GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Savana (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Malfunction Indicator Lamp (MIL) Inoperative

Circuit Description

The malfunction indicator lamp (MIL) is used to indicate to the driver of the vehicle that an engine control system condition has occurred that may cause an increase in emissions and/or cause a driveability concern.

When the ignition is first turned ON, the gasoline control module (VCM/PCM or ECM) will illuminate the MIL for a bulb and system check. When the engine is started and operating the gasoline control module will turn the MIL OFF if no DTCs are set.

The Alternative Fuels ECM connects into the normal MIL Control circuit path and does not affect the gasoline control module operation of the MIL. The Alternative Fuels ECM is connected into the MIL Control circuit path in order to command the MIL ON when a CNG DTC sets. Both the Alternative Fuels ECM and the gasoline control module simultaneously test certain DTCs regardless of the fuel system that is in operation and may illuminate the MIL.

The instrument panel gage fuse supplies the positive voltage source for the MIL. The gasoline control module and/or the Alternative Fuels ECM can supply the ground path.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the symptom is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

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 mis-diagnosis.

    2.

  2. With the ignition ON and the engine OFF, the gasoline control module (VCM/PCM or ECM) should continuously illuminate the MIL.

    3.

  3. The scan tool will command BOTH the gasoline control module and the Alternative Fuels ECM to turn the MIL OFF.

    4.

  4. This step checks if a gasoline control module system failure is the cause of the inoperative MIL. The most likely cause of no MIL and a no start is a loss of voltage to the gasoline control module.

    5.

  5. This step tests for an open MIL input circuit. The gasoline control module should always command the MIL ON when the ignition is ON and the engine is OFF.

    6.

  6. This step tests for an open MIL Control circuit or IP.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Were you sent here from another diagnostic procedure?

Go to Step 2

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

2

Turn the ignition ON, with the engine OFF.

Is the MIL continuously illuminated?

Go to Step 3

Go to Step 5

3

While communicating with the Alternative Fuels ECM, command the MIL OFF.

Is the MIL illuminated?

Go to Malfunction Indicator Lamp (MIL) Always On

Go to Step 4

4

The fault is not present. Refer to Diagnostic Aids.

Is an operational concern still present?

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

Go to Intermittent Conditions

5

Crank the engine for at least 10 seconds.

Does the engine start and run?

Go to Step 6

Go to Step 9

6

Observe the operation of the other instrument panel (IP) gages and/or lamps.

Do all the IP indicators, except the MIL, operate correctly?

Go to Step 7

Go to Step 13

7

  1. Turn the ignition ON with the engine OFF.
  2. Observe the MIL Input -Gasoline/PCM data parameter in the Alt. Fuels Data List.

Is a gasoline MIL Input request present?

Go to Step 8

Go to Step 11

8

  1. Turn the ignition OFF and wait 30 seconds.
  2. Disconnect the Alternative Fuels ECM connectors.
  3. Turn the ignition ON leaving the engine OFF.
  4. Connect a fused jumper between the MIL CONTROL (AF ECM) circuit terminal and a known good ground.

    5. If the jumper fuse opens repair the short to voltage on the AF ECM MIL Control circuit.

Is the MIL illuminated?

Go to Step 14

Go to Step 10

9

The fault is in gasoline control module system. Refer to Diagnostic System Check - Engine Controls in the G Van Service Manual.

Are all diagnosis and repairs of the gasoline system complete?

Go to Step 17

--

10
  1. Test the MIL Control circuit between the Alternative Fuels ECM connector terminal and the IP for an open.
  2. Repair the circuit as necessary.

Was a circuit repair necessary?

Go to Step 17

Go to Step 13

11
  1. Test the MIL INPUT circuit between the Alternative Fuels ECM connector terminal and the gasoline control module connector terminal for an open or a short to voltage.
  2. Repair the circuit as necessary.

Was a circuit repair necessary?

Go to Step 17

Go to Step 12

12
  1. Check terminal contact at the gasoline control module connector.
  2. Repair terminal contact as necessary.

Was a terminal repair necessary?

Go to Step 17

Go to Step 15

13

The fault is in the instrument panel. Refer to Instrument Cluster Replacement in the Instrument Panel, Gages, and Console section of the G Van Service Manual.

Are all diagnosis and repairs of the base vehicle system complete?

Go to Step 17

--

14
  1. Check terminal contact at the Alternative Fuels ECM connector.
  2. Repair terminal contact as necessary.

Was a terminal repair necessary?

Go to Step 17

Go to Step 16

15

Important: The following control module replacement procedures must all be completed before attempting to operate the vehicle.

The following procedures are located in the G Van Service Manual.

  1. Replace the gasoline control module (VCM/PCM or ECM). Refer to Powertrain Control Module Replacement in Engine Controls .
  2. Program the replacement control module with the correct vehicle software and calibration.

    3. Important: In order to perform the variation learn procedure a switch-over to gasoline operation is necessary. Remove the CNG 20-amp fuse in order to perform the switch-over.

  3. Perform the Crankshaft Position System Variation Learn in Engine Controls.

Is the replacement complete?

Go to Step 17

--

16
  1. Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  2. Program the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

Go to Step 17

--

17
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. Using a scan tool, clear DTC information from the Alternative Fuels ECM and the gasoline control module (VCM/PCM or ECM).
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the fuel indicator lamp and 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 FIL/MIL illumination and no stored DTCs?

System OK

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