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Quick Tip: Ignition Coil Primary Voltage Test

Applies To: MODIS Edge™, MODIS Ultra™, VANTAGE® Ultra, VERUS® Edge, VERUS® PRO, ZEUS®

Vehicle: 2013 Ford® Fusion. Having a hard time figuring out whether or not an ignition coil is bad? In this Diagnostic Quick Tip, National Field Trainer Jason Gabrenas shows you how to perform a primary side voltage test to quickly test a coil.



When trying to diagnose ignition problems on a vehicle, there's a couple of different ways we can test it with a scope. We're going to talk about a Coil Primary Voltage Test here.

With the vehicle hooked up already I have the number one lead attached to the ground side of the coil and all we've got to do is just fire it up.

I've got it on a hundred volt scale and a 20 millisecond time base so let's see what we get when we turn it on.

I'll pause it after a few seconds and we'll shut off the vehicle. It looks like we've got a good pattern here.

Let me move my cursors here so we can make some measurements and we'll talk about what we're looking at.

Where cursor one is that should be alternator voltage, right line voltage, and we're looking at about 15 volts here so we're a little higher than average, but still within spec.

Right here where it makes the sharp dive down, that's when the ECM grounds the coil to turn it on and start charging the coil.

Over here is where it releases, so that's the first voltage to jump the gap and start firing the spark plug.

On these multi-strike ignition vehicles, like this one is, these first two here, that's going to be controlled by the PCM, the duration of the sparks.

So if you want to make a measurement to see how it is working spark-wise, take a measurement here, the last line. Here, it's 1.17 milliseconds. Somewhere in the range of about one millisecond is a pretty good spark line.

This is a nice and easy test that you can do, it only takes one channel on the scope. Just backprobe the coil and that makes it pretty easy to test.

Last Updated: January 17, 2020

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