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

Circuit Description

The powertrain control module (PCM) uses the 5-volt reference B circuit as a sensor feed for the A/C refrigerant 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 ignition is ON.

Conditions for Setting the DTC
    • The PCM detects a voltage out of tolerance condition on the 5-volt reference B circuit.
    • 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.

Diagnostic Aids

Important: Inspect the PCM and engine grounds for being secure and clean.

Check for the following conditions:

    • A poor connection at the PCM-- Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
    • A damaged harness--Inspect the wiring harness for damage. If the harness appears to be OK, observe the A/C pressure display on the scan tool while moving connectors and wiring harnesses related to the A/C refrigerant pressure sensor. A change in the A/C pressure display will indicate the location of the malfunction.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

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 complete the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2
  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 12

Go to Step 3

3
  1. Turn the ignition OFF.
  2. Disconnect the fuel tank pressure (FTP) sensor connector.
  3. Turn the ignition ON, with the engine OFF.
  4. Connect a DMM between the 5-volt reference and sensor ground in the FTP sensor connector.

Does the voltage measure more than the specified value?

5 V

Go to Step 4

Go to Step 5

4
  1. Leave the DMM connected.
  2. Turn OFF the ignition.
  3. Disconnect the PCM connector.
  4. Turn ON the ignition, with the engine OFF.

Does the voltage measure more than the specified value?

5 V

Go to Step 8

Go to Step 11

5

Does the voltage measure less than the specified value?

5 V

Go to Step 6

Go to Step 9

6
  1. Leave the DMM connected to the 5-volt reference and sensor ground circuits at the FTP connector.
  2. Disconnect the A/C pressure sensor connector.

Does the voltage measure near the specified value?

5 V

Go to Step 10

Go to Step 7

7
  1. Turn the ignition OFF.
  2. Leave the FTP and the A/C pressure sensors disconnected.
  3. Disconnect the PCM connector.
  4. Test the 5-volt reference for a short to chassis or sensor ground.
  5. 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 12

Go to Step 11

8

Locate and repair the short to voltage in the following circuits. Refer to Wiring Repairs in Wiring Systems:

    • 5 V reference circuit.
    • FTP sensor signal circuit
    • A/C pressure sensor signal circuit

Did you complete the repair?

--

Go to Step 12

--

9

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

Did you complete the replacement?

--

Go to Step 12

--

10

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 12

--

11

Important: The replacement PCM must be reprogrammed. Refer to Powertrain Control Module Replacement . Replace the PCM.

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Select the diagnostic trouble code (DTC) 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 13

Go to Step 2

13

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