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.
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. |
| • | The reduced engine power is active. |
| • | DTC P0638 runs continuously once the above conditions are met. |
Conditions for Setting the DTC
The required PWM to move the throttle blade is at 100 percent for more than 1.2 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 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 move to the open and 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 . |
Step | Action | Value(s) | 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 - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||
2 | Observe the DTC information. Is DTC P0689 also set? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 3 | ||||||
3 | Observe the DTC information. Are DTCs P0122, P0123, P0222, P0223 also set? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 4 | ||||||
4 |
Important: 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.
Did you find any of these conditions with the throttle body? | -- | Go to Step 15 | Go to Step 5 | ||||||
5 |
Important: Disconnecting the throttle body connector will cause additional codes to set.
Is the voltage less than the specified value for both circuits? | 2 V | Go to Step 6 | Go to Step 8 | ||||||
6 |
Does the DMM indicate maximum voltage above the specified value? | 2 V | Go to Step 7 | Go to Step 9 | ||||||
7 |
Does the DMM indicate maximum voltage above the specified value? | 2 V | Go to Step 11 | Go to Step 10 | ||||||
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 17 | Go to Step 16 | ||||||
9 | Test the TAC motor control 1 circuit 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 17 | Go to Step 13 | ||||||
10 | Test the TAC motor control 2 circuit 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 17 | Go to Step 13 | ||||||
11 |
Important: The DMM and test leads must be calibrated to 0 ohms to prevent misdiagnosis. Refer to the DMM User Manual for the calibration procedure.
Is the resistance less than the specified value? | 4 ohms | Go to Step 12 | Go to Step 14 | ||||||
12 | 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 17 | Go to Step 15 | ||||||
13 | Test for an intermittent and for a poor connection at the engine control module (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 17 | Go to Step 16 | ||||||
14 | 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 17 | -- | ||||||
15 | Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement . Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||
16 | Replace the ECM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||
17 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 18 | ||||||
18 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK | ||||||
