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-A/BT-8637 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 . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines if the condition exists. A TP sensor DTC may cause this DTC to set.
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Depending on the condition, DTC P0638, P2100, P2101, and/or P2119 may set individually or together.
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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.
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 - Vehicle? | -- | Go to Step 2 | |||||
Did DTC P0121, P0122, P0123, P0221, P0222, or P0223 fail this ignition? | -- | Go to Step 3 | ||||||
Did DTC P0638, P2100, P2101, and/or P2119 fail this ignition? | -- | Go to Step 5 | Go to Step 4 | |||||
4 |
Did the DTC fail this ignition? | -- | Go to Step 5 | Go to Diagnostic Aids | ||||
5 |
Is the voltage within the specified range? | 0.40-0.80 V | Go to Step 6 | Go to DTC P0121 | ||||
6 | Observe the TP sensor 2 voltage parameter with a scan tool. Is the voltage within the specified range? | 4.7-4.1 V | Go to Step 7 | Go to DTC P0221 | ||||
7 |
Notice: Refer to Disengaging Connectors Notice in the Preface section. Important: Disconnecting the throttle body harness connector causes additional DTCs to set. Is the voltage within the specified range for both circuits? | 2-4 V | Go to Step 8 | Go to Step 9 | ||||
Important: The test lamp intensity should be bright. Compare to another vehicle. Does the test lamp turn ON and then OFF each time the ignition is cycled? | -- | Go to Step 12 | Go to Step 11 | |||||
9 | Is the voltage less than the specified value? | 2 V | Go to Step 10 | Go to Step 14 | ||||
10 | 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 . Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 13 | ||||
11 |
Is there continuity? | -- | Go to Step 15 | Go to Step 13 | ||||
12 | Test for shorted terminals and for poor connections at the throttle body connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 16 | ||||
13 | Test for shorted terminals and for poor connections at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 17 | ||||
14 | Repair the appropriate TAC motor control circuit for a short to voltage. Refer to Wiring Repairs . Did you complete the repair? | -- | Go to Step 18 | -- | ||||
15 | Repair the short between the TAC motor control circuits 1 and 2. Refer to Wiring Repairs . Did you complete the repair? | -- | Go to Step 18 | -- | ||||
16 | Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement . Did you complete the replacement? | -- | Go to Step 18 | -- | ||||
17 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 18 | -- | ||||
18 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 19 | ||||
19 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||
