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How to Check the Electrical Spark

The ignition system of an internal combustion engine delivers an electrical spark from a coil through a wire between the coil and the center post of a distributor cap. The spark then travels through a rotor on top of the distributor shaft and back through individual posts in the outer ring of the distributor cap, through the spark plug wires and to the spark plugs. If an engine does not start, or is constantly missing on one or more cylinders, chances are good that the problem is with the electrical spark to the spark plug. The only way to find the problem is to check the electrical spark at the distributor and coil, working out toward the plugs.

Instructions

1

Push the spring-loaded hooks on each side of the distributor cap down and turn them counterclockwise 90-degrees with a screwdriver to loosen the distributor cap. Lift the distributor cap off. Pull the rotor off the end of the distributor shaft.

2

Attach a jumper switch to the vehicles starter. Bump the switch until one of the lobes on the distributor cam aligns with the nylon cam follower on the ignition points. At this point the points should be open rather than together. Inspect the ignition points to ensure that there is no corrosion that will prevent electricity from flowing through them. If there is any corrosion on the points, replace them.

3

Slide a feeler gauge between the two open contacts on the ignition points. Ensure that the thickness of the feeler gauge is equal to the factory-specified gap setting. If the gauge does not fit between the contact points, or the gauge is too loose, adjust the points by loosening the adjustment screw, adjusting the mounting plate toward or away from the distributor shaft until you achieve the proper point gap. Tighten the mounting screw.

4

Inspect the rotor for burning or corrosion anywhere on the brass, mounted on the plastic rotor. If you find any burning corrosion, clean the rotor with a file or replace the rotor if the pitting from the corrosion is too deep.

5

Inspect the distributor for the same type of burning or corrosion. Clean or replace the distributor cap as needed. Reinstall the rotor on top of the distributor shaft. Reinstall the distributor.

6

Unplug the spark plug wires from the spark plugs by twisting them with a plug wire pliers and pulling them off the top of the spark plugs. Turn the vehicles ignition switch to the "on" position. Remove the thick black wire between the coil and the distributor, known as the coil wire, from the end of the coil. Hold the coil wire close to the coil with the plug wire pliers while operating the starter with the jumper switch. A spark should jump from the coil to the coil wire. If there is no spark between the coil wire and the coil, replace the coil and retest.

7

Plug the coil wire back into the coil and disconnect it from the center post of the distributor. While holding the coil wire with one hand and operating the jumper switch with the other, ensure that there is a spark between the coil wire and the distributor cap. If there is no spark, replace the coil wire with a new one, then retest.

8

Plug the coil wire back into the distributor. Pull one of the spark plug wires out of the top of the distributor. Hold it with the plug wire pliers and test as before. Repeat for all of the spark plug wires. If there is no spark between the distributor cap and the wires, replace the distributor cap and rotor, then retest.

9

Replace all the spark plug wires on the spark plugs. Start the engine. Remove the spark plug wires one at a time from the spark plugs and listen to engine performance. If an engine is missing, there is no perceptible change in engine idle when the plug wires removed, there is no spark between the wire and plug and there is a spark between the plug wire and the distributor, replace the spark plug wire.