Under the Hood: Seeking source of surprise jolt

Question: I was checking the transmission fluid on my truck and darn! I got a really strong shock from something. The engine was running, of course, and I don't believe I touched anything near the spark plug wires or distributor other than the dipstick. How and why did this happen?

Answer: It sounds like you tangled with some ignition secondary system spark, which is about as unpleasant a surprise as there is! My hunch is your hand brushed up against a vacuum hose located near the transmission dipstick and the hose provided an unintended path for ignition spark. You may have also brushed against an injured spark plug wire.

There are two halves to an ignition system, the primary and secondary. The primary system consists of the ignition control module, engine position and/or PCM (powertrain control module) inputs, one side of the ignition coil and related low voltage wiring. The secondary circuit consists of the other half of the ignition coil, the distributor cap and rotor, spark plug wires, and spark plugs. In a nutshell the primary system requests the spark and the secondary system makes and delivers it.

It's difficult for secondary system spark to jump the spark plugs, making any possible escape opportunity between the coil and plugs to engine metal or to you very attractive. Any cracks, splits or pinholes in the secondary delivery parts (cap, rotor, wires) can provide this path, and should a spark plug wire or plug have excessive resistance, there will be even greater spark than usual trying to escape. Spark plug wires can develop corroded terminals or damage (resistance and insulation tears) if they are tugged on excessively during removal.

My hunch is you have a faulty spark plug wire with an insulation fault and possible high resistance. One might not assume a rubber vacuum hose can conduct electricity but it can happen if it's touching the spark plug wire at the point of insulation damage and you offer an additional pathway by touching it.

I'd inspect the area around the distributor looking for a vacuum hose touching a spark plug wire and/or a visible insulation fault in one of the plug wires. Note which spark plug wire it is (by cylinder number – this may come in handy should a misfire or diagnostic trouble code later appear) and reroute the parts away from each other.

A minor insulation fault in a spark plug wire may be tolerated if there isn't anything nearby for spark to jump to. The possibility of excessive resistance in that wire is of concern as it could lead to a cylinder misfire.

How about visually inspecting the terminals at each end of any suspect wires for corrosion, and if possible check them for excessive electrical resistance? A typical spark plug wire contains about 5000-10,000 ohms of resistance per foot of length. Checking other secondary system parts such as the cap, rotor and plugs for excessive wear or damage is a good idea as well.



Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.


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