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 1 signal with the ignition switch in the ON and START positions. The PCM uses a Class II ignition mode message and voltage input from the ignition switch to test the ignition 1 voltage circuit. If the PCM detects an improper ignition 1 signal, DTC P1634 will set.

Conditions for Running the DTC

No class 2 serial data circuit problem.

Conditions for Setting the DTC

The PCM detects an improper ignition 1 voltage signal.

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 information 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

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Engine Electrical?

--

Go to Step 2

Go to Diagnostic System Check - Engine Electrical

2

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the ignition 1 signal with a scan tool.

Is the ignition 1 signal near the specified value?

12 V

Go to Step 4

Go to Step 3

3

Inspect the fuse to the ignition 1 voltage circuit.

Is the fuse open?

--

Go to Step 11

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Connect a test lamp between the ignition 1 voltage circuit and good ground.
  4. Turn the ignition switch to the ON and START positions.

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

--

Go to Step 17

Go to Step 5

5

Did the test lamp illuminate for either ignition switch position?

--

Go to Step 18

Go to Step 6

6

  1. Turn OFF the ignition.
  2. Disconnect the ignition 1 relay from the underhood fuse block.
  3. Connect a test lamp between the battery positive voltage circuit of the ignition 1 relay and a good ground.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 12

7

  1. Connect a 5-amp fused jumper wire between the battery positive voltage circuit of the ignition 1 relay and the ignition 1 voltage circuit.
  2. Connect the test lamp between the ignition 1 voltage circuit at the PCM and a good ground.

Does the test lamp illuminate?

--

Go to Step 8

Go to Step 10

8

  1. Turn ON the ignition, with the engine OFF.
  2. Connect a test lamp between the control circuit of the ignition 1 relay and the ground circuit of the ignition 1 relay.

Does the test lamp illuminate?

--

Go to Step 16

Go to Step 9

9

Connect a test lamp between the control circuit of the ignition 1 relay and a good ground.

Does the test lamp illuminate?

--

Go to Step 14

Go to Step 13

10

Test the ignition 1 voltage circuit for an open and for a high resistance. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 21

--

11

Test the ignition 1 voltage circuit and the voltage circuit of the start 1 relay for a short to ground. Refer to Testing for Short to Ground and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 21

Go to Step 20

12

Test the ignition 1 relay battery positive voltage circuit for an open and for a high resistance. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 21

--

13

Test the ignition 1 relay control circuit for an open and for a high resistance. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 21

Go to Step 15

14

Test the ignition 1 relay ground circuit for an open and for a high resistance. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 21

--

15

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

Did you find and correct the condition?

--

Go to Step 21

Go to Step 18

16

Test for an intermittent and for a poor connection at the ignition 1 relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 21

Go to Step 19

17

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

Did you find and correct the condition?

--

Go to Step 21

Go to Step 20

18

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

Did you complete the replacement?

--

Go to Step 21

--

19

Replace the ignition 1 relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

--

Go to Step 21

--

20

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 21

--

21

  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 22

22

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK