Quick Tip: Fuel Pump Current Ramp Test

A failing fuel pump can be a cause of multiple drivability issues.

If it's not providing enough fuel, you could have misfires, could have some sort of other drivability issue, You could even have intermittent no start problems occur.

So there's an easy test to do that'll help us diagnose that.

It's called a fuel pump current ramp test.

Let's walk through it.

All right.

So the easiest way to do this is to use a guided component test function.

So if you have a snap on tool with a scope, it's going to have this guided component test.

So we already have the vehicle ID, It's a 2011 Hyundai Tucson.

Once we get into the guided component test, it's going to confirm the vehicle.

And then you'll see all the systems that we have tests available for.

So in this case, there's, what, 7 systems on this vehicle.

It's going to vary depending on the vehicle, But if I want to look at the fuel pump, it's going to be under engine and once I'm in there, I see all the different components in the engine and also subsystems.

So in this case we see fuel system, we have fuel injector, fuel pressure and fuel pump.

So we'll go into the pump and you can see we have component information, a voltage test and a current ramp test.

So the current ramp test is the one we want.

You can gives us some precautions.

Observe all fuel system safety precautions when connecting and disconnecting fuel lines.

Keep a dry chemical fire extinguisher near the work area, etcetera.

Fuel pump can be checked using a scope, waveform viewer and a low amp probe.

So I have a low amp probe connected on the vehicle right now and you can do this a few different ways.

We can do it.

If there's a fuse, you can pull the fuse out, put a jumper in, put the clamp around that, or a relay, same thing.

Or if you can actually get close to the pump, you could do it that way as well.

And what we're going to look at is the different bumps that are on the screen, which will denote the commutator segments on the electric motor as they go by the brushes.

So what we want to do is we'll just hit view meter in this case cars already running.

And what the scope is going to do is it's going to set up the scale and it's also going to set up the time base for us.

Now we can see, we we do see some bumps, they're relatively flat.

So not a lot of variation going on.

Some of that's going to depend on how much fuel's in the tank, how fast the pump is running, etcetera.

But I kind of want to change a few of these settings.

Like I said, it sets it up automatically, gets us in the neighbourhood, but I'd like to change a few things up just to make it a little easier.

First thing I'm going to do is put this trigger up here so I can keep it in place on the screen there.

Sometimes it's a little tricky.

There we go, Try and keep it in place as much as we can.

There we go.

All right, now it's fairly good.

Left to right.

Now, most fuel pumps, most brushed automotive fuel pumps that we'll find these days are going to have 8 commutator segments.

So if we count out eighty of these bumps, that's one time around.

So I see 123-4567, almost 8.

So we're really close to having eight in this 10 millisecond window that it sets up.

So I'm going to actually bring it out a little bit further.

I'm going to set it set to A10 millisecond.

Let's set it to a 20 millisecond window.

That gives us double the width so we can investigate a little bit better.

Now you can see I have a lot of them on here, so I'm going to hit stop, stop the recording.

Now we can take a look at these.

Each of them should be fairly even.

We don't want to see any dropouts, no weird fuzziness, no weird noise going on.

Overall, this one looks pretty good for a a vehicle of this age, this 12 year old looks like they're all fairly even.

Not too much current flow, you know it's not pulling out too much current.

Let's see how fast it's turning.

That's another aspect that we want to check.

So I'm going to turn on my cursors here and what we want to do is we want to put cursor number one one of them and then so that's number 12345678 and then there's #1 again.

So I'll put the cursor in roughly the same spot and you'll see how it kind of repeats to the the bumps look very similar when they repeat.

Number one had kind of has that little dip there.

Number one has that dip there.

That's how you know where it's going to repeat.

Now what we want to do is we want to do a little bit of a math calculation to see how fast this is turning because the volume of the fuel will be dictated by how fast the pump is turning.

So it looks like in this case I see 10.59 milliseconds for one turn.

If I pull up my calculator, if we do 60,000 milliseconds in a minute divided by how long it takes to turn one time 10.59 milliseconds, it's going to give me my revolutions per minute.

In this case, add an idle, it's warmed up 56165 RPM give or take.

That's pretty good.

Good fuel pumps going to run 5 to 6000 RPM or more.

You'll run into a drivability issue if you get down to 3000 or less because like I said, fuel volume is dictated by how fast the pump turns.

Also, I want to note, if you're looking at these bumps and there's one missing, one drops out, that could be a cause of an intermittent no start problem.

So that's where it's it spins and it stops on a good one, then it'll start again, then it stops on a good one, then it starts again, then it stops on that bad commutator segment.

Current can't flow through the motor, can't turn the motor, then that way the vehicle won't start.

And those are the ones you fix by like tapping the hammer on the bottom of the fuel tank vibrates the fuel pump enough to move it to a good segment, and then it's able to start again.

So if you ever run into something like that, or you suspect that the fuel pump is on its way out to be a good check to do, just kind of make sure everything looks even everything looks good and it'll help you diagnose those fuel pumps.