DTC P1646. GMC Truck GMC T-Series Medium Duty Truck F6/F7/F8 2004

GM Service Manual Online

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2004 🢒 GMC T-Series Medium Duty Truck F6/F7/F8 🢒 DTC 🢒 DTC P1646

DTC P1646 Without RPO CTF

Circuit Description

The engine control module (ECM) provides 5 volts for the accelerator pedal position (APP) sensor 3.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

•	The circuit voltage is more than 5.5 volts.

•	The circuit voltage is less than 4.5 volts.

•	The above conditions are met for 0.7 seconds.

Action Taken When the DTC Sets

The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

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.

•	Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Inspect for the following conditions:

•	A poor connection at the ECM connector

•	A poor connection at the battery connector

•	A harness short

•	A defect at the sensor

Step

Action

Values

Yes

Schematic Reference: Engine Controls Schematics

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

Turn the ignition ON.
Clear the DTCs.
Turn the ignition OFF for 30 seconds.
Turn the ignition ON.

Does the scan tool display the DTC?

Go to Step 3

Go to Intermittent Conditions

Turn the ignition OFF.
Disconnect the APP 3 sensor.
Turn the ignition ON.
Measure the voltage of the APP 3 sensor 5-volt reference circuit.

Does the voltage measure within the specified range?

4.5-5.5 V

Go to Step 4

Go to Step 6

Disconnect the APP sensor.
Measure the resistance of the APP 3 sensor with a DMM.
Depress the accelerator pedal while observing the resistance of the APP 3 sensor.

Does the resistance of the APP 3 sensor change while moving the accelerator pedal?

Go to Step 6

Go to Step 5

Replace the APP sensor.

Did you complete the replacement?

Go to Step 8

Inspect for the following conditions:

•	A poor connection at the APP sensor or the ECM.

•	An open in the APP sensor harness.

•	A short to ground in the APP sensor harness.

•	A short to voltage in the APP sensor harness.

Did you find and correct the condition?

Go to Step 8

Go to Step 7

Replace the ECM.

Did you complete the replacement?

Go to Step 8

Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

Go to Step 9

Go to Step 2

Observe the stored information, Capture Info with a scan tool.

Does the scan tool display any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK

DTC P1646 With RPO CTF

The engine control module (ECM) supplies 5 volts reference voltage through the reference circuit 3 to the accelerator pedal position (APP) sensor. The ECM monitors the voltage on the 5-volt reference 3 circuit. If the ECM detects that the voltage is out of tolerance, this DTC will set.

Conditions for Running the DTC

•	The ignition is ON.

•	The battery voltage is between 10-16 volts.

Conditions for Setting the DTC

The 5-volt reference circuit 3 voltage is more than 5.5 volts or less than 4.5 volts.

Action Taken When the DTC Sets

•	The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails

•	The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.

Condition for Clearing the DTC

•	A current DTC, Last Test Failed, clears when the diagnostic runs and passes.

•	Clear the MIL and the DTC with a scan tool.

Step

Action

Values

Yes

Schematic Reference: Engine Controls Schematics

Did you perform the Diagnostic System Check - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

Observe the Failure Records data for this DTC.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Failure Records.

Did the DTC fail this ignition?

Go to Step 3

Go to Intermittent Conditions

Turn OFF the ignition.
Disconnect the accelerator pedal position (APP) sensor connector.
Turn ON the ignition, with the engine OFF.
Observe the APP Sensor 3 parameter with a scan tool.

Is the APP Sensor 3 parameter within the specified value?

4.5-5.5 V

Go to Step 5

Go to Step 4

Test the signal circuit between the engine control module (ECM) and the APP Sensor 3 for the following conditions:

•	A short to ground circuit

•	A short to battery voltage

Repair the circuits as necessary.

Did you find and correct the condition?

Go to Step 8

Go to Step 6

Replace the APP sensor. Refer to Accelerator Pedal Position Sensor Replacement .

Did you complete the replacement?

Go to Step 8

Turn OFF the ignition.
Disconnect the ECM.
Inspect for connections at the harness connector of the ECM.
Repair the connections as necessary

Did you find and correct the condition?

Go to Step 8

Go to Step 7

Replace the ECM. Refer to Engine Control Module Replacement/Restore Fuel Rate Data .

Did you complete the replacement?

Go to Step 8

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

Did the DTC fail this ignition?

Go to Step 2

Go to Step 9

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK