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Quick Tip: Fuel Pump Current Ramp Test

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

Having a hard time diagnosing a fuel delivery problem? In this Diagnostic Quick Tip, National Field Trainer Jason Gabrenas shows how to quickly verify the condition of a fuel pump, using a lab scope and amp probe.



Diagnosing a fuel pump can be pretty troublesome, especially if it's way back in the tank and you've got to drain the fuel tank and pull the pump out and all that good stuff.

Wouldn't it be nice to have a nice easy way to actually check the fuel pump from anywhere in the circuit and see how it's operating just by looking at the pattern on a lab scope?

It's called a Fuel Pump Current Ramp Test, and I'm going to show you how to do it right now. I already have a low amps probe hooked up in the circuit.

I pulled out one of the fuses, put a jumper wire in there and put the amps probe around that, and really you can tap in anywhere on the circuit to do this test. As long as the amp probe is around one wire.

I've got it [the lab scope] set up on a two amp scale, 10 millisecond window. Let's go crank the car over and see what we get for a pattern.

Here's our pattern, and this is an indication of a fuel pump that seems to be on its way out. This guy came in, he had a, a bit of a stumbling issue when going up hills, so let's take a look and see what the problem might be.

A fuel pump is basically just an electric motor, right? So it's got windings and it's got brushes and it's got these things called commutators that the brushes ride on, and that's what transfers the electricity.

Almost every automotive fuel pump you're ever going to run into has eight commutators in it. So if we can count out eight bumps here, each of those bumps is supposed to denote one of the commutators in the motor.

If we count out eight of those bumps [on the scope screen], that's going to be one revolution of this motor. So let's see, that looks like one, two, three, four, five, six, seven, eight. I guess that looks pretty good.

If you have at least eight bumps in a 10 millisecond window, you know the thing's turning sufficiently fast because then it's turning at least 6,000 rpm.

Most regular fuel pumps are going to run between five and six thousand rpm and anything around 3,000 RPM or so, you're going to run into some drivability issues.

Also, you want to make sure all those bumps are nice and even which this one definitely is not.

We're seeing a lot of wear on this fuel pump as we're looking at it so I'm definitely going to recommend replacing a fuel pump as soon as possible on this so the customer can avoid being left on the side of the road.

That was a pretty quick, easy way to diagnose it and just by looking at the pattern, you can see how worn out a fuel pump is.

Last Updated: January 17, 2020

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