TAC module provides diagnostic information to the PCM through a dedicated serial data
line. The TAC achieves throttle positioning by providing a pulse width modulated voltage
to the TAC motor as directed by the PCM. The TAC module is not serviced and just be
replaced with the throttle body assembly.
Powertrain Control Module
The PCM determines the drivers intent and then calculates the appropriate throttle
response. This information is sent to the TAC module through a dedicated serial data
line.
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 PCM updates the learned minimum
pedal value. |
| • | Minimum TP values--At key-up the PCM 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 PCM 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 PCM 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 PCM detects a condition with the TAC system, the PCM may enter a reduced
engine power mode. Reduced engine power may cause one or more of the following conditions:
| • | Acceleration limiting--The PCM will continue to use the accelerator
pedal for throttle control; however, the vehicle acceleration is limited. |
| • | Limited throttle mode--The PCM will continue to use the accelerator
pedal for throttle control; however, the maximum throttle opening is limited. |
| • | Throttle default mode--The PCM will turn off the throttle actuator
motor and the throttle will return to the spring loaded default position. |
| • | Forced idle mode--The PCM 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 PCM will disable fuel and de-energize the
throttle actuator. |
