GM Service Manual Online
For 1990-2009 cars only

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?

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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.

  1. Turn OFF the ignition.
  2. Visually inspect the throttle body for the following conditions:
  3. • Throttle valves that are NOT in the rest position
    • Throttle valves that are binding open or closed
    • Throttle valves that are free to move open or closed WITHOUT 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.

  1. Disconnect the throttle body harness connector.
  2. Turn ON the ignition, with the engine OFF.
  3. Measure the voltage of the throttle actuator control (TAC) motor control 1 and TAC motor control 2 circuits of the throttle actuator motor with a DMM connected to ground.

Is the voltage less than the specified value for both circuits?

2 V

Go to Step 6

Go to Step 8

6

  1. Turn OFF the ignition.
  2. Allow the engine control module (ECM) to completely power down. This may take several minutes and can be verified by the loss of communication with the scan tool.
  3. Connect the DMM between the TAC motor control 1 circuit and a good ground.
  4. Using the min/max function, select the proper voltage range and record the maximum voltage with a DMM.
  5. Turn ON the ignition.

Does the DMM indicate maximum voltage above the specified value?

2 V

Go to Step 7

Go to Step 9

7

  1. Turn OFF the ignition.
  2. Allow the ECM to completely power down. This may take several minutes and can be verified by the loss of communication with the scan tool.
  3. Connect the DMM between the TAC motor control 2 circuit and a good ground.
  4. Using the min/max function, select the proper voltage range and record the maximum voltage with a DMM.
  5. Turn ON the ignition.

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 14

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.

  1. Turn OFF the ignition.
  2. Disconnect the ECM connector containing the TAC motor control circuits.
  3. Measure the resistance of the TAC motor control 1 and TAC motor control 2 circuits between the ECM connector and the throttle body connector. Refer to Circuit Testing in Wiring Systems.

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

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17

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

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