GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2006 🢒 TrailBlazer - 2WD 🢒 Engine 🢒 Engine Controls - 4.2L 🢒 Electronic Ignition (EI) System Diagnosis

Diagnostic Instructions
    •  Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    •  Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Circuit/System Description

The ignition system uses individual ignition coil assemblies for each cylinder. The powertrain control module (PCM) controls each individual ignition coil by transmitting timing pulses on the ignition control (IC) circuit of each ignition coil to enable a spark event. Each ignition coil has the following circuits:

    • An ignition voltage circuit
    • A ground circuit
    • An IC control circuit

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Controls Connector End Views
    •  Powertrain Control Module Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Output Controls

Special Tools Required

J 26792 HEI Spark Tester

Circuit/System Verification

Observe the engine controls schematic for the ignition coils, and review the ignition system specifications to verify the following:

    • The ignition coils are correctly connected
    • The proper firing order
    • The proper spark plug type
    • The proper spark plug gap and torque
        Refer to Ignition System Specifications .

Circuit/System Testing

Important: Verify that the engine is in good mechanical condition before continuing with this diagnostic.

  1. Turn ON the ignition.
  2. Inspect the fuse that supplies voltage to the ignition coils.
    3. ⇒If the fuse is open, test the spliced circuits and the individual ignition coils for a short to ground.
  3. Disconnect the affected ignition coil.
  4. Connect a test lamp between the ignition voltage circuit and a good ground.
  5. With the test lamp still connected, measure for battery voltage between the ignition voltage circuit and a good ground.
    6. ⇒If the voltage is not within 0.5 volt of battery voltage, repair the high resistance or the open in the ignition voltage circuit.
  6. Turn OFF the ignition, and all electrical accessories. Allow sufficient time for all of the control modules to power down before taking a resistance measurement.
  7. Measure for a proper value of less than 5 ohms of resistance between the ground circuit of the affected ignition coil and a good ground.
    8. ⇒If the resistance is more than 5 ohms, repair the high resistance or the open in the ground circuit.
  8. Exchange the affected ignition coil with the ignition coil of a good cylinder.
  9. Start the engine and observe the scan tool.
    10. ⇒If the misfire transfers with the suspect ignition coil, replace as needed.
    ⇒If the misfire does not transfer with the suspect ignition coil, review the component testing procedures below and replace the spark plug, if needed.

Component Testing
    • Use the Spark Plug Inspection procedure to verify the integrity of the spark plugs. Refer to Spark Plug Inspection .
    • Use the J 26792 to verify the output of each ignition coil.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Spark Plug Replacement
    •  Ignition Coil Replacement