GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC K Sierra - 4WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Malfunction Indicator Lamp (MIL) Always On

Circuit Description

The malfunction indicator lamp (MIL) informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.

This vehicle utilizes a powertrain control module (PCM) and an alternative fuels engine control module (ECM) in order to control engine operation. Both modules need the ability to illuminate the MIL. In order to allow both modules to illuminate the MIL the alternative fuels ECM contains circuitry that will operate the MIL for both modules. The alternative fuels ECM will command the MIL ON whenever the PCM provides a ground path on the alternative fuels ECM MIL Input circuit.

When the ignition is first turned ON, the PCM will continously illuminate the MIL for a bulb and system check. When the engine is started and operating the PCM will turn the MIL OFF if no PCM DTCs are set.

The alternative fuels ECM is wired into the base vehicle MIL Control circuit path and will command the MIL ON whenever a PCM MIL request or an alternative fuels DTC is set. Both the alternative fuels ECM and the PCM simultaneously test certain DTCs and may illuminate the MIL.

The instrument panel gage fuse supplies the positive voltage source for the MIL. The alternative fuels ECM supplies the ground path.

Test Description

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

  1. The System Check must be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in misdiagnosis.

    2.

  2. With the ignition ON and the engine OFF, the PCM should continuously illuminate the MIL.

    3.

  3. The scan tool will command both the PCM and the alternative fuels ECM to turn the MIL OFF.

    4.

  4. This step tests if the MIL Input circuit is requesting MIL illumination. There should not be a MIL Input while the MIL is commanded OFF.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Engine Controls (Alternative Fuels)?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2

Turn ON the ignition, with the engine OFF.

Is the MIL continuously illuminated?

Go to Step 3

Go to Malfunction Indicator Lamp (MIL) Inoperative

3

With the scan tool in the Alternative Fuels - Special Function menu command the MIL OFF.

Is the MIL illuminated?

Go to Step 5

Go to Step 4

4
  1. Observe the diagnostic trouble code (DTC) information from the PCM.
  2. Observe the diagnostic trouble code (DTC) information from the alternative fuel ECM with a scan tool.

Does the scan tool display any MIL Request DTCs?

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

Go to Intermittent Conditions

5

Observe the MIL Input (Gasoline/PCM) data parameter in the Alt. Fuels Data List while the MIL is commanded OFF.

Is a gasoline MIL Input request present?

Go to Step 7

Go to Step 6

6
  1. Test the MIL Control circuit between the alternative fuels ECM connector terminal and the IP for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.
  2. Test the IP for an internally shorted condition.
  3. Repair the circuit or IP as necessary.

Did you find and correct the condition?

Go to Step 11

Go to Step 9

7
  1. Test the MIL INPUT circuit between the alternative fuels ECM connector terminal and the gasoline PCM connector terminal for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.
  2. Repair the circuit as necessary.

Did you find and correct the condition?

Go to Step 11

Go to Step 8

8
  1. Connect the alternative fuels ECM connectors.
  2. Disconnect the PCM connectors.
  3. Turn ON the ignition with the engine OFF.

    4. Important: DTCs may set with the PCM disconnected. Clear DTCs after all diagnosis is complete.

  4. Observe the MIL Input (Gasoline/PCM) data parameter in the Alt. Fuels Data List .

Is a gasoline MIL Input request present?

Go to Step 9

Go to Step 10

9

Replace the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .

Did you complete the replacement?

Go to Step 9

--

10

Important: KL6 models require a switchover to gasoline with a scan tool in order to perform the Crankshaft Position Variation Learn Procedure.

KL8 models do not require the Crankshaft Position Variation Learn Procedure.

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

Go to Step 11

--

11
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. With a scan tool, clear DTC information from the alternative fuels ECM and the PCM.
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the MIL.
  7. Observe vehicle performance and driveability.
  8. After operating the vehicle inspect BOTH control modules for stored DTCs.

Does the vehicle operate on CNG with normal driveability, no MIL illumination and no stored DTCs?

System OK

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)