GM Service Manual Online
For 1990-2009 cars only

Circuit Description

Voltage is supplied to the malfunction indicator lamp (MIL). The Engine Control Module (ECM) turns the MIL ON by grounding the MIL control circuit. There should be a steady MIL with the ignition ON and the engine OFF.

MIL Operation

The MIL is located on the instrument panel cluster (IPC).

MIL Function

    • The MIL informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.
    • The MIL illuminates during a bulb test and a system test.
    • A DTC will be stored if a MIL is requested by the ECM.

MIL Illumination

    • The MIL will illuminate with the ignition switch ON and the engine not running.
    • The MIL will turn OFF when the engine is started.
    • The MIL will remain ON if the self-diagnostic system has detected a malfunction.
    • The MIL may turn OFF if the malfunction is not present.
    • If the MIL is illuminated and then the engine stalls, the MIL will remain illuminated as long as the ignition switch is ON.
    • If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON.

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step tests for a short to voltage on the MIL control circuit. With the fuse removed there should be no voltage on the MIL control circuit.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Verify whether the instrument cluster is operational. If the instrument panel (IP) is completely inoperative, refer to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages and Console.
  2. Command the MIL ON and OFF with a scan tool.

Does the MIL turn ON and OFF when commanded with a scan tool?

--

Go to Intermittent Conditions

Go to Step 3

3

Inspect the fuse that supplies battery voltage to the cluster.

Is the fuse open?

--

Go to Step 10

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Remove the fuse that supplies battery voltage to the cluster.
  3. Disconnect the Engine Control Module (ECM).
  4. Turn ON the ignition with the engine OFF.
  5. Measure the voltage from the MIL control circuit in the ECM harness connector to a good ground.

Is the voltage less than the specified value?

1.0 V

Go to Step 5

Go to Step 11

5

  1. Turn OFF the ignition.
  2. Install the fuse that supplies battery voltage to the cluster.
  3. Turn ON the ignition with the engine OFF.
  4. Connect a 3-amp fused jumper wire between the MIL control circuit in the ECM harness connector and a good ground.

Is the MIL illuminated?

--

Go to Step 9

Go to Step 6

6

  1. Turn OFF the ignition.
  2. Remove the instrument panel cluster (IPC). Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the battery voltage circuit of the IPC harness connector with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 12

7

Test the MIL control circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 15

Go to Step 8

8

Test for an intermittent and for a poor connection at the IPC. 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 15

Go to Step 13

9

Test for an intermittent and for a poor connection at the ECM. Refer to 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 15

Go to Step 14

10

Test all circuits and components that are supplied by this fuse for excessive current draw. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Intermittent Conditions

11

Repair the short to voltage in the MIL control circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 15

--

12

Repair the open in the battery voltage circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 15

--

13

Replace the IPC. Refer to Instrument Cluster Replacement in Instrument Panel, Gauges, and Console.

Did you complete the replacement?

--

Go to Step 15

--

14

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

15

  1. Turn OFF the ignition for 30 seconds.
  2. Start the engine.

Does the MIL operate correctly?

--

Go to Step 16

Go to Step 2

16

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