GM Service Manual Online
For 1990-2009 cars only

Circuit Description

Ignition voltage is supplied directly to the malfunction indicator lamp (MIL). The engine control module (ECM) controls the lamp by grounding the control circuit through an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line which is monitored by the ECM. When the ECM is commanding a component ON, the voltage of the control circuit should be low, near 0 volts. When the ECM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the fault line status will change causing the DTC to set.

Conditions for Running the DTC

    • The engine speed is more than 400 RPM.
    • The ignition voltage is between 6-18 volts.

Conditions for Setting the DTC

    • The ECM detects that the commanded state of the driver and the actual state of the control circuit do not match.
    • The conditions are present for a minimum of 5 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.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests for a short to ground in the MIL control circuit. With the ECM disconnected and the ignition ON the MIL should be OFF.

  2. 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.

  3. This vehicle is equipped with a ECM that utilizes an electrically erasable programmable read only memory (EEPROM). When the ECM is being replaced, the new ECM must be programed.

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

  1. Verify whether the instrument panel (IP) is operational. If the instrument panel (IP) is completely inoperative, refer to Instrument Cluster Inoperative in Instrument Panel, Gauges, and Console.
  2. Verify whether the MIL turns ON and OFF when commanded.
  3. Turn ON the ignition, with the engine OFF.
  4. Command the MIL ON and OFF using the scan tool. Repeat the commands as necessary.

Does the MIL turn ON and OFF when commanded?

--

Go to Step 3

Go to Step 4

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 to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

Inspect the fuse that supplies battery positive voltage to the MIL.

Is the fuse open?

--

Go to Step 10

Go to Step 5

5

  1. Turn OFF the ignition.
  2. Disconnect the ECM. Refer to Engine Control Module Replacement .
  3. Turn ON the ignition.

Is the MIL OFF?

--

Go to Step 6

Go to Step 11

6

  1. Turn OFF the ignition.
  2. Remove the fuse that supplies voltage to the MIL.
  3. Turn ON the ignition, with the engine OFF.
  4. Connect a DMM from the MIL control circuit to a good ground.

Does the voltage measure the specified value?

0 V

Go to Step 7

Go to Step 12

7

  1. Turn OFF the ignition.
  2. Reinstall the fuse that supplies voltage to the MIL.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the MIL control circuit in the ECM harness connector with a fused jumper wire connected to ground. Refer to Using Fused Jumper Wires in Wiring Systems.

Does the MIL illuminate?

--

Go to Step 15

Go to Step 8

8

  1. Turn OFF the ignition.
  2. Remove the instrument panel cluster (IPC). Refer to Instrument Cluster Replacement in Instrument Panel, Gauges, and Console.
  3. Probe the MIL battery positive voltage circuit at the IP harness connector with a test lamp connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 9

Go to Step 13

9

Test the MIL control circuit for an open or high resistance and repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 18

Go to Step 14

10

Repair the short to ground in the battery positive voltage circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 18

--

11

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

Did you complete the repair?

--

Go to Step 18

--

12

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

Did you complete the repair?

--

Go to Step 18

--

13

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

Did you complete the repair?

--

Go to Step 18

--

14

Inspect for poor connections at the IP and repair as necessary. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 16

15

Inspect for poor connections at the ECM and repair as necessary. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 17

16

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

Did you complete the repair?

--

Go to Step 18

--

17

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the repair?

--

Go to Step 18

--

18

  1. Use a scan tool 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 19

Go to Step 2

19

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