Ignition System Troubleshooting

Introduction

This procedure covers diagnosis of a no-spark condition. Before starting, verify that a no-spark condition actually exists by performing the initial no-spark test in TS-01: Troubleshooting — Engine Will Not Start.

Many ignition faults can be traced to specific components, but if the DME computer itself is faulty there is no definitive way to test it directly. The best approach is to confirm that the DME has power and is receiving all required input signals. If all inputs check out and the DME still fails to produce spark or control the fuel pump, the DME is likely the cause. Substituting a known-good DME is the most reliable test for a suspected DME failure.

Tools

• Multi-meter
• Test lead jumpers

No-spark condition — possible causes

Work down the troubleshooter above. The two checks you can do yourself — the distributor cap and rotor, then the coil — come first; if both pass, the fault is the engine cranking/running signal to the DME or the DME itself.

Bad distributor cap or rotor

Cracks, carbon tracking, moisture inside the cap, a worn rotor button or pitted terminals all kill spark. Inspect per the Distributor Cap and Rotor Inspection below.

Bad ignition coil

Confirm with the Ignition Coil Test (live spark) and the Ignition Coil Resistance Check below.

Bad cranking/running signal or faulty DME

If the cap, rotor and coil are all good, the DME may not be receiving the engine cranking/running signal, or the DME itself may be at fault. There is no direct test for the DME — confirm it has power and all input signals, then substitute a known-good unit. Check per FUEL-16.

Distributor Cap and Rotor Inspection

1. Grasp the distributor cap and attempt to move it back and forth. If it moves easily, the retaining screws are not properly engaged or the distributor cap gasket is compressed and needs replacement.
2. Using IGN-01, remove the distributor cap.
3. Inspect the distributor cap for the following conditions and replace if any are found: a. Cracks b. Carbon tracking c. Moisture buildup on the inside of the cap d. Debris e. Worn rotor button f. Pitted, worn, or charred terminals
4. Inspect the rotor for charring or pitting; replace if necessary.
5. Check that the rotor retaining screw is in place. These screws can work loose and fall out, causing the rotor to become misaligned.
6. Ensure the dust cover is securely in place behind the rotor. This cover keeps debris from the belt housing area out of the distributor cap and is important for reliable spark.

Ignition Coil Test

1. Disconnect the ignition coil output wire at the distributor cap.
2. Connect a spark plug to the end of the disconnected coil output wire.
3. Connect a ground wire to the threaded portion of the spark plug.
4. Disconnect the ignition coil ground wire from the negative terminal on the coil (green wire).
5. Connect one end of a jumper wire to the ignition coil negative terminal.
6. Turn the ignition switch to ON.
7. Tap the other end of the jumper wire against a good ground (for example, the battery negative terminal) and observe the spark plug. Good sparks at the plug that correspond to each tap confirm the coil is functioning.
8. If no good spark is produced, check for approximately 12 VDC from the coil positive terminal (black wire) to ground, and approximately 12 VDC from the coil negative terminal (green wire) to ground, with the ignition switch in the ON position.

Ignition Coil Resistance Check

Resistance measurements confirm coil condition independent of the live spark test.

1. Primary coil resistance: Connect an ohmmeter between the positive terminal (black wire) and the negative terminal (green wire). Specification: 0.4–0.6 ohms.
2. Secondary coil resistance: Connect an ohmmeter between the coil output terminal and the negative terminal. Specification: 5,000–7,200 ohms.