A while back I got a pretty good shock from one of my Camry’s spark plug wires. I didn’t think much about it at the time other than to never touch them again when the engine is running. Yesterday I mentioned it to a friend, and he told me the voltage was probably 10,000 volts, or even higher.

My questions are: Is this true? If so, how come I wasn’t badly hurt? You hear about people being injured or killed by just 110 volts every so often. Why did the shock happen anyway?

Terry Durham, San Jose, Calif.



I hate it when that happens! Secondary ignition system voltage can range from about 5,000 to as high as 40,000 volts and can give you quite a jolt when encountered. Depending on the ignition system type, typical voltage at steady speed will range between 5,000 to 10,000 volts and jump to perhaps 15,000 during acceleration. Higher voltage is usually the result of a high-resistance (faulty) spark plug wire.

Why weren’t you fried like a zucchini by this much voltage? Electrical current is what will hurt you, and there was very little present. Your car’s ignition coil(s) transforms battery voltage to many thousand volts by creating and collapsing a magnetic field in what’s called the primary winding, a coil of wire with about 200 loops. The created magnetic lines of force cut across a second winding containing several thousand loops, stepping up voltage and reducing current by the same ratio. The secondary winding and subsequent circuit leads to the spark plugs, and the high voltage ionizes (jumps) the spark plug gap. While painful and certainly to be avoided, a vehicle’s secondary ignition circuit current is below the level considered dangerous.

Why were you shocked? The spark plug wire you touched contained an insulation fault – perhaps a small split or crack in the rubber coating. The wire might have been routed such that enough distance (air) prevented spark from leaking to ground, causing an engine misfire. Odds are that as you touched the wire, another part of your body was simultaneously contacting a metal engine part or body component, creating a more desirable path to ground than the gap of the spark plug.

The spark plug wire also might have contained high resistance, due to a corroded end terminal or torn conductor. The higher voltage now created by the ignition coil – in an attempt to overcome this resistance – more aggressively seeks a ground path, through you.

Should you repair the offending spark plug wire? Assuming you keep your hands clear in the future, and the car runs smoothly at all times, probably not. I’m more concerned about excessive wire resistance than the probably minor insulation fault. If you have access to a digital volt-ohm meter, a useful and affordable home mechanic’s tool, each spark plug wire can be tested for resistance. A typical TVRS (television/radio suppression) spark plug wire should have less than 10,000 ohms resistance, per foot of length.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190.

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AP-NY-06-24-05 0617EDT