GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Corvette 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 DTC P1639

Circuit Description

The powertrain control module (PCM) uses the 5-volt reference B circuit as a sensor feed to the following sensors:

    • The fuel tank pressure (FTP) sensor
    • The A/C pressure sensor

The PCM monitors the voltage on the 5-volt reference B circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.

Conditions for Running the DTC

The engine is running.

Conditions for Setting the DTC
    • The PCM detects a voltage out of tolerance condition on the 5-volt reference B 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 number below refers to the step number on the diagnostic table.

  1. This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read only memory (EEPROM). When the PCM is being replaced, the new PCM 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 and/or 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 and/or Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Intermittent Conditions

3
  1. Inspect the PCM 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.

Was a problem found?

--

Go to Step 12

Go to Step 4

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

Is voltage near the specified value indicated?

5 V

Go to Step 6

Go to Step 5

5

Is voltage more than the specified value?

5 V

Go to Step 9

Go to Step 7

6
  1. Reconnect A/C pressure sensor.
  2. Disconnect FTP sensor.
  3. Using a DMM, measure the voltage between the 5-volt reference B circuit and engine ground.

Is voltage near the specified value indicated?

5 V

Go to Intermittent Conditions

Go to Step 10

7
  1. Monitor DMM while disconnecting the FTP sensor.
  2. If voltage changes when the sensor is disconnected, replace the component. Refer to Fuel Tank Pressure Sensor Replacement

Was a component replaced?

--

Go to Step 12

Go to Step 8

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Check the 5-volt reference circuit for a short to ground or any sensor low reference circuit.
  4. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 12

Go to Step 11

9
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  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 FTP sensor signal circuit.
  5. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 12

Go to Step 11

10

Replace the A/C pressure sensor. Refer to Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement in HVAC.

Is the action complete?

--

Go to Step 12

--

11

Replace the PCM. Refer to Powertrain 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