Article 976 - Spark Plug High Voltage Wires - (All)

TSB #57 - January 6, 1967

A majority of replaced spark plug high voltage wires returned through the Warranty Parts Return Center are found to be non-defective. These wires are usually replaced because of a system related problem or complaint. The following brief test procedure will accurately determine if a spark plug wire replacement will correct or assist in the correction of a customer complaint.

A. RESISTANCE CHECK: Spark plug wires which exhibit excessively high resistance in the conductor, pin staple or terminals will cause spark plug misfire and subsequent rough engine operation and therefore must be replaced. Spark plug wires should be replaced if any of the following conditions exist:
    1. A wire which measures open circuit (infinite resistance) when checked.
    2. A wire that has an intermittent connection so that it exhibits an open circuit condition while being flexed in the terminal area.
    3. Any wire whose terminal to terminal resistance exceeds 1,000 ohms per inch of wire. Example: A 15 inch wire should not exceed 15,000 ohms resistance.

B. LEAKAGE TEST: In addition to high resistance, insulation failure (Puncture and leakage of the spark plug wire to ground) of the plug wire can also cause plug misfire and rough engine operation. Dielectric failure or leakage of the plug wire can be checked in the following manner.
    1. Fabricate a leakage tester by using a 30-36 inch piece of 14 gauge insulated wire with two insulated alligator clips attached to the wire ends.
    2. Remove and open circuit the plug wire at the spark plug. Note: Position the plug boot so that the wire will not fire to ground through the open boot end.
    3. Attach one clip of the fabricated wire to ground and with the engine running, move the remaining clip on the wire along the entire length and circumference of the spark plug wire. Punctures and/or leakage of the wire insulation will be evidenced by the presence of a bright spark or arc through the insulation of the spark plug wire to the alligator clip to ground. See lllus. S1162-A.
■ Spark plug wires which exhibit leakage should be replaced, however corona should not be mistaken as spark leakage. Corona is merely the glowing that appears around the wire circumference and is caused by the electrical stress in the air adjacent to the spark plug wire. This is not a malfunction.

In addition to checking the wires for resistance and leakage, it is also equally important that the wire terminations are properly seated at the distributor cap and spark plug.