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. |
| • | A message center or an indicator displays Reduced Engine Power. |
| • | Under certain conditions the control module commands the engine OFF. |
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 . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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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.
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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.
Step | Action | Value | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||
2 | Observe the DTC information. Are DTCs P0121, P0122, P0123, P0221, P0222, P0223 also set? | -- | Go to Step 3 | |||||||
Did you find any of these conditions with the throttle body? | -- | Go to Step 13 | Go to Step 4 | |||||||
4 |
Important: Disconnecting the throttle body connector will cause additional codes to set.
Is the voltage within the specified range for both circuits? | 2-4 V | Go to Step 6 | Go to Step 5 | ||||||
5 | Is the voltage less than the specified value for either circuit? | 2 V | Go to Step 9 | Go to Step 8 | ||||||
Does the test lamp flash ON, then turn OFF? | -- | Go to Step 7 | Go to Step 11 | |||||||
7 |
Is the resistance less than the specified value? | 5 ohms | Go to Step 10 | Go to Step 12 | ||||||
8 | Test the TAC motor control circuit that measured above the specified value for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 11 | ||||||
9 | Test the TAC motor control circuit that measured below the specified value for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 11 | ||||||
10 | Test for an intermittent and for a poor connection at the throttle body connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 13 | ||||||
11 | Test for an intermittent and for a poor connection at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 14 | ||||||
12 | Repair the TAC motor control circuit with the high resistance. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 15 | -- | ||||||
13 | Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | ||||||
14 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | ||||||
15 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 16 | ||||||
16 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||
