GM Full Size Vans 1967-1986 Repair Guide

Spark Plug Wires



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Checking plug wire resistance through the distributor cap with an ohmmeter

At every tune-up/inspection, visually check the spark plug cables for burns cuts, or breaks in the insulation. Check the boots and the nipples on the distributor cap and/or coil. Replace any damaged wiring.

Every 50,000 miles (80,000 km) or 60 months, the resistance of the wires should be checked with an ohmmeter. Wires with excessive resistance will cause misfiring, and may make the engine difficult to start in damp weather.

To check resistance, remove the distributor cap, leaving the wires in place. Connect one lead of an ohmmeter to an electrode within the cap. Connect the other lead to the corresponding spark plug terminal (remove it from the spark plug for this test). Replace any wire which shows a resistance over 30,000 ohms. Generally speaking, however, resistance should not be over 25,000 ohms, and 30,000 ohms must be considered the outer limit of acceptability.

It should be remembered that resistance is also a function of length. The longer the wire, the greater the resistance. Thus, if the wires on your car are longer than the factory originals, the resistance will be higher, possibly outside these limits.


See Figures 2 and 3

Click image to see an enlarged view

Fig. Fig. 2: Mark the wires and cap before disconnecting the wires

Click image to see an enlarged view

Fig. Fig. 3: Pull the wire from the boot, not the wire itself

When installing new wires, replace them one at a time to avoid mixups. Start by replacing the longest one first. Install the boot firmly over the spark plug. Route the wire over the same path as the original. Insert the nipple firmly onto the tower on the distributor cap, then install the cap cover and latches to secure the wires.