Proof of good fuel pump voltage using amperage.

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4 years 3 months ago #35696 by blackbart
I’ve seen several SD fuel pump diagnostic videos where Paul states “because of the high amperage draw it proves the power and ground is good. “ I need help with this. Obviously you can’t pull a 20A startup with a bad connection. But where is the limit? Say you have a crusty connector and it’s still flowing 11A. I tried asking this on the public utube channel, and you can imagine the input.
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4 years 3 months ago - 4 years 3 months ago #35698 by Chad

blackbart wrote: I’ve seen several SD fuel pump diagnostic videos where Paul states “because of the high amperage draw it proves the power and ground is good. “ I need help with this.


Someone else may be able to explain it better. But, it boils down to Ohm's Law. I=V/R, where I=Current, V=Voltage, and R=Resistance. You can't have the expected Current flow if Voltage and/or resistance is not what is expected.

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

I have learned more by being wrong, than I have by being right. :-)
Last edit: 4 years 3 months ago by Chad.
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4 years 3 months ago - 4 years 3 months ago #35705 by Andy.MacFadyen
As with Chad's post, Ohms Law the key is working out what you should expect to see either by measuring the resistance of a known good or doing some research. You can build up a working knowledge of the current draw you can expect see from common parts such as injectors, relays, starter solenoids, wiper and blower motors. The curent draw from tungsten & halogen bulbs can roughly be worked out from the wattage 48 watts/12v = 4 amps 21 watts/12v = 1.7 amps, 5 watts/12v = about 0.5 amps.

Last month I was looking at a common rail diesel measured at the fuel pump fuse the curent was about 7 amps, but the fuel flow was poor and aerated and pressure took time to build.

It turned out it was a two pump system (in-tank scavenge/primer pump and under hood pump) both pumps powered through a common fuse and should be drawing about 11 amps combined total. Disconnecting the underbonnet pump and the curent dropped to zero.

At the tank multi-plug with pump still connected back probing showed the power feed was showing a heathy 12.6 volts and the ground zero volts. With pump powered up the voltage on the ground pin should be low as possible but still expect to see some voltage.

Fitted new in-tank pump (about 1/2 the size of the under hood pump) curent at the common fuse rose to the expected 10 to 11 amps. Back probing showed the power at 12.3 volts and the ground at 0.05 volts, which is okay.

Paul has a really good video on diagnosing on this on a rusty old GM dump truck

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Last edit: 4 years 3 months ago by Andy.MacFadyen.
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4 years 3 months ago #35721 by Noah
Excellent responses from everyone.
I'd like to share what I've learned because this is a concept that puzzled me in the past.
The key to the amperage confirming circuit integrity is the initial turn on spike.
The way Paul explained it to me was that you would not reach that high peak amperage at the beginning of the wave form if they was a circuit problem.
Electric motors have almost no resistance at rest, so when power and ground are first applied you get a big spike in amperage. As the motor turns and generates counterelectromotive force, the amperage is reduced to the nominal draw of the motor.

"Ground cannot be checked with a 10mm socket"
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4 years 3 months ago #35735 by Andy.MacFadyen

Noah wrote: Excellent responses from everyone.
I'd like to share what I've learned because this is a concept that puzzled me in the past.
The key to the amperage confirming circuit integrity is the initial turn on spike.
The way Paul explained it to me was that you would not reach that high peak amperage at the beginning of the wave form if they was a circuit problem.
Electric motors have almost no resistance at rest, so when power and ground are first applied you get a big spike in amperage. As the motor turns and generates counterelectromotive force, the amperage is reduced to the nominal draw of the motor.


The very high in-rush is one of the big disadvantages of electric motors making use of it as a diagnostic test is neat.

Inrush current link www.motiongenesis.com/MGWebSite/MGGetSta...orWithRLCircuit.html

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4 years 3 months ago #35742 by blackbart
I understand this if you know what amperage to expect.
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4 years 3 months ago #35760 by Noah

blackbart wrote: I understand this if you know what amperage to expect.


Most of the fuel pumps I've tested are around 10 amps.

You can get an idea by the size of the fuse that protects the circuit. The fuse is usually double the expected current draw. So a 20 amp fuse would protect a circuit with an expected load of about 10 amps.

"Ground cannot be checked with a 10mm socket"
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4 years 3 months ago #35761 by blackbart
With that in mind, say you have a 20A fuse and expected 10A but You read 7A on the scope. Do you decide to do voltage drop tests at that point? I've been surprised at how high some new pumps draw.
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4 years 3 months ago #35763 by Chad
I expect to see anywhere from 6-10 amps. I wouldn't question 7 amps, at all.

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

I have learned more by being wrong, than I have by being right. :-)
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4 years 3 months ago #35767 by Andy.MacFadyen
Don't be surprised when one throws a curved ball, Paul has a classic video on an in- pump at Rosedale Tech the pump was replaced with a generic pump and the pump polarity got reversed so the pump was running backwards.

Eric O at the SMA Channel has one where the in-tank pump plumbing had an internal leak. In both cases the pump gave no electrical symptoms detectable from ouside the tank. I have had couple similar cases where the main fuel filter located inside the tank had come apart.

Another one that can throw you is if you are testing a relay and its' case flooded with water.

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