DTC P0638: Throttle Actuator Control Command Performance: Description

WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The Throttle Actuator Control (TAC) module contains a direct current actuator motor controlled by the ECM used to move the throttle plate (in either direction) and two throttle position sensors to determine throttle plate position. The throttle plate is held at 7 percent rest position to a mechanical stop by a constant force return spring. This spring will hold the throttle plate to the stop when there is no current flowing to the actuator motor. There is another return spring, which creates constant force on the throttle plate only when the throttle plate is moved towards the full closed position. The ECM will monitor both the actuator motor and TP sensors for circuit faults as well as check to make sure both springs can return the throttle to the 7 percent position no matter where the throttle plate remains. DTC P0638 will set if the duty cycle of the throttle actuator motor needs to be commanded greater than a calibrated amount for a certain length of time in order to achieve the desired throttle plate position (in either direction).