P0638

DTC P0638

Circuit Description

The engine control module (ECM) controls the throttle valve by applying a varying voltage to the control circuits of the throttle actuator control (TAC) motor. The ECM monitors the duty cycle that is required to actuate the throttle valve. The ECM monitors the throttle position (TP) sensors 1 and 2 to determine the actual throttle valve position. If the ECM detects that the duty cycle to control the throttle valve to the desired position is more than a predetermined value, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0638 Throttle Actuator Control (TAC) Command Performance

Conditions for Running the DTC

* The ignition is ON or the engine is operating.
* The ignition 1 voltage is more than 7 volts.
* DTC P0638 runs continuously once the above conditions are met for more than 1 second.

Conditions for Setting the DTC

* The ECM detects that the commanded duty cycle for the range test high is more than 80 percent for more than 0.6 second.
OR
* The ECM detects that the commanded duty cycle for the range test low is more than 80 percent for more than 5 seconds.

Action Taken When the DTC Sets

* The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
* The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC

* The control module turns OFF the malfunction indicator lamp (MIL) after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
* A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
* A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
* Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

* Use the J 35616 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.
* The lower connector of the ECM is connector C1 and the upper connector of the ECM is connector C2. Refer to Engine Controls Component Views.
* Inspect for a condition in which the throttle valve may have been held open. For example, ice may have formed in the throttle bore causing the throttle valve not to close. Observe the Freeze Frame/Failure Records.
* The throttle valve is spring loaded to a slightly open position. The throttle valve should be open approximately 3-5 percent. This is referred to as the rest position.
* The throttle valve should not be completely closed or open any more than a specified amount.
* The throttle valve should move open and to the closed position without binding under the normal spring pressure.
* The throttle should NOT be free to move open or closed WITHOUT spring pressure.
* The scan tool has the ability to operate the throttle control system using the TP function located in the TAC System menu. This function operates the throttle valve through the entire range in order to determine if the TAC system operates correctly. Additional DTCs may set using this function.
* For an intermittent condition, refer to Intermittent Conditions. Intermittent Conditions

Test Description

Step 1 - Step 7:

Step 8 - Step 18:

Step 19:

The numbers below refer to the step numbers on the diagnostic table.
2. This step determines if the condition exists. A TP sensor DTC may cause this DTC to set.
3. Depending on the condition, DTC P0638, P2100, P2101, and/or P2119 may set individually or together.
8. When the ignition is turn ON, the ECM operates the throttle actuator control motor to verify the integrity of the system prior to start-up. This can be seen by the momentary flash of the test lamp as the ignition is turned ON.