P0638

Wiring Diagram:

P0638- THROTTLE ACTUATOR CONTROL COMMAND PERFORMANCE
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 a 7% 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% 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).

DTC PARAMETERS
DTC P0638 will set if the ECM needs to command an 80% or greater duty cycle to the throttle actuator motor in order to achieve a certain throttle position when:
^ Condition exists for longer than 0.6 seconds
^ The 80% or greater duty cycle needed could be to open or close the throttle plate
^ Battery voltage is greater than 7.0 volts

DTC P0638 diagnostic runs continuously once the above conditions have been met.

P0638 is a type A DTC

NOTE: The MIL will only illuminate if the fault is present for longer than 5 seconds.

DIAGNOSTIC AIDS

IMPORTANT: If any TP sensor 1 or 2 DTC is set. diagnose that DTC first. Then if DTC P2101 is also set, diagnose that DTC next.

A short to ground or voltage on circuit 581 or 582 will set P0638 and P2101. An open in either. will set P2101.

If DTC P0638 is set, the ECM will default to one of the two limp home modes (random injector disable with engine speed limited to 1800 rpm). The REDUCED POWER telltale will be illuminated and the ECM will remain in this condition for the entire ignition cycle even if the fault [5 corrected.

If no TP sensor 1, TP sensor 2 or P2101 DTC is set with P0638; the only possible cause for P0638 is weakened return springs or a binding throttle plate. Replace throttle body assembly. Make sure the throttle body is re]earned by performing the "Throttle Body Relearn Procedure" under DTC P2176.