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 MIL Request PCM DTCs are set.
The alternative fuels ECM connects 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.
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The System Check must be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in mis-diagnosis.
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With the ignition ON and the engine OFF, the PCM should continuously illuminate the MIL. A 2 second bulb check is not considered continously illuminated.
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The scan tool will command both the PCM and the alternative fuels ECM to turn the MIL OFF.
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This step checks if a PCM 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 control modules.
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This step tests for an open MIL input circuit. The PCM should always command the MIL ON when the ignition is ON and the engine is OFF.
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This step tests for an open MIL Control circuit or IP.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Engine Controls Schematics | |||
Did you perform the Diagnostic System Check - Engine Controls (Alternative Fuels)? | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) | |
Turn ON the ignition, with the engine OFF. Is the MIL continuously illuminated? | Go to Step 3 | Go to Step 4 | |
With the scan tool in the Alternative Fuels - Special Function menu, command the MIL OFF. Is the MIL illuminated? | Go to Intermittent Conditions | ||
Crank the engine for at least 10 seconds. Does the engine start and run? | Go to Step 5 | ||
5 | 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 6 | Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages and Console in the C/K Pickup Service Manual |
Is a gasoline MIL Input request present? | Go to Step 7 | Go to Step 9 | |
If the jumper fuse opens repair the short to voltage on the alternative fuels ECM MIL Control circuit. Is the MIL illuminated? | Go to Step 11 | Go to Step 8 | |
8 | Test the MIL Control circuit between the alternative fuels ECM connector terminal and the IP for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | Go to Step 14 | Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages and Console in the C/K Pickup Service Manual |
9 | Test the MIL Input circuit between the alternative fuels ECM connector terminal and the gasoline PCM connector terminal for an open or a short to voltage. Refer to Circuit Testing and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | Go to Step 14 | Go to Step 10 |
10 | Inspect for proper terminal contact at the PCM connector. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | Go to Step 14 | Go to Step 12 |
11 | Inspect for proper terminal contact at the harness connector of the alternative fuels ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | Go to Step 14 | Go to Step 13 |
12 |
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 14 | -- |
13 | Replace the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement . Did you complete the replacement? | Go to Step 14 | -- |
14 |
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) |
