Quick Tip: Fuel Injector Voltage and Current Tests

A bad fuel injector could cause a host of drivability problems on a vehicle.

Let's walk through how to test both voltage and current flow through the injector using guided component tests.

OK, so as I said, we'll go through guided component test because that's the easiest way to do it.

Vehicles already Id'd in here and then once you load into a vehicle is going to give you all of the different systems that we have information on the test.

In this case we have 7 systems, could be up to 12 systems.

This information also goes back to 1981.

So there's a lot of information in here.

Let's go into engine though, because that's where the fuel injector is.

Once we enter a system, it's going to give us a list of components on that system and also the subsystems that are available on this engine.

And in this case, we'll go to fuel system and then we'll look at the fuel injector first things component information.

So this will tell us about how it works.

So it says these are normally closed solenoids responsible for delivering atomized fuel into the combustion chambers.

When energized, the coil winding inside the fuel injector will create an electromagnetic field.

This will pull the pintle off the seat and allow the injector to spray.

Fuel injector on time is known as pulse width and it's calculated by the ECM or PCM based on engine speed, load and temperature.

So this is good to know.

This is a standard gasoline fuel injector on this, but there's multitude of different types of fuel injectors you could find out there and they're all going to test differently and they're all going to look differently.

So in this case, this is just a standard top feed gasoline fuel injector.

So it's good to know that.

And if we go back, everything else on here is a test.

So we could do resistance test, current ramp test, which we'll look at in a second.

But the first thing I want to go to is the signature test because we can see a lot about that component by looking at the signature that it makes.

So we'll go into signature test, then it tells us where to connect the test leads.

So of course this is on a tool with a scope.

So we have our colour coded leads, we have our yellow lead goes to the injector control.

I'm on cylinder 1 so that means I need to hook to the brown wire gives us the pin out and gives us the wire colours and black to a known good ground.

So I have it grounded on the battery here it says should be used to check waveform signature integrity and here's what it's supposed to look like.

We'll hit view meter and see if it matches and it looks like it's pretty darn close.

You can see the dip down, the dip up.

Now, I want to take a little bit closer look at this.

I'm going to change some of the settings.

Like I said, the tool does automatically set it up for us, but sometimes you want to make a little bit of an adjustment to see a little bit better.

So we'll pop into the meter and then we will bring our time down a little bit more, bring it down to, say 20 milliseconds, move this over a little bit.

All right.

So we can see the voltage coming in.

What we should see is we should see alternator voltage at the first line.

Let me turn on my cursors to get a measurement here.

All right, so stop it.

OK, so you see I have a line that comes in, a line that comes down, and a line that goes up.

So first line should be constant power from the ECM.

So our from the fuse actually.

So we should see constant alternator voltage.

So in this case 14.2 volts.

Engines running looks good.

When it turns it on, it pulls it down to ground.

So we want to see a nice sharp pull down down to pretty much zero.

And in this case I see .2 volts right there.

Now it grounds it to turn it on for a set amount of time, so we'll just move our cursors again and take a look.

So from there to there, how long is the computer commanding it open?

In this case I see 2.45 milliseconds, so little under 2 1/2 milliseconds to fire and add an idle.

That's probably pretty good.

Next thing we see is this big spike.

So what this spike is, is, as it said, it builds an electromagnetic field when it's open.

So when it pulls that pintle off the seat when it's holding that open, it is going to be storing all that electricity inside.

It works on a very similar principle to an ignition coil.

So when all that electricity is then released, it has to go somewhere so it goes back in that align.

So if I measure how tall this is at the top, probably right about there, I'm seeing 52.3 volts.

So it's 14 volts in, 52 volts out.

Now this is going to vary depending on what type of fuel injector it is, how many windings are in it, how much voltage has gone into it, how long it's open.

Way too many variables to say Here's what good is but what you want.

What you would want to do on a vehicle is just compare injectors across the different cylinders to see is 1 higher or lower than the other.

Now that's checking it voltage wise, verifying the circuit's OK We can also check it mechanically by checking the current flow through the injector.

So I'm going to turn on another channel here, Channel 2.

I'm going to set that to current.

I already have it hooked up on the car.

Oh, got to be recording.

So I'm going to set that to low amps and we can see looks a little small there, so I'm going to bring it up to 2 amp Max.

All right, little noisy.

So we'll put the filter on and we'll get it roughly there.

All right.

So we can see how they match up.

So should have no current flow through the circuit or very minimal current flow through the circuit when it's off.

When it goes to turn on, we want to see a nice smooth ramp like this.

It kind of dips down a little bit.

That dip right there in the middle, that dip is where the injector actually opens and starts spraying fuel.

That little dip is going to be where the pintle pulls off the seat.

And then when it's time to close the injector, we see we also get another little spike of current in there.

And then it comes down, drops off and levels off.

So we can see how current and voltage relate to each other.

Now what you want to do, if you're checking a single injector and you suspect you have a mechanical problem, watch that little dip, you just see it's roughly the same time every time.

If it was moving up and down, maybe over here over there and kind of moving erratically up that ramp, that indicates a mechanical problem like a bad spring inside.

It's kind of rattling around in there and you'll be able to see that by how the current flows through that.

So a couple simple tests we can do with the guided component test to help us verify whether or not an injector's good.