# Which way does current flow in (automotive) spark-plug wires?

Every modern gasoline engine, that I'm aware of, uses an ignition coil to generate "spark". Usually, a current is introduced into the primary of the ignition coil and then interrupted (by a solid-state switch of some sort) causing a spike in voltage at the secondary of the ignition coil. Depending on how the secondary winding is connected to the spark-plug wires (through the distributor), the resultant current could presumably be in either direction.

Is there a good reason for current to flow "towards" the spark plug versus "away" from the spark plug? Is it the same way on all vehicles? How about small engines (lawnmowers, snowblowers, chainsaws, outboards)?

All of this is significant because the cost of automotive timing lights is largely due to the cost of the inductive (clamp on) pickup. A positive or negative "edge" from the inductive pickup fires a Xenon flash tube which illuminates the timing marks. Some alternate way of developing an edge (relative to chassis ground) could be significant.

• With inductively coupled timing lights selling for $17, it's not clear that the clamp is a standout cost compared to the product, flash tube circuitry, housing, packaging and marketing costs... and the fact that there's no longer a large market of consumers who have anything in their vehi^h^h rolling computers that can be adjusted with the aid of a timing light. Jan 7, 2017 at 23:53 • I believe the HT leads are positive and make a circuit to the grounded engine body. I can't see how thing makes a difference to timing light construction though. Jan 8, 2017 at 0:03 • Ignition of both the spark plug and the Xenon tube works in both polarities. Using a pencil or carbon tip midstream with the plug wire removed, show indicate a spray of arc towards the plug. MBA/B's used positive ground. Most cars now use negative ground. But the polarity can be controlled separately by coil design independent of battery polarity. Jan 8, 2017 at 2:15 • That a timing light can be put together for$12, points to the fact that every dollar matters! My (6th generation) Honda Accord has programmed electronic fuel-injection...the manual states that idle-timing should be checked for 12 degrees BTDC (+/- 2 degrees). If it's not, replace the engine computer. Jan 14, 2017 at 1:58
• Another thing about the flash-tube timing-lights is that if you drop them once, they are about as useful as a wheel chock. Jan 15, 2017 at 2:10