GM Astro/Safari 1985-1996 Repair Guide

Spark Plug Wires


See Figures 1 and 2

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

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Fig. Fig. 2: Checking individual plug wire resistance with a digital 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.

If the spark plug wires have become unserviceable due to time and wear, it is probably a good idea to replace the distributor cap and rotor as well.

Every 30,000 miles (48,000 km) or so, 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. Generally, the useful life of the cables is 30,000-45,000 miles (48,000-72,000 km), though some late-model vehicles (such as 1996 Astro or Safari equipped with platinum-tip plugs) use newer long-life wires which could last up to 100,000 (161,000 km) miles in some circumstances.

To check the resistance, remove the distributor cap (you'll have to remove the engine cover for access), leaving the wires in place. Connect 1 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 or which fluctuates value if the wire is moved/bent slightly.

It should be remembered that resistance is also a function of length; the longer the wire the greater the resistance. If the wire resistance is below 30,000 ohms, then compare the ohmmeter reading to the appropriate specification for that wire's length. Replace any wire which exceeds the appropriate resistance for its length:

0-15 in. (0-38 cm)-3000-10,000 ohms
15-25 in. (38-64 cm)-4000-15,000 ohms
25-35 in. (64-89 cm)-6000-20,000 ohms
Over 35 in. (89 cm)-5000-10,000 ohms per 12 in. (30 cm)

When installing a new set of spark plug wires, replace the wires 1 at a time so there will be no mixup. Start by replacing the longest cable first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original. Insert the distributor end of the wire firmly into the distributor cap tower, then seat the boot over the tower. Repeat the process for each wire.