Instruct students on how to inspect Secondary Wiring Connections
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SECONDARY WIRING AND COMPONENTS: Inform students that the spark plug wires should be inspected carefully for signs of chafing and cracks that can cause high-voltage leaks and arching. Let students know that often there are signs of white or grayish powdery deposits on the secondary cables at the point where they cross or are near metal parts. Explain that the deposits are evidence that the high-voltage leak burned dust collected on the cable. These conditions result in an intermittent engine miss, especially under damp conditions.
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Have students look at both ends of the plug wires to make sure the boots are neither split nor brittle and that the terminal ends are clean, tight, and fit their connection securely. If the terminal ends are dark or rusty in appearance, tell students to replace the plug wires.
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Explain Carbon Tracking: Carbon tracking is carbonized dust that can form on spark plugs, inside wire boots, coil, and distributor cap. The carbon track will form a low resistance path to ground and cause a misfire. If a spark plug shows signs of carbon tracking, tell the students to replace the plug and wire together since the carbon track is also on the inside of the plug wire and will transfer to the new plug after a few thousand miles.
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Teach students that if the vehicle is leaking engine oil on the spark plug wires, this should also be corrected. Oil-soaked plug wire boots will lead to arcing at the cylinder head. Instruct students to always use the correct plug wire spacers and route plug wires correctly to prevent problems with crossfire and EMI (Electromagnetic interference). Electromagnetic interference (EMI) can cause problems with the vehicle's onboard computer. Also, let students know it makes sense to replace plug wires in sets and that are made specifically for the vehicle, the reason being that if one plug wire is defective, then the others will probably fail soon. Explain that it is usually more cost effective to purchase a set of wires instead of buying two or three.
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Instruct students that secondary cables must be connected according to the firing order. Have students refer to the manufacturer's service manual to determine the correct firing order and cylinder numbering. Let students know that an occasional glow around the spark plug cables, known as a corona effect, is not harmful but indicates that the cable should be replaced.
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Inform students that spark plug cables from consecutively firing cylinders should cross rather than run parallel to one another. Tell student that spark plug cables running parallel to one another can induce firing voltages in one another and cause the spark plugs to fire at the wrong time.
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IGNITION COIL INSPECTION AND TESTS: Teach students that the ignition coil should be inspected for cracks, carbon tracking, and corrosion at the secondary connection, as well as the primary wiring connectors. Let them know that the ignition coil can also be checked with an ohmmeter.
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Explain to students how to test the coil with a DVOM: Tell students to make sure that the power to the coil is off, and then check the coil primary winding. Most primary windings range from 0.5 to 2 Q resistance. The secondary winding can be checked on single secondary terminal coils by checking from one primary terminal to the secondary terminal. The expected resistance should be 6,000 to 20,000 ohms. Also, tell students to check the wiring for a short to the case. Be sure and let them know to check the service information for exact specifications for these tests.
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Instruct students that the ohmmeter/DVOM tests on the primary and secondary windings should indicate the general condition of the coil. However, let them know that these tests do not guarantee that the coil will work fine. Some defects will only show up on scope tests, such as defective insulation around the coil windings, which causes high-voltage leaks.
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INSTRUCT STUDENTS ON HOW TO REMOVE AND REPLACE SPARK PLUGS:
Tell students to unscrew each spark plug, using the special spark-plug socket, a ratchet and extension. Have them replace only one spark plug at a time.
Teach them to check each of the plugs as they remove them. Explain that this will indicate the general running condition of the engine. In an engine running normally, the spark plug tip should be brown or grayish tan in color. If it is any other color, or if there are deposits on the tip, there is a problem with the way the engine is running. Let them know that the cars service manual should have a chart that will help you determine the running condition of the engine based on the appearance of the spark plug tip.
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Instruct them that the gap between the spark plug electrodes is a critical dimension that must be correct. Let them know that most spark plug gaps are set at the factory, but sometimes gaps can be wrong, even on new spark plugs. In most cases, they will want to check the gaps of the new plugs, assuring they are to the specification listed in their service manual. Let them know that in some cases, particularly with iridium spark plugs, the manufacturer recommends against checking and adjusting the spark plug gaps. Tell them to always follow the recommendations of the spark plug manufacturer.
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Tell them to thread one of the new plugs into the spark plug hole until they can no longer turn it with your fingers, and then use a torque wrench to tighten it to the specification listed in their service manual. Inform them that it is a good idea to slip a short length of rubber hose over the end of the plug to use as a tool to thread it into place. Explain that the hose will grip the plug well enough to turn it, but will start to slip if the plug begins to cross-thread in the hole, and that this will prevent damaged threads.
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Last, instruct students to attach the ignition coil or spark plug wire to the new spark plug, assuring it is fully seated on the plug, and repeat the procedure for the remaining spark plugs.
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