GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 1998 🢒 Regal 🢒 DTC 🢒 DTC P1639 5 Volt Reference 2 Circuit

Object Number: 248164 Size: SF
Engine Data Sensors-A/C Refrig Press, TP, MAP, ECT, IAT
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The PCM uses the 5 Volt Reference B circuit as a sensor feed for the A/C Refrigerant Pressure Sensor and the Fuel Tank 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 Setting the DTC
    • The PCM detects a voltage out of tolerance condition on the 5 Volt Reference B circuit.
    • Above condition for longer than 10 seconds.

Action Taken When the DTC Sets

    • The PCM will not illuminate the Malfunction Indicator Lamp (MIL).
    • The PCM will store conditions which were present when the DTC set as Fail Records data only. This information will not be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC
    • A history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids

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

Check for the following conditions:

    • Poor connection at the PCM.
         Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
    • 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 Fail 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

Number(s) below refer to the step number(s) 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

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Visually/physically check the PCM and engine grounds. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 14

Go to Step 3

3

Are any other DTCs set?

--

Go to Diagnose the other DTCs first

Go to Step 4

4
  1. Ensure that the A/C is OFF.
  2. Observe A/C High Side Pressure display on the scan tool.

Does the scan tool display voltage greater than the specified value?

4.8V

Go to Step 6

Go to Step 5

5

Does the scan tool display voltage less than the specified value?

0.1V

Go to Step 8

Go to Diagnostic Aids

6
  1. Turn the ignition switch OFF.
  2. Disconnect the PCM.
  3. Turn ON the ignition switch.
  4. Measure voltage between the 5 volt reference B circuit and the PCM ground circuit at the PCM harness connector.

Is voltage more than the specified value indicated?

5.5V

Go to Step 7

Go to Step 12

7
  1. Locate and repair the short to voltage in the following circuit(s):
    2. • 5 volt reference B circuit
    • A/C pressure sensor signal circuit
    • Fuel tank pressure sensor circuit
  2. Refer to Wiring Repairs in Wiring Systems.

Is action complete?

--

Go to Step 14

--

8
  1. Turn the ignition switch OFF.
  2. Disconnect the A/C pressure sensor.
  3. Turn the ignition switch ON.
  4. Connect the DMM between the 5 volt reference B circuit and ground.

Does the DMM display a voltage near the specified value?

5V

Go to Step 13

Go to Step 9

9
  1. Leave the A/C pressure sensor disconnected.
  2. Leave the DMM connect between the 5V reference B circuit and ground.
  3. Turn the ignition switch OFF.
  4. Disconnect the fuel tank pressure sensor.
  5. Turn the ignition switch ON.

Does the DMM display a voltage near the specified value?

5V

Go to Step 11

Go to Step 10

10
  1. Turn OFF the ignition switch.
  2. Disconnect the PCM.
  3. Check the 5 volt reference B circuit for a short to ground or to the sensor ground circuit.
  4. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 14

Go to Step 12

11

Replace the fuel tank pressure sensor. Refer to Fuel Tank Pressure Sensor Replacement .

Is action complete?

--

Go to Step 14

--

12

Important: The replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming .

Replace the PCM.

Is action complete?

--

Go to Step 14

--

13

Replace the A/C refrigerant pressure sensor.

Is action complete?

--

Go to Step 14

--

14
  1. Review and record scan tool Fail Records data.
  2. Clear DTCs.
  3. Operate the vehicle within Fail Records conditions.
  4. Using a scan tool, monitor Specific DTC info for DTC P1639 until the DTC P1639 test runs.

Does the scan tool indicate DTC P1639 failed this ignition?

--

Go to Step 2

System OK