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 (APP) sensing |
| • | Throttle positioning to meet driver and engine demands |
| • | Throttle position (TP) sensing |
| • | Cruise control functions |
| • | Manage TAC electrical power consumption |
The TAC system includes the following components:
| • | The throttle body assembly |
| • | The powertrain control module (PCM) |
Accelerator Pedal Position (APP) Sensor
The accelerator pedal contains 2 individual accelerator pedal position
(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 powertrain control
module (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 actuator control (TAC) module |
The throttle body functions similar to a conventional throttle body with the
following exceptions:
| • | There is no longer an idle air control (IAC) motor and associated air
passages. |
| • | 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 powertrain control module (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 (TAC) Module
The throttle actuator control (TAC) module is the control center for the TAC
system. The TAC system uses electronically erasable programmable read only memory
(EEPROM) and is self-diagnosing. The TAC module provides diagnostic information to
the powertrain control module (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 (PCM)
The powertrain control module (PCM) determines the drivers intent and then calculates
the appropriate throttle response. This information is sent to the throttle actuator
control (TAC) module through a dedicated serial data line.
Modes of Operation
Normal Mode
During the operation of the throttle actuator control (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 powertrain control module (PCM)
updates the learned minimum pedal value. |
| • | Minimum throttle position (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 TP, 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 TP, 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. |
