Owner's Problem Description
The owner reports that their 1994 Geo Prizm won't start despite having power for lights, radio, and windows. The vehicle clicks when attempting to start, indicating potential issues with the ignition system. The previous technician has already ruled out the battery and starter as causes. The owner suspects either the ignition coil or the internal ignitor is faulty and seeks guidance on how to determine which component needs replacement.
Professional Technical Analysis
The 1994 Geo Prizm is equipped with a 1.6L I4 engine and a manual transmission, with an estimated mileage of 150,000 miles. The symptoms reported include a single click sound when the ignition key is turned, indicating a potential issue with the ignition system rather than the starting system. Since the battery has been confirmed to be functional and providing sufficient voltage (typically around 12.6V at rest), and auxiliary systems such as lights and radio are operational, focus shifts to the ignition components. It's essential to test both the ignition coil and the internal ignitor, as a failure in either can prevent spark generation. The ignition coil should show a primary resistance of 0.5-2.0 ohms and secondary resistance of 7,000-15,000 ohms. Meanwhile, the ignitor (also known as the ignition control module) often fails due to thermal stress, particularly in older models. If the vehicle clicks but does not crank, it may also suggest an issue with the starter relay or wiring. A thorough diagnosis involving checking the continuity of the wiring and testing the ignition circuit is crucial. Given the vehicle's age and common failure patterns, inspection of the ignition system is critical for identifying the root cause.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Coil Failure: The ignition coil may be defective, failing to provide sufficient voltage to create a spark. Typical resistance values should be checked; primary resistance should range from 0.5 to 2.0 ohms and secondary resistance from 7,000 to 15,000 ohms. A fault here often leads to no spark at the plugs, causing the no-start condition. - No spark detected at the spark plugs upon testing with a spark tester.
- Ignition Control Module (ICM) Failure: The ICM could be failing due to heat, which is common in older vehicles. It often fails to send the signal to fire the ignition coil. Testing the ICM for continuity and proper operation is crucial; a faulty ICM can prevent the engine from starting. - Check engine light may be illuminated, and no voltage signal detected at the ignition coil connector.
- Starter Relay Issue: The clicking sound may indicate a problem with the starter relay not engaging the starter motor properly. If the relay is faulty, it may need to be replaced to ensure proper engagement of the starting system. Testing the relay for continuity is recommended. - Clicking noise when attempting to start, but no further engagement of the starter motor.
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 OBD-II Scan: Begin by using a professional OBD-II scanner or the GeekOBD APP to check for any stored trouble codes in the PCM. This can help identify issues with the ignition system or related components, and retrieve freeze frame data for further insights.
- STEP 2 - Ignition Coil Testing: Check the primary and secondary resistance of the ignition coil using a digital multimeter. Ensure it falls within the specified range (0.5-2.0 ohms for primary, 7,000-15,000 ohms for secondary). If the values are outside these ranges, replace the ignition coil.
- STEP 3 - ICM Testing: Remove the ignition control module to test for proper function. Check for continuity and ensure that it is sending the correct signal to the ignition coil when the ignition is turned on. A faulty ICM should be replaced.
- STEP 4 - Relay Check: Inspect the starter relay by testing it for continuity. If the relay is not functioning correctly, replace it to ensure that the starter motor receives the necessary power to engage properly.
