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Quick Tip: Fuel Injector Voltage and Current Tests

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

Having a hard time diagnosing a faulty fuel injector? In this Diagnostic Quick Tip, National Field Trainer Jason Gabrenas shows how to get a certain diagnosis using a lab scope to check both voltage and amperage.



When diagnosing a misfire on some vehicles, sometimes you might suspect that it's a fuel injector that's giving you problems.
You need to check it both electrically and mechanically to see whether or not it's working properly, or maybe you have a stock fuel injector.
We're going to show you a couple of quick and easy ways to do it using a lab scope. Let me go start the car and we'll analyze the pattern.
Let's take a look at what we captured here. The yellow line is the voltage going through that fuel injector and the green line is the amperage going through this fuel injector.
If I take a measurement at cursor one for the voltage, we should see pretty close to the alternator voltage. So 13.5 volts, that's pretty good.
Then this squared-off corner right here is where the PCM takes and grounds that fuel injector to turn it on.
Measurement at cursor two is roughly ground, so it's going to change slightly from depending on where we put it.
That's pretty darn close to ground so if I take cursor one and put it there, and cursor two and put it at the end of that ground period, that's going to give us our injector pulse width; 2.2800000000000002 milliseconds in this case.
Alternator voltage comes in and the PCM grounds it for a set amount of time - the pulse width. If I take cursor two and move it over, that's all that voltage that was just stored inside of that fuel injector.
And once it releases the ground, all that voltage goes back into the line. We're looking at 117.3 volts in this case.
And then over here we're going to see another little bump, we'll call that coil oscillation. That's the voltage just ringing back and forth within that coil. And then the voltage should go back to about alternator voltage.
Now as far as the current flowing through, this should ramp up at about the same time as the PCM grounds that injector. It shouldn't have a real high squared-off ramp; that'll indicate high resistance and a lot of amperage trying to open that injector. Maybe the injector is sticking.
If we look about halfway up this ramp right here, there's a little dip. That dip is the point where the fuel injector is actually opening and starting to spray fuel. And then this is a current limited injector.
So once it gets to a set current that the PCM dictates, it's just going to level off and then close at the same time that the injector closes.
If this wasn't a current limited injector, you'd see it continue up like a shark fin until the ground is released.
As you can see, just by doing a couple of quick little tests on a fuel injector, you can see how it's working both electrically and mechanically, and that will help you narrow it down when you think you might have a misfiring fuel injector.

Last Updated: January 17, 2020

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