Ignition System Troubleshooting – 2.5L NA

Overview

This guide outlines the steps to diagnose a no-spark condition in the ignition system of all Porsche 944 variants with an 8-valve engine, covering both pre-1985.5 and 1985.5 onwards models. A no-spark condition can prevent the engine from starting, and this procedure helps identify issues with components like the distributor cap, rotor, ignition coil, or DME computer. Follow these steps carefully to pinpoint the cause and restore proper ignition function.

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Tools Required

• Multimeter
• Test lead jumpers
• Spark plug
• Ground wire
• Ohmmeter

Parts Required

• Distributor cap (if replacement needed) – 944 602 902 00
• Distributor cap gasket (if replacement needed) – 944 602 139 00
• Rotor (if replacement needed) – 944 602 901 00
• Ignition coil (if replacement needed) – 944 602 115 00

Required Additional Procedures

Steps in these procedures are required to complete this process:

• Engine Will Not Start Troubleshooting
• Distributor Cap Removal
• Fuel System Troubleshooting

Step-by-Step Procedure

Step 1: Verify No-Spark Condition

1. Confirm the engine has a no-spark condition by following the initial test in the Engine Will Not Start Troubleshooting procedure. This ensures you’re addressing the correct issue before proceeding.

Step 2: Inspect Distributor Cap and Rotor

2. Grasp the distributor cap and try to move it side to side. If it moves easily, tighten the retaining screws or replace the distributor cap gasket (944 602 139 00) if compressed.
3. Remove the distributor cap by following the Distributor Cap Removal procedure.
4. Inspect the distributor cap for:
   Cracks
   Carbon tracking (dark streaks or burn marks)
   Moisture buildup inside
   Debris
   Worn rotor button
   Pitted, worn, or charred terminals
5. Replace the distributor cap (944 602 902 00) if any issues are found.
6. Inspect the rotor for charring or pitting. Replace the rotor (944 602 901 00) if damaged.
7. Ensure the rotor retaining screw is secure, as it can loosen and cause misalignment.
8. Verify the dust cover is securely placed behind the rotor to prevent debris from entering the distributor cap, which can cause poor or no spark.

Step 3: Test Ignition Coil

9. Disconnect the ignition coil output wire from the distributor cap.
10. Connect a spark plug to the end of the disconnected ignition coil output wire.
11. Attach a ground wire to the threaded portion of the spark plug.
12. Disconnect the ignition coil ground wire (green wire) from the negative terminal of the coil.
13. Connect one end of a ground wire to the ignition coil negative terminal.
14. Turn the ignition switch to the ON position.
15. Tap the other end of the ground wire on a good grounding point, such as the battery negative terminal, and observe the spark plug for sparks that match the tapping frequency.
16. If sparks are consistent and strong, the ignition coil is functioning correctly. Proceed to Step 4.
17. If no or weak sparks are observed, use a multimeter to check for approximately 12 VDC at the coil positive terminal (black wire) to ground with the ignition switch ON.
18. Check for approximately 12 VDC at the coil negative terminal (green wire) to ground. If voltage is incorrect, inspect wiring or connections.

Step 4: Perform Ignition Coil Resistance Check (Optional)

19. Using an ohmmeter, measure the primary coil resistance between the positive (black wire) and negative (green wire) terminals of the ignition coil. The resistance should be 0.4 to 0.6 ohms.
20. Measure the secondary coil resistance between the coil output terminal and the negative terminal. The resistance should be 5000 to 7200 ohms.
21. If resistance values are outside these ranges, replace the ignition coil (944 602 115 00).

Step 5: Check Engine Cranking/Running Signal to DME

22. Follow the Fuel System Troubleshooting procedure to verify the engine cranking/running signals to the DME computer. This ensures the DME is receiving the necessary inputs to produce spark.

Step 6: Evaluate DME Computer

23. If all previous steps show correct components and signals but no spark is produced, the DME computer may be faulty.
24. Follow the Fuel System Troubleshooting procedure to test the DME computer inputs and outputs.
25. If possible, borrow a known good DME computer to test the vehicle, as this is the most reliable way to confirm a faulty DME.
26. If the borrowed DME resolves the issue, replace the original DME computer.

Post Procedure Checklist

• Verify all connections to the distributor cap, rotor, and ignition coil are secure.
• Ensure no debris or moisture is present in the distributor cap.
• Confirm the rotor retaining screw and dust cover are properly installed.
• Test the engine to ensure it starts and runs smoothly, indicating a resolved spark issue.
• Check for any warning lights or irregular engine behavior after starting.

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manuals
• YouTube