GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Controls - 6.6L 🢒 DTC P1639

Circuit Description

The engine control module (ECM) uses the 5-volt reference 2 circuit as a sensor feed to the following sensors:

    • The boost sensor
    • The accelerator pedal position accelerator pedal position (APP) sensor 2
    • The barometric pressure (BARO) sensor

The ECM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is out of tolerance, the ECM will set diagnostic trouble code (DTC) P1639.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC
    • The ECM detects a voltage out of tolerance condition on the 5-volt reference 2 circuit.
    • The above condition is present for longer than 10 seconds.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

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.

Test Description

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

  1. If the circuit is shorted to the transmission control module (TCM) malfunction indicator lamp (MIL) request circuit, the scan tool will show skewed BARO and BOOST sensor readings just after the ignition is turned ON. The sensor readings will then return to normal in under 1 second.

    2.

  2. This vehicle is equipped with a ECM which utilizes an electrically erasable programmable read only memory (EEPROM). When the ECM is being replaced, the new ECM must be programmed.

DTC P1639 - 5 Volt Reference B Circuit

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step  2

Go to Diagnostic System Check - Engine Controls

2
  1. Observe the Freeze Frame/Failure Records data 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 as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Intermittent Conditions

3
  1. Visually and physically inspect the ECM and engine grounds. Ensure that the grounds are clean and secure.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 4

4
  1. Turn OFF the ignition.
  2. Disconnect the BARO sensor electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Using a DMM, measure voltage between the 5-volt reference circuit and the engine ground.

Does the voltage measure near the specified value?

5 V

Go to Step 6

Go to Step 5

5

Does the voltage measure more than the specified value?

5 V

Go to Step 9

Go to Step 7

6
  1. Reconnect BARO sensor.
  2. Disconnect the APP sensor.
  3. Using a DMM, measure voltage between the 5-volt reference B circuit of APP 2 and the engine ground.

Does the voltage measure near the specified value?

5 V

Go to Intermittent Conditions

Go to Step 10

7
  1. Monitor DMM while disconnecting the following sensors:
    2. • The APP sensor
    • The boost sensor
  2. If voltage changes when one of the sensors is disconnected, replace the component. Refer to Accelerator Pedal Position Sensor Replacement or Boost Sensor Replacement .

Did you find and correct the condition?

--

Go to Step 12

Go to Step 8

8

  1. Turn OFF the ignition.
  2. Disconnect the ECM.
  3. Check the 5-volt reference circuit for the following conditions:
    4. • A short to ground.
    • A short to any low reference circuit.
    • A short to the fuel pressure regulator control circuit.
    • A short to the TCM MIL request circuit.
    • A short to the ignition 1 relay control circuit.
  4. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

9
  1. Turn OFF the ignition.
  2. Disconnect the ECM.
  3. Turn ON the ignition, with the engine OFF.
  4. Check the following circuits for a short to voltage:
    5. • The 5-volt reference circuit
    • The boost sensor signal circuit
    • The BARO sensor signal circuit
  5. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

10

Replace the BARO sensor. Refer to Barometric Pressure Sensor Replacement .

Did you complete the replacement?

--

Go to Step 12

--

11

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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 13

Go to Step 2

13

With a scan tool, observe the stored information, Capture Info.

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

--

Go to Diagnostic Trouble Code (DTC) List

System OK