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Home >> Cadillac >> 2006 >> STS Base, 3.6 7, AWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System - 3.6L - DTC P0342 To P0690 >> DTC P0638 >> Test Description
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Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. 2:  This step determines if the condition exists. A TP sensor DTC may cause this DTC to set.
  2. 3:  Depending on the condition, DTC P0638, P2100, P2101, and/or P2119 may set individually or together.
  3. 8:  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.
P0638 Throttle Actuator Control (TAC) Command Performance

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  Go to Diagnostic System Check - Vehicle
2 
  1. Turn On the ignition, with the engine OFF.
  2. Clear the DTCs with a scan tool.
  3. Start the engine.
  4. Perform the following tests:
    • Rapidly depress the accelerator pedal from the rest position to the wide open throttle position (WOT) and release pedal. Repeat the procedure several times.
    • Slowly depress the accelerator pedal to WOT and then slowly return the pedal to closed throttle. Repeat the procedure several times.
  5. Observe the DTC information with a scan tool.
Did DTC P0121, P0122, P0123, P0221, P0222, or P0223 fail this ignition?		 - Go to Diagnostic Trouble Code (DTC) List - Vehicle Go to Step 3 
3  Did DTC P0638, P2100, P2101, and/or P2119 fail this ignition? - Go to Step 5  Go to Step 4 
4
  1. Observe the Freeze Frame/Failure Records for this DTC.
  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 5  Go to Diagnostic Aids
5
  1. Turn OFF the engine.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the TP sensor 1 voltage parameter with a scan tool.
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
    NOTE: Refer to Disengaging Connectors Notice .
    IMPORTANT: Disconnecting the throttle body harness connector causes additional DTCs to set.
  1. Disconnect the throttle body harness connector. Refer to Throttle Body Assembly Replacement .
  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 control motor with a DMM connected to the engine control module (ECM) housing.
Is the voltage within the specified range for both circuits?		 2-4 V Go to Step 8  Go to Step 9 
8 
  1. Turn OFF the ignition.
  2. Connect the J 34730-405  Injector Test Lamp between the TAC motor control 1 and TAC motor control 2 circuit of the throttle body harness connector. See Special Tools .
    3. IMPORTANT: The test lamp intensity should be bright. Compare to another vehicle.
  3. Turn ON the ignition for about 5 seconds and then turn OFF the ignition while you observe the test lamp. Repeat this procedure, as necessary.
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
  1. Turn OFF the ignition.
  2. Disconnect the ECM.
  3. Test for continuity between the TAC motor control 1 and TAC motor control 2 circuit of the throttle body harness connector.
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
  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 19 
19 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 System OK
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