Owner's Problem Description
The owner inherited a 1986 Chevy Celebrity with a 2.8L V6 engine. It initially ran well but then refused to start after a day. The engine cranks but shows no spark. The owner confirmed voltage at the coil and fuel delivery. After checking the computer, it only displays a flashing '12' code. The OBD-I tool failed to retrieve any codes, suggesting connection issues, leading the owner to suspect the computer is dead.
Professional Technical Analysis
This 1986 Chevy Celebrity, equipped with a 2.8L V6 engine and a two-barrel carburetor, is exhibiting a no-start condition after experiencing initial functionality. The vehicle has approximately 120,000 miles on it, and it was running smoothly until the owner changed the air filter. Symptoms include engine cranking without spark, confirmed voltage at the coil, and the computer displaying an endless '12' code. The flashing '12' indicates the computer is in diagnostic mode, but the lack of codes retrieved by the OBD-I tool suggests a communication issue or a malfunctioning computer. The ignition system's primary voltage should be verified, with a pink wire typically providing 12V when the key is on. The secondary ignition circuit should also be inspected for spark presence at the plugs. The absence of a pulse on the negative side of the coil during cranking indicates a potential issue with the pickup coil or ignition module. Given the vehicle's age, common points of failure include the ignition control module or wiring degradation due to heat exposure. If the pickup coil is not sending a signal to the ignition module, it will not create the necessary pulse to the coil, leading to a no-spark condition. The safety of the vehicle is a concern as it currently does not start, and further diagnostics are essential to avoid unnecessary part replacements.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Control Module (ICM) Failure: The ICM could be malfunctioning, leading to a lack of signal to the coil. Testing should confirm if the ICM is receiving a signal from the pickup coil, as this is essential for proper ignition timing. Symptoms of failure often include intermittent starting issues followed by a complete no-start condition. A common failure mode for these modules occurs after prolonged heat exposure, leading to thermal breakdown. The ICM should be confirmed for proper operation with a multimeter, checking for expected resistance values between 500-1500 ohms on the pickup coil side. - Symptoms include no spark at the plugs, potential stored codes related to ignition timing, and the ICM may be too hot to touch during testing.
- Pickup Coil Malfunction: The pickup coil may not be generating the necessary signal for the ICM. This failure can result from wire insulation breakdown or physical damage due to age. Testing should confirm the resistance of the pickup coil, which should typically read between 500-1500 ohms. If the resistance is outside this range, the pickup coil should be replaced. Additionally, check for proper alignment and secure connections to ensure reliable operation. - No pulse on the negative side of the coil during cranking suggests a lack of signal from the pickup coil, leading to no ignition spark.
- Computer Malfunction: The behavior of the computer flashing the '12' code indicates it is in diagnostic mode but unable to proceed with diagnostics. This could be a sign of internal failure or communication issues within the computer. Testing can include checking for power and ground connections to the computer, ensuring they are secure and free of corrosion. If the computer is confirmed to have power but still shows no codes, replacement may be necessary. - OBD-I tool indicates no codes despite power to the computer, suggesting a failure in the computer itself.
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 - Initial Voltage Check: Verify the primary voltage at the coil using a multimeter. The pink wire at the coil should show 12V with the key in the 'ON' position. If voltage is present, proceed to check the negative side of the coil for a pulsing signal while cranking the engine. This will confirm whether the ICM is functioning correctly.
- STEP 2 - Check Pickup Coil Resistance: Disconnect the pickup coil connector and measure the resistance across its terminals. The expected resistance should be between 500-1500 ohms. If the reading is outside this range, the pickup coil likely needs replacement. Ensure to check the wiring for any visible damage or corrosion.
- STEP 3 - Inspect Computer Connections: Check power and ground connections to the computer. Ensure there is no corrosion and that the connectors are securely attached. If all connections are good but the computer still does not function, it may require replacement.
- STEP 4 - Test Spark at Plugs: If the above tests are satisfactory, check for spark at the spark plugs. Remove a plug, reconnect it to the coil wire, and ground it to the engine block. Crank the engine and look for a spark. This will confirm whether the ignition system is functioning as intended.
