GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The powertrain control module (PCM) receives three ignition inputs from the ignition switch--ignition 0 signal, ignition 1 signal, and crank signal. The PCM receives the ignition 0 signal with the ignition switch in the CRANK position. The ignition switch sends this signal through the Off/Run/Crank voltage circuit. The PCM uses a class 2 ignition mode message and voltage input from separate systems to test the Off/Run/Crank voltage circuit. If the PCM detects an improper ignition 0 signal, DTC P1633 will set.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1633 Ignition 0 Switch Circuit

Conditions for Running the DTC

No class 2 serial data circuit problem.

Conditions for Setting the DTC

The PCM detects an improper IGN 0 input.

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 only.

Conditions for Clearing the DTC

    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using the scan tool Clear DTC Information function.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Disconnect the powertrain control module (PCM).
  2. Connect a test lamp between the Off/Run/Crank voltage circuit and a good ground.
  3. Turn the ignition switch to the ON and START positions.

Does the test lamp illuminate with the ignition switch in both the ON and START positions?

Go to Step 6

Go to Step 3

3

Did the test lamp illuminate for either ignition switch position?

Go to Step 7

Go to Step 4

4

Test the Off/Run/Crank voltage circuit for an open and for a high resistance. Refer to Testing for Continuity and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 5

5

Test for an intermittent and for a poor connection at the ignition switch. Refer to Testing for Intermittent Conditions and Poor Connections and Circuit Testing in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 7

6

Test for an intermittent and for a poor connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Circuit Testing in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 8

7

Replace the ignition switch. Refer to Ignition and Start Switch Replacement in Steering Wheel and Column.

Did you complete the replacement.

Go to Step 9

--

8

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 9

--

9

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Does the DTC fail this ignition?

Go to Step 2

Go to Step 10

10

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK