Throttle Actuator Control (TAC) System Description
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
Internal diagnostics
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
A signal 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 blade
The throttle actuator motor
The throttle position (TP) sensor 1 and 2
The TAC module
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.