GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2001 🢒 Eldorado 🢒 DTC 🢒 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 the diagnostic trouble code (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 Diagnostic System Check - Engine Controls

Go to Intermittent Conditions

3
  1. Visually and physically inspect the powertrain control module (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 12

Go to Step 4

4
  1. Turn the ignition OFF.
  2. Disconnect the fuel tank pressure (FTP) 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 10

Go to Step 5

5

Does the voltage measure more than the specified value?

5 V

Go to Step 6

Go to Step 7

6
  1. Turn OFF the ignition.
  2. Disconnect the PCM and check for a short to voltage on the following circuits:
    3. • 5-volt reference circuit
    • The FTP sensor signal circuit
  3. Repair the circuits as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

7

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 at the FTP sensor connector. Refer to Circuit Testing in Wiring Systems.

Does the resistance measure at the specified value?

∞ or OL

Go to Step 8

Go to Step 9

8
  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.

Does the resistance measure at the specified value?

∞ or OL

Go to Step 11

Go to Step 9

9

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 12

--

10

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

Did you complete the replacement?

--

Go to Step 12

--

11

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12
  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 13

Go to Step 2

13

With the 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