Aircraft Spark Plug Wire Testing

A faulty aircraft ignition harness must be repaired.

An aircraft spark plug wire is also known as a magneto or ignition harness. An ignition harness transfers high voltage from the distributor at the magneto to the spark plugs. The harness also protects the center conductor from damage. Shielding is used around each harness wire to prevent electromagnetic interference from reaching the aircraft's radio and navigation equipment. In many cases, ignition problems first blamed on the magnetos or spark plugs can be traced to a faulty wire in the ignition harness. Testing the wires helps determine the problem's source.

Things You'll Need

• 3/4 or 7/8 inch open end wrench
• High voltage ignition tester

Instructions

1. Disengage the engine cowls and panels to gain full access to the ignition harness.

2. Remove the ignition leads from each spark plug. Use either a 7/8-inch or 3/4-inch open-end wrench, depending on the spark plugs installed on your aircraft.

3. Inspect the end of each lead. There should be a metal spring extending out from the center of the lead. The spring is connected to the center conductor of the ignition harness, and makes electrical contact with the spark plug. The harness is defective if the spring is missing.

4. Inspect the entire length of the ignition harness. Look for damage to the braided outer shielding or cuts into the harness. Pay particular attention to the point where the harness is clamped to the engine.

5. Connect a high-voltage tester to the under-test harness lead. Connect the wire marked "+12V" on the tester to the aircraft's positive battery terminal. Connect the tester's black high-voltage lead to either the tested harness's shield or the engine ground. Connect the tester's red high-voltage lead to the center conductor of the under-test ignition lead.

6. Press the "Test" button on the high-voltage tester. An ignition harness lead is good if you observe a continuous spark in the tester's window. If you do not see a spark, release the "Test" button and rotate the propeller one quarter turn. Retest the same harness lead. The absence of a spark in the window a second time indicates a bad ignition harness lead.

7. Test the other harness ignition leads using a similar procedure. Ensure all leads in both harnesses achieve normal test results.

8. Reinstall and tighten the spark plug leads on the tested harnesses.

9. Reinstall the aircraft engine cowl.

10. Perform the aircraft engine run-up and test.

Tips & Warnings

• You can ignition tester through an aviation supply company, or borrow one from an aircraft mechanic.

References

• Integrated Publishing: Aviation Maintenance and Miscellaneous Manuals: Ignition Harness
• Aircraft Maintenance Technology: Ignition Systems: Some Basics on Electromagnetic Interference
• "Aviation Maintenance Technician Series: Power Plant:" Ignition Harness: Dale Crane: 2005