Owner's Problem Description
The owner reports that the 1997 GMC Jimmy is turning over but does not start. Despite hearing the starter engage, there is no ignition from the coil to the distributor. Previous attempts to diagnose the issue included replacing the ignition control module, distributor, rotor, and cap. The vehicle has never started since the problem was first noticed, leading to frustration as the owner has extensive experience with motor replacements and hopes to resolve the issue soon.
Professional Technical Analysis
The 1997 GMC Jimmy, equipped with a 4.3L V6 engine, is experiencing a no-start condition despite normal cranking behavior, indicating potential ignition system failure. With an estimated mileage of around 150,000 miles, the vehicle has had recent replacement of the ignition control module and distributor components. The symptoms suggest that while the starter is functioning, there is no spark reaching the distributor from the ignition coil. A thorough investigation of the ignition circuit is critical, with particular attention to the coil driver module. The wiring harness should be examined for continuity and proper voltage at the B+ (pink wire) while cranking. The white wires should show pulse signals, indicating the ignition control module's output. An oscilloscope can be used to check for the pulse signal on the white wires, which is necessary for the ignition system's proper operation. Additionally, the ground connection (black wire) must be verified to ensure it's intact. Previous repairs indicate that the owner has replaced several components, which may point to an underlying issue with the wiring or connectors leading to the ignition system. The current condition poses a safety risk as the vehicle is inoperable, and further testing is needed to identify the root cause of the no-start condition.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Control Module Failure: The ignition control module may have a thermal failure or internal fault preventing proper signal output to the coil, despite being a new part. Typically, these modules fail to deliver a strong enough pulse to trigger the ignition coil, which is critical for spark generation. It’s essential to check the module's input and output signals using a multimeter or oscilloscope to ensure it is functioning correctly. - Check engine light may illuminate, and diagnostic codes could indicate ICM failure or signal issues, especially if P0350-P0358 codes are present.
- Faulty Wiring or Connections: Issues with the wiring harness or connectors leading to the ignition coil can cause intermittent failures. These may include broken wires, corroded connectors, or poor ground connections, which must be inspected thoroughly. A visual inspection combined with resistance testing on affected wires can help identify breaks or high resistance that could impede proper operation. - Visual inspection reveals possible fraying or corrosion at connections; voltage drop tests may show inconsistent voltage at the ignition coil.
- Ignition Coil Failure: A new ignition coil could also be defective, particularly if it was not properly tested before installation. Coils can fail due to internal winding issues or insulation breakdown, which prevents spark generation. Testing the coil's primary and secondary resistance with a multimeter should reveal values within manufacturer specifications, typically around 0.4-2.0 ohms for primary and 6,000-15,000 ohms for secondary. - Testing reveals resistance out of specification, indicating a potential internal fault in the ignition coil.
- PCM (Powertrain Control Module) Issues: If the PCM is malfunctioning, it may not send the correct timing signals to the ignition system, leading to a no-fire situation. Testing the PCM for fault codes and verifying its operation through live data monitoring can help determine if it’s functioning as intended. A faulty PCM can sometimes show no visible signs but may require replacement. - Diagnostic tools indicate no communication with the PCM or fault codes related to ignition timing.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Scan for Diagnostic Codes: Utilize a professional OBD-II scanner or GeekOBD APP to retrieve any stored diagnostic trouble codes (DTCs) that may indicate faults in the ignition system. Focus on codes related to the ignition control module and any misfire codes. The presence of codes such as P0350-P0358 could highlight issues with the ignition coil or control module.
- STEP 2 - Visual Inspection of Wiring: Conduct a thorough visual inspection of the wiring harness to the ignition coil and control module. Look for signs of wear, fraying, or corrosion in connectors. Use a multimeter to check for continuity and resistance in the wires. Ensure all ground connections are secure and clean to prevent voltage loss.
- STEP 3 - Testing the Ignition Coil: Test the ignition coil's primary and secondary resistance using a multimeter. Primary resistance should be between 0.4-2.0 ohms and secondary resistance should be within 6,000-15,000 ohms. If the readings are out of specification, the coil may need to be replaced. Additionally, check for spark output at the coil while cranking the engine.
- STEP 4 - PCM Testing: If no issues are found with the ignition coil or wiring, check the PCM operation. Use GeekOBD APP to review live data for ignition timing signals and other parameters. If communication is lost or signals are erratic, further diagnostic testing of the PCM may be necessary to determine if it is functioning properly.
