GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 4WD 🢒 Engine 🢒 Engine Controls - 4.2L 🢒 DTC P1639

Circuit Description

The powertrain control module (PCM) provides a 5-volt reference to the following sensors:

    • The throttle position (TP) sensor 2
    • The accelerator pedal position (APP) sensor 1
    • The manifold absolute pressure (MAP) sensor

The PCM monitors voltage on the 5-volt reference circuit. If the voltage is higher or lower than required, DTC P1639 will set.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC
    • The PCM detects a voltage that is higher or lower than required on the 5-volt reference 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) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

    Important: If the vehicle has a cranks but will not run condition proceed to step 3.

  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 as possible 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. Turn OFF the ignition.
  2. Disconnect the throttle body electrical connector. Refer to Throttle Body Assembly Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage from the 5-volt reference circuit of the throttle position (TP) sensor 2 to a good ground with a DMM.

Is the voltage near the specified value?

5 V

Go to Step  5

Go to Step 4

4

Is the voltage more than the specified value?

5 V

Go to Step 8

Go to Step 6

5
  1. Connect the throttle body electrical connector.
  2. Disconnect the manifold absolute pressure (MAP) sensor. Refer to Manifold Absolute Pressure Sensor Replacement .
  3. Measure the voltage from the 5-volt reference circuit of the MAP sensor to a good ground with a DMM.

Is the voltage near the specified value?

5 V

Go to Intermittent Conditions

Go to Step 9

6
  1. Monitor the DMM while disconnecting all other devices connected to this 5-volt reference circuit one at a time.
  2. If voltage changes when one of the above components are disconnected, replace the component. Refer to the appropriate replacement procedure.

Was a component replaced?

--

Go to Step 12

Go to Step 7

7
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Test this 5-volt reference circuit for a short to ground or for any sensor low reference circuit. Refer to Circuit Testing in Wiring Systems.
  4. If a condition is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 10

8
  1. Turn ON the ignition, with the engine OFF.
  2. Test the following circuits for a short to voltage:
    3. • This 5-volt reference circuit
    • The MAP sensor signal circuit
  3. If a condition is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 10

9

Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement .

Did you complete the replacement?

--

Go to Step 12

--

10

Test this 5-volt reference circuit for a short to the TAC motor control 1 or 2 circuits.

Did you find and correct a condition?

--

Go to Step 12

Go to Step 11

11

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12

  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 13

Go to Step 2

13

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