P0638

DTC P0638

CIRCUIT DESCRIPTION




The engine control module (ECM) opens the throttle blades by applying a pulse width modulated voltage to the throttle valve motor. The ECM reverses the polarity on the throttle valve motor control circuits in order to close the throttle blades. The ECM increases the pulse width as necessary to open the throttle blades. The ECM monitors the throttle position (TP) sensors 1 and 2 to determine the actual blade position. If the ECM detects the required pulse width modulation (PWM) to move the throttle blade exceeds a predetermined value, DTC P0638 sets.

CONDITIONS FOR RUNNING THE DTC
- The ignition is ON.
- The battery voltage is more than 7 volts.

CONDITIONS FOR SETTING THE DTC
The required PWM to move the throttle blades exceeds a predetermined value.

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 and/or the Failure Records.
- The control module commands the TAC system to operate in the Reduced Engine Power mode.
OR
- Under certain conditions the control module commands the engine OFF.
- The message center displays Reduced Engine Power.

CONDITIONS FOR CLEARING THE MIL/DTC
- The control module turns OFF the malfunction indicator lamp (MIL) after 3 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
- Inspect for a condition in which the throttle valves may have been held open.
- Inspect for conditions in which ice may have formed in the throttle bore.
- The throttle valves are spring loaded to a slightly open position. This is referred to as the rest position.
- The throttle valves should not be completely closed, nor should they be open any more than the specified amount.
- The throttle valves 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 will operate the throttle valve through the entire range in order to determine if the throttle body and system operate correctly.
- For intermittent conditions, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-11:

Steps 12-14:

The numbers below refer to the step numbers on the diagnostic table.
3. The throttle valves are spring loaded in a slightly open position and should move in either direction without binding. The throttle valves should always be under spring pressure.
6. When the ignition is turn ON, the ECM operates the throttle 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.