GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2001 🢒 Regal 🢒 DTC 🢒 DTC P1639

Circuit Description

The powertrain control module (PCM) uses the 5-volt reference 2 circuit as a sensor feed for the A/C refrigerant pressure sensor and 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, DTC P1639 will set.

Conditions for Running the DTC

The engine must be running.

Conditions for Setting the DTC
    • The PCM detects voltage out of tolerance 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.

DTC P1639 - 5 Volt Reference 2 Circuit

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. Inspect the PCM and engine grounds. Ensure that the grounds are clean and secure.
  2. If a condition is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 4

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

Does the voltage measure near the specified value?

5 V

Go to Step 5

Go to Step 6

5
  1. Turn OFF the ignition.
  2. Reconnect the A/C pressure sensor.
  3. Disconnect the FTP sensor.
  4. Connect a DMM between the 5-volt reference at the FTP sensor connector and a good ground.

Does the voltage measure near the specified value?

5 V

Go to Intermittent Conditions

Go to Step 13

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. Leave the DMM connected from step 4.
  3. Disconnect the PCM. Refer to Powertrain Control Module Replacement .
  4. Turn ON the ignition, with the engine OFF.

Does the voltage measure at the specified value?

0 V

Go to Step 15

Go to Step 12

8
  1. Turn OFF the ignition.
  2. Leave the DMM connected from step 4.
  3. Disconnect the FTP sensor.
  4. Turn ON the ignition, with the engine OFF.

Does the voltage measure near the specified value?

5 V

Go to Step 14

Go to Step 9

9

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

  1. Turn OFF the ignition.
  2. Using a DMM, inspect for continuity between the 5-volt reference circuit and the low reference circuit of the A/C pressure sensor or the FTP sensor. Refer to Circuit Testing in Wiring Systems.

Is the resistance at the specified value?

∞ or OL

Go to Step 10

Go to Step 11

10
  1. Leave the ignition OFF.
  2. Using a DMM, inspect 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 15

Go to Step 11

11

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

Did you complete the repair?

--

Go to Step 17

--

12

Locate and repair the short to voltage in the 5-volt reference circuit or the FTP signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

--

13

Replace the A/C pressure sensor. Refer to Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement in Heating, Ventilation, and Air Conditioning.

Did you complete the replacement?

--

Go to Step 17

--

14

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

Did you complete the replacement?

--

Go to Step 17

--

15

Test the 5-volt reference circuit for an open, short to chassis ground or poor connection at the PCM. Refer to Circuit Testing and Wiring Repairs or Connector Repairs in Wiring Systems.

Did you find a problem?

--

Go to Step 17

Go to Step 16

16

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 17

--

17
  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 18

Go to Step 2

18

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