GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Sonoma Pickup - 4WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 DTC P1639

Circuit Description

The powertrain control module (PCM) uses the 5-volt reference 2 circuit as a sensor feed for the fuel tank pressure (FTP) sensor. The PCM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.

Conditions for Running the DTC

The engine must be running.

Conditions for Setting the DTC
    • The PCM 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 number below refers to the step number on the diagnostic table.

  1. The ignition must be turned OFF and the PCM powered down or the test may become inaccurate.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you complete 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 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.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 4

4
  1. Turn the ignition OFF.
  2. Disconnect the FTP sensor connector.
  3. Connect a DMM between the FTP sensor signal circuit and a good ground.
  4. Turn the ignition ON, with the engine OFF.

Does the voltage measure near the specified value?

0 V

Go to Step 5

Go to Step 7

5
  1. Turn the ignition OFF.
  2. Disconnect the FTP sensor connector.
  3. Connect a DMM between the 5-volt reference and a good ground.
  4. Turn the ignition ON, with the engine OFF.

Does the voltage measure near the specified value?

5 V

Go to Step 11

Go to Step 6

6

Does the voltage measure more than the specified value?

5 V

Go to Step 7

Go to Step 8

7

  1. Turn OFF the ignition.
  2. Disconnect the PCM and check for a short to voltage on the following circuits:
    3. • The 5-volt reference circuit
    • The FTP sensor signal circuit
  3. If you find a problem repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and repair a problem?

 

Go to Step 13

Go to Step 12

8

Important: The ignition must be turned OFF for this test.

  1. Turn OFF the ignition.
  2. Using a DMM, check for continuity between the 5-volt reference circuit and the low reference circuit at the FTP sensor connector. Refer to Circuit Testing in Wiring Systems.

Is the resistance at the specified value?

∞ or OL

Go to Step  9

Go to Step  10

9
  1. Leave the ignition OFF.
  2. Using a DMM, check for continuity between the 5-volt reference circuit and a good chassis ground. Refer to Circuit Testing in Wiring Systems.

Is the resistance at the specified value?

∞ or OL

Go to Step  12

Go to Step  10

10

Repair the circuit for a short to chassis ground or a short to FTP sensor low reference. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 13

--

11

Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement .

Did you complete the replacement?

--

Go to Step 13

--

12

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 13

--

13
  1. Select the diagnostic trouble code (DTC) option and the Clear DTC Information option using the scan tool.
  2. Idle the engine at the normal operating temperature.
  3. Select the diagnostic trouble code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this test ran and passed?

--

Go to Step 14

Go to Step  2

14

Select the Capture Info option and the Review Info option using the scan tool.

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

--

Go to Diagnostic Trouble Code (DTC) List

System OK