Some simple but very useful tools will go far when troubleshooting an ignition problem.
An inductive-type spark tester will allow you to check for spark at each plug without disconnecting the wires. This inexpensive tool uses a probe that is applied to a plug wire with the engine running or cranking. The current flowing in the wire (if present) will cause the probe's light to flash. This will allow you to check all cylinders in a short time.
A test light will permit checks of circuit integrity. Even more useful (but more expensive) is a VOM (volt-ohmmeter), which can be very helpful for finding short or open circuits as well as performing component tests.
A spark plug boot tool is the best way to disconnect plug leads on modern engines. These are usually difficult to remove and the tool will enable removal without damaging the wire or boot and may help to protect the user against electrical shock.
- If the engine stops running or will not start, the problem is probably in the distributor cap, ignition coil or other upstream components.
- Check for spark at each wire. Use an inductive tester while cranking the engine. If one is not available, disconnect and ground the plug caps (one at a time) and check for spark while cranking the engine.
- If there is no spark at any plug, check the distributor cap and rotor.
Secondary Spark Test
- Remove the plug for the cylinder in question and check condition. "General Information And Maintenance" provides illustrations of spark plug condition and the various causes.
- Connect the boot and ground the plug on the engine. Start the engine and check for spark at the plug.
- Remove the plug and insert a screwdriver or metal rod into the cap and hold it about 1 / 4 " from the engine. If a spark jumps, the problem was the spark plug.
- If there is still no spark, check condition of the plug wire. Replace it if burned, cracked, or shows insulation damage. Resistance should not exceed 12,000 ohms/foot.
- If no test equipment is available, try fitting the wire from another cylinder and repeating the spark test.
- If plug and wire are in working condition, check the distributor cap. Check the cap for cracks, terminals for corrosion, internal contacts for erosion, carbon paths, or other obvious faults.
Spark Drop Test
In the event of a misfire or poor running condition in which no spark at one or more plugs is suspected, check for spark at each plug.
NOTEIf any service was performed on the engine recently, be sure that the spark plug wires are connected to the correct cylinders before proceeding. It is possible for an engine to run (poorly) with wires switched. Refer to the firing order illustrations in this section.
- An inductive tester, as mentioned above, is the easiest way to check. If not available, proceed as follows.
- With the engine running, disconnect the wires from the distributor cap terminals one at a time. Be sure to properly insulate yourself before handling the wires. Do not leave the wire(s) disconnected for more than a few seconds.
- As each one is disconnected, engine rpm should decrease or the engine should run roughly. If no difference is noted when a plug wire is disconnected, check the spark plug, wire, and distributor cap terminal for that cylinder.
- Check the spark plug cable connections for good contact at the coil, distributor cap and spark plugs. Push on them to ensure they are fully seated. Check that the boots are in good condition and fit tightly on their terminals. Boots that are cracked or torn should be replaced. Distributor terminals and plug boots should be smeared with dielectric grease to prevent corrosion.
- Clean plug wires with a non-flammable solvent to remove oil, grease or dirt. Wipe dry. Check for cracks, brittle condition, melted insulation or other obvious damage. Replace any damaged wire(s).
- Some engines are fitted with heat shields pressed into the cylinder head to protect the boots from damage. They should not be removed. When fully pressed in, the boot should show a small air gap between the boot lip and the top of the shield.
- Check that plug wires are correctly routed, secure in wire holders (if fitted) and clear of danger areas such as exhaust manifolds or moving parts.
A rough inspection of the ignition system can be carried out by running the engine in the dark. Look for arcing from wires, cap, coil, etc. to ground.
WARNINGWhen performing ignition system tests, note the following:
Engines should not be run for more than a minute or so with one or more spark plug wires disconnected. To do so risks damage to the catalytic converter.