Owner's Problem Description
The owner reports that their 1995 Cadillac DeVille won't start despite replacing the entire distributor. The mechanic suspects the computer might be faulty. The battery, alternator, and fuel pump are confirmed to be in good condition. The vehicle shows no error codes except for an AC-related code. Attempts to start it after spraying starting fluid into the throttle body have been unsuccessful.
Professional Technical Analysis
The 1995 Cadillac DeVille is equipped with a 4.6L V8 engine and an electronically controlled ignition system. The vehicle is likely experiencing a no-start condition due to an ignition system failure, given the owner’s report of no spark. Primary checks should include verifying power to the ignition coil and measuring the coil's primary and secondary winding resistances, which should be approximately 0.5-1.5 ohms for primary and 8.0-12.0 kOhms for secondary. The distributor's pickup coil and ignition control module (ICM) must also be tested, as failures in these components are common in vehicles of this age and can lead to no-spark conditions. Environmental factors such as heat can exacerbate issues with these components, especially if they are already prone to failure due to age. The absence of diagnostic trouble codes (DTCs) complicates the issue; however, checking for a consistent ignition signal from the ICM during cranking is critical. The vehicle's current condition indicates that it may be unsafe to operate until the ignition issue is resolved, as a no-start can lead to potential hazards when diagnosing on the road. With approximately 150,000 miles on the odometer, wear and tear on electrical connections and components should also be considered, as they can lead to intermittent failures. Ensuring all grounds, particularly at the coil and ICM, are secure is essential in this diagnosis. Given the previous attempts to start the vehicle, rechecking the ignition circuit and ensuring proper installation of the new distributor components is crucial.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Coil Failure: The ignition coil may not be providing the necessary spark due to internal shorting or failure. Primary resistance should be checked (0.5-1.5 ohms), and secondary resistance (8.0-12.0 kOhms) must also be verified. A faulty ignition coil can prevent spark generation, leading to a no-start condition. - No spark detected at the plugs, voltage at the coil primary terminal inconsistent.
- Ignition Control Module (ICM) Failure: The ICM may not be generating the necessary signals to trigger the ignition coil due to thermal failure or internal component breakdown. This is common in older GM vehicles. Testing the module's output signal is essential to determine functionality. - No signal from ICM during cranking, possible stored DTCs P0351-P0358.
- Distributor Installation Issues: If the new distributor is not properly aligned or the ignition timing is off, it can result in no spark. Verify that the installation aligns with the engine’s timing marks and that the rotor is positioned correctly. - Rotor misalignment or improper timing can prevent spark delivery.
- Wiring and Ground Issues: Poor connections or corroded grounds in the ignition system can lead to inadequate power supply to the ignition components. Inspect wiring harnesses for damage and ensure all ground connections are clean and secure. - Loose or corroded connections may result in intermittent ignition issues.
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 - Visual Inspection: Begin with a thorough visual inspection of the ignition system components, including the ignition coil, ICM, and wiring. Look for loose connections, frayed wires, or signs of corrosion that could affect performance. Ensure that the new distributor is correctly installed and aligned with the engine’s timing marks.
- STEP 2 - Check Power to Ignition Coil: Using a multimeter, verify that the ignition coil is receiving power. Check the voltage at the primary terminal; it should be around 12V with the ignition on. If no voltage is present, trace back to the ICM and wiring for faults.
- STEP 3 - Test Ignition Coil and ICM: Measure the resistance of the ignition coil’s primary and secondary windings. If readings are out of specification, replace the coil. Additionally, test the ICM's output signal with an oscilloscope to confirm it is sending a signal to the ignition coil during cranking.
- STEP 4 - Perform a Cranking Test: With the vehicle in a safe environment, attempt to start the engine while monitoring for spark at the plugs. If no spark is observed, further investigate the wiring harness and ground connections for faults.
