Throttle Actuator Control (TAC) System Description: Notes

The throttle actuator control (TAC) system is used to improve emissions, fuel economy, and driveability. The TAC system eliminates the mechanical link between the accelerator pedal and the throttle plate. The TAC system eliminates the need for a cruise control module and idle air control motor. The following is a list of TAC system components:

• The accelerator pedal assembly includes the following components:
  • The accelerator pedal
  • The accelerator pedal position (APP) sensor 1
  • The APP sensor 2
• The throttle body assembly includes the following components:
  • The throttle position (TP) sensor 1
  • The TP sensor 2
  • The throttle actuator motor
  • The throttle plate
• The engine control module (ECM)

The ECM monitors the driver demand for acceleration with 2 APP sensors. The APP sensor 1 signal voltage range is from about 0.98-4.16 volts as the accelerator pedal is moved from the rest pedal position to the full pedal travel position. The APP sensor 2 range is from about 0.49-2.08 volts as the accelerator pedal is moved from the rest pedal position to the full pedal travel position. The ECM processes this information along with other sensor inputs to command the throttle plate to a certain position.

The throttle plate is controlled with a direct current motor called a throttle actuator control motor. The ECM can move this motor in the forward or reverse direction by controlling battery voltage and/or ground to 2 internal drivers. The throttle plate is held at a 7 percent rest position using a constant force return spring. This spring holds the throttle plate to the rest position when there is no current flowing to the actuator motor.

The ECM monitors the throttle plate angle with 2 TP sensors. The TP sensor 1 signal voltage range is from about 4.86-0.86 volts as the throttle plate is moved from 0 percent to wide open throttle (WOT). The TP sensor 2 voltage range is from about 0.82-4.14 volts as the throttle plate is moved from 0 percent to WOT.

The ECM performs diagnostics that monitor the voltage levels of both APP sensors, both TP sensors, and the throttle actuator control motor circuit. It also monitors the spring return rate of both return springs that are housed internal to the throttle body assembly. These diagnostics are performed at different times based on whether the engine is running, not running, or whether the ECM is currently in a throttle body relearn procedure.

Every ignition cycle, the ECM performs a quick throttle return spring test to make sure the throttle plate can return to the 7 percent rest position from the 0 percent position. This is to ensure that the throttle plate can be brought to the rest position in case of an actuator motor circuit failure. Observe, under cold conditions, the ECM commands the throttle plate to 7 percent with the ignition ON and the engine OFF to release any ice that may have formed on the throttle plate.