Ford Mustang/Capri 1979-1988 Repair Guide

Spark Plug Wires



See Figure 1

Visually inspect the spark plug cables for burns, cuts, or breaks in the insulation. The cables should be supple enough to be formed into three-inch-diameter loops. Check the spark plug boots and their terminals, paying particular attention to rusted or misshapen terminals. Replace any damaged wiring. If no physical damage is obvious, the wires can be checked with an ohmmeter for excessive resistance. This is particularly important when a break occurs only in the conductive core, with no external signs of breakage. Resistance should be no more than 5,000 ohms per foot of cable length. Therefore, it is normal for resistance readings of the longer wires to be somewhat higher than those of the shorter wires.

On models equipped with electronic ignition, apply a small amount of silicone dielectric compound (D7AZ-19A331-A or the equivalent) to the inside of the terminal boots whenever an ignition wire is disconnected from the plug, or coil/distributor cap connection.

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Fig. Fig. 1: Measure the resistance of disconnected spark plug wires using an ohmmeter


See Figure 2

When installing a new set of spark plug cables, replace the cables one at a time so there will be no mix-up. Start by replacing the longest cable first. Install the boot firmly over the spark plug and route the cable exactly the same as the original. (Improper arrangement of the wiring can induce voltage between the cables, resulting in misfiring and poor performance.) Insert the terminal firmly into the tower on the distributor cap. Repeat the process for each cable.

When removing spark plug wires, be sure to twist and pull only on the rubber boots. Never pull on the wire itself, otherwise the wire may break.

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Fig. Fig. 2: Note the different lengths of the spark plug wires and number them before removal to avoid confusion