GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2002 🢒 DeVille 🢒 Engine 🢒 Engine Controls - 4.6L 🢒 DTC P1639

Circuit Description

The PCM uses the 5-volt reference 2 circuit as a sensor feed for the following sensors:

    • The AC pressure sensor
    • The fuel tank pressure (FTP) sensor

The PCM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is outside an acceptable range, 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.
    • Above condition is present for longer than 10 seconds.

Action Taken When the DTC Sets
    • The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.

Conditions for Clearing the MIL/DTC
    • The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using 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. 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 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 the ignition ON, 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 fuel tank pressure (FTP) sensor.
  4. Connect a DMM between the 5-volt reference at the FTP sensor connector and a good ground.
  5. Turn ON the ignition, with the engine OFF.

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 me 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 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 check for continuity between the 5-volt reference circuit and a good chassis ground. Refer to Wiring Repairs 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 complete the repair?

--

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

Did you complete the replacement?

--

Go to Step 17

--

15

Check the 5-volt reference circuit for an open, short to chassis ground or poor connection at the PCM. Refer to Circuit Testing and SIR/SRS Wiring Repairs or Wiring 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. 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 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