Signs & Symptoms of a Bad Coil High Voltage Wire

If the engine stops and starts erratically while driving, or if it cuts out altogether and refuses to restart, you may have a bad wire between the coil and the distributor. Constant movement, caused by engines vibration and the wire's tendency to flex slightly, fatigues the wire's central core. This sometimes causes an eventual break in the core close to the crimped end. Although the two halves may still make contact most of the time, they lose contact at odd intervals when the wires vibrate. This leads to eventual failure caused by a high-voltage arc leaping across the minute gap. To isolate the fault, turn the ignition key on without attempting to start the engine and then remove the distributor cap. Ensure that the points are open and adjust a multimeter to the volts setting. Touch one probe to the insulated side of the points and the other to the engine block. If you get a reading close to 12 volts, have an assistant wiggle the cable between the coil and the distributor while monitoring the readout. If it fluctuates, the cable is defective and must be replaced.

If the engine starts normally, then runs erratically once you accelerate, apart from low engine compression or a bad fuel system, the problem is most likely an electrical fault caused by cable deterioration between the coil and the distributor. Remove one of the spark plug leads. Insert the tip of a thin insulated screwdriver into the end of the plug boot so that it makes contact with the inner plug spring clip. Hold the lead in one hand and the insulated handle in the other -- do not touch the screwdriver shaft and keep your body away from the car to prevent a dangerous high-voltage shock. Move the shaft close to the head of a suitable grounded bolt on the engine. Play close attention to the gap between the screwdriver and the bolt and have an assistant crank the engine by three or four revolutions with the ignition key. A powerful blue spark should leap across the gap. If it is a weak yellow color, either the coil is faulty or you have a bad coil to distributor cable.

Distributed resistance cables are equipped with a stranded fiberglass core impregnated with a latex-based graphite to provide the maximum amount of radio frequency suppression while providing excellent electrical conductivity. Although the internal resistance is relatively low, the core still generates between 3,000 and 12,000 ohms per foot of cable -- and that electrical resistance generates heat. After extended use, this heat breaks down the latex bond holding the graphite particles together, causing greater resistance, a misfiring engine, and eventual failure. This type of cable should be replaced every three or four years.

Fixed resistor wires have a core made of copper and, sometimes, multiple steel strands. To provide RFI suppression, a fixed resistor is fitted inside each spark plug boot. Of the three wire types, a fixed resistor wire has the highest fatigue-related failure rate as described under the “Symptoms 1” heading. Fixed resistor wires should be replaced every two years of regular use to prrevent roadside breakdowns.

Inductance wires have a core made from copper/nickel alloy to provide maximum conductivity. The core itself is wound with a spiral of extra thin wire of the same alloy, which creates an RFI-inhibiting magnetic field. Apart from durability and its RFI suppressing qualities, inductance wire only generates approximately 500 ohms of resistance per foot. This means that less current is needed to provide a healthy spark across the plug electrodes. Although cable failure on this type is relatively rare, the copper/nickel alloy is also subjected to possible fatigue as described under the “Symptoms 1” heading.