The purpose of this test is to investigate and examine the current draw waveform from a vehicle fuel pump.
The orientation of the amp clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.
This known good waveform has the following characteristics:
If you zoom in on the waveform you can see the pump waveform in even more detail. It shows a current pulse for each sector of the commutator. The majority of fuel pumps have 6 to 8 sectors. Our example has 8, but if you are unsure you can count the pulses quickly and easily in PicoScope 6.
By using the vertical rulers you can mark the waveform where the pattern appears to be repeated.
Align the two rulers at this point and apply the Falling Edge Measurement. This will reveal the number of sectors present on the commutator.
With the rulers aligned you can also use the Frequency and RPM indicator which will indicate the speed of the fuel pump. In this example, the measured speed is 8190 RPM.
A known good fuel pump waveform will generally have a seesaw pattern with relative consistency and minimal variation between the highs and lows. This can make it difficult to find the number of sectors on the commutator. A bad waveform will show large or irregular drops in the pattern, with large differences between the highs and lows. These obvious inconsistencies can help identify a worn spot on the commutator or a short in the armature.
When you have a fuel pump issue you will experience loss of power. The pump may still pressurize the system and the engine may even start, but when the demand from the fuel pump is high, there will be a distinct lack of power. This can be down to a number of issues: there could be a blockage in the fuel lines, the fuel filter could be blocked or the non-return valve could no longer be operating correctly. A repeated feature on the waveform can indicate wear and an impending failure.
Time rulers used to mark sections on the commutator.
Frequency and RPM indicator measurement taken between the rulers
Reference waveform from known good fuel pump
Our waveform in Figure 4 shows a very uneven current draw on a number of sections when compared to a known good reference waveform from the waveform library. (Click here for more information regarding reference waveforms.)
This issue could be down to a poor conducting commutator or worn bushes. We would normally expect a current draw from this type of pump to be around 2.5 A, but the above waveform is above 0.5 A. From what we can see there is enough evidence to warrant removing the pump for further investigation.
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