Fuel Pump Testing with a Low Amp Probe

The low amp probe and oscilloscope have revolutionized the way diagnostic work is approached by todays technician. Digital storage oscilloscopes, or DSO for short, began showing up in shops with some regularity during the mid to late 90s. Technicians quickly found the DSO to be an invaluable tool for diagnosing cam and crank sensors, throttle position sensors, and even secondary ignition systems. As the DSO becomes more and more popular, the larger cabinet style ignition scopes are beginning to disappear from the shop.

The oscilloscope alone is not very useful for testing electric fuel pumps, or any other DC motor for that matter. Fuel pumps, blower motors, radiator fan motors, and even window motors are all examples of DC motors and all have problems with intermittent failures. In this article I will be dealing primarily with fuel pumps, but the information applies to almost any DC motor found on a vehicle today.

By itself, the scope is not any more useful than a voltmeter when it comes to checking for a defective fuel pump. You can use it to check power supply and perform a voltage drop test on the ground circuit. If the pump had good power and ground but did not work, then it was bad. That was about the extent of a technicians testing capabilities until the low amp probe came onto the scene. When testing DC motors, amperage draw can tell us much more than a voltage trace on a scope ever could. To add icing to the cake, connecting a low amp probe is super easy and does not require piercing or cutting any wires.

If the vehicle has a dedicated fuel pump fuse, meaning that the fuel pump fuse does not also feed other components, simply put a fused jumper wire in place of the fuel pump fuse and connect your low amp probe. On any Ford product, connect the low amp probe around one of the hot wires at the inertia switch. If you understand how to read a relay diagram, you can use a fused jumper in place of the relay and run the pump, however, do NOT try this method unless you know with absolute certainty which wires to jump at the relay connection. Jumping the wrong relay wires can damage the PCM. You can not damage anything by jumping the fuel pump fuse with a fused jumper wire, so that is the preferred method. If all else fails, and the vehicle does not have a dedicated fuel pump fuse, locate the fuel pump harness and connect around the main power feed wire to the fuel pump.

Electrical and Mechanical Parts

There are many different designs of electric fuel pumps made by various manufacturers and aftermarket parts supplies. Of the two pumps shown above, the one on the left is a typical domestic in-tank pump. The one on the right is a European frame mount pump. Both motors have a different design but achieve the same results, and that is to turn the mechanical fuel pump built in to the housing.

The electric fuel pump has two halves. The first half is the electric motor, and the second half is a mechanical pump that is driven by the electric motor. Either of these two halves can fail, but most intermittent fuel pump failures are due to a problem with the DC motor. The actual pump is only a small part of most electrical fuel pump assemblies. The electric motor is usually double or more the size of the mechanical pump. The picture to the above is the mechanical part of the domestic fuel pump assembly.

The electric DC motor has several working parts including the stator windings or field magnet, armature, commutator bars, and brushes. The brushes deliver power to the commutator bars which are connected to windings on the armature. When these windings are energized, they become magnetic and repel against the same polarity force in the stator, just as two magnets of the same polarity will repel against one another, causing the motor to turn. Since the commutator bars are attached to the axle they will also turn and eventually break contact with the brush thereby de-energizing that field wire. Then the next pair of commutator bars will contact the brushes, energize its field wires and create a new magnetic field to push against the stator or field magnet (some electric motors use stationary magnets as a stator while others use stator windings to create an electro-magnet). This cycle repeats itself as long as the pump has power supplied to it. Most fuel pumps have a total of four sets of commutator bars and consequently four sets of windings on the armature. This means that in one complete revolution of the motor, the brushes will have energized four different sets of windings at the appropriate times to create repelling forces and spin the armature on its axle.

The top picture above shows the European pump disassembled and the picture below is the domestic pump. Notice how small the mechanical pump is on the European assembly when compared to the electric motor. On the European pump, the brushes contact the sides of the commutator bars, and on the domestic pump, the brushes are in the top of the assembly and contact the top of the commutator bars.

Viewing the Pattern with a Scope and Low Amp Probe

The contact points where the brushes meet the commutator bars and energizes the fields on the armature can easily be seen on a scope with the use of a low-amp probe. Each time a pair of brushes contacts a commutator bar set, current begins to flow into the field windings attached to that set of commutators and each time the brush breaks contact, current flow drops. Take a look at the image below of a normal fuel pump amperage waveform from a 1997 Dodge Caravan. Notice that amperage fluctuates rhythmically from 5 to 6 amps as the motor rotates. Look closely at each bump and you can see that the pattern repeats it self every 8 cycles. This is because the motor has four pairs of commutator bars and 2 brushes. Each commutator-brush contact point will produce a unique signature.