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Ignition timing: The forgotten test

Monday, April 1, 2019 - 07:00
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The timing of this ignition event should raise a question: When a vehicle is running shouldn’t the ignition timing be advanced? The answer is yes and the conclusion is that something is broken.

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Measuring ignition timing

If you own a PicoScope, measuring timing of an event is relatively easy. The rulers can be used to mark two consecutive top dead center pressure events to give the scope a 720° reference. Then a cursor can be dragged to line up with the timing event that you desire to measure. A box will appear at the top of the scope screen and the difference in degrees will be displayed.

Figure 7 - A 720 degree event is measured to be 527.2 millisecond

If you are using a scope that does not offer this option, such as a Snap-On product, this task can still be performed relatively easily with a little bit of math. First, use your cursors to mark a 720° event from top dead center to top dead center. The scope will display the amount of time that the 720° event took (Figure 7). In this case that measurement is 527.2 milliseconds. Second, divide the amount of time of the event displayed on the scope by 720. This will tell us how much time each degree of crankshaft rotation is responsible for. In our example, 527.2 milliseconds divided by 720 degrees equals .73 milliseconds per crankshaft degree. Third, leave the first cursor at top dead center and move the second cursor to the timing event you wish to measure (Figure 8). A new time measurement will be displayed on the scope screen. In our case that number is 29.46 milliseconds. Finally, divide this new time measurement by the number obtained during the second step. In our example, 29.46 milliseconds divided by .73 milliseconds equals 40 degrees. This number represents the amount of timing advance, or retard, for the given capture. In this case the ignition timing is retarded 40 degrees. Remember, no matter which tool or method you are using, if the event occurs to the right of top dead center this indicates a retarded timing event and to the left of top dead center would indicate an advanced event.

Figure 8 - The ignition firing event is measured and occurs at 29.46 milliseconds after top dead cente


Ignition timing is just as important as it always has been for the proper operation of a spark ignition internal combustion engine even though technological advancements have eliminated the technician’s ability to adjust, or even check, base ignition timing. The obsolescence of timing lights, timing marks and timing adjustments have resulted in an industry mentality that tends to forget this important issue.

Technically, ignition timing should never have to be checked on a modern vehicle. The engineers, as a result, did not give us the ability to do so. However, in the engineers’ defense, every potential failure cannot be anticipated. Yet components do break and we technicians have to adjust our diagnostics to these unforeseen situations. Who knows, maybe some day we will see the return of timing marks on a crank pulley for the purpose of diagnostics. I doubt it. Maybe we can get a diagnostic trouble chart that actually leads us to a vehicle fault in a timely and accurate manner. There is a saying that has something to do with which hand fills up faster: “Wish in one hand and…"

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