Diagnostic Instructions

DTC Descriptors

DTC P0638: Throttle Actuator Control (TAC) Command Performance

DTC P1551: Throttle Valve Rest Position Not Reached During Learn

DTC P2100: Throttle Actuator Control (TAC) Motor Control Circuit

DTC P2101: Control Module Throttle Actuator Position Performance

DTC P2119: Throttle Closed Position Performance

DTC P2176: Minimum Throttle Position Not Learned

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Motor Control 1	P0638, P1551, P2100, P2101, P2176	P0638, P1551, P2100, P2101, P2176	P0638, P1551, P2100, P2101, P2176	P0638, P1551, P2100, P2101, P2176
Motor Control 2	P0638, P1551, P2100, P2101, P2176	P0638, P1551, P2100, P2101, P2176	P0638, P1551, P2100, P2101, P2176	P0638, P1551, P2100, P2101, P2176

Circuit/System Description

The engine control module (ECM) is the control center for the throttle actuator control (TAC) system. The ECM determines the drivers intent and then calculates the appropriate throttle response. The ECM achieves throttle positioning by providing a pulse width modulated voltage to the TAC motor. The TAC system uses the following circuits:

Two processors are also used to monitor the TAC system data. Both processors are located within the ECM. Both processors monitor each other's data to verify that the indicated throttle position is correct.

Conditions for Running the DTC

P0638, P2100, P2101

P1551 and P2119

P2176

Conditions for Setting the DTC

P0638

The motor control pulse width is more than 80 percent for more than 4 seconds.

P1551, P2100, and P2101

The indicated throttle position does not match the predicted throttle position for more than 0.3 second.

P2119

The ECM determines that the throttle blade did not return to the rest position within 720 milliseconds.

P2176

The difference between the predicted and the actual throttle position is more than a calibrated amount for more than 1.5 seconds.

Action Taken When the DTC Sets

Conditions for Clearing the DTC

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Throttle Actuator Control (TAC) System Description

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

1. Ignition ON, use a scan tool to observe the throttle position (TP) sensor 1 indicated position. Throttle position should increase as pedal is applied and decrease as pedal is released. When no accelerator pedal activity is detected, the throttle position will return to the spring loaded default position.

Note: A low voltage condition may set this DTC.

2. Review the Freeze Fame/Failure Records and ensure sufficient ignition voltage was present when the DTC was set.

⇒	If voltage was low, verify with the owner that the battery was not discharged. If the battery has not been discharged, refer to Power Mode Mismatch.

Circuit/System Testing

Warning: Turn OFF the ignition before inserting fingers into the throttle bore. Unexpected movement of the throttle blade could cause personal injury.

1. Inspect the Q38 throttle body for the following conditions:

•	A throttle blade that is NOT in the rest position

•	A throttle valve that is binding open or closed

•	A throttle valve that is free to move open or closed WITHOUT spring pressure

Note: Disconnecting the throttle body harness connector causes additional DTCs to set.

2. Ignition OFF, disconnect the harness connector at the Q38 throttle body.
3. Ignition ON, verify that a test lamp does not illuminate between each motor control circuit terminal A and B and ground.

⇒	If the test lamp illuminates, test the affected motor control circuit for a short to voltage. If the circuit tests normal, replace the K20 ECM.

4. Ignition ON, verify that a test lamp does not illuminate between each motor control circuit terminal A and B and B+.

⇒	If test lamp illuminates, test the affected motor control circuit for a short to ground. If the circuit tests normal, replace the K20 ECM.

5. Ignition ON, test for 3.5-4.7V between the Motor control circuit terminal B and ground.

⇒	If voltage is not within the specified range, test for an open or high resistance. If the circuit tests normal, replace the K20 ECM.

6. Ignition OFF, allow sufficient time for the K20 ECM to completely power down. Connect a DMM between the motor control 2 terminal A circuit and ground.

Note: When measuring peak voltage, ensure that the DMM is on the proper voltage scale that the response time is set to 1 ms.

7. Using the peak min max function, measure the voltage on the motor control circuit terminal A as the ignition is turned ON. Voltage should be within 1V of ignition voltage.

⇒	If voltage is not within 1V of ignition voltage, test for an open or high resistance. If the circuit tests normal, replace the K20 ECM.

8. If all circuits test normal, replace the Q38 throttle body

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.