Fuel pump waveform/AC coupling/ questions and problems?!

[scbird94]

Hey all, looking for some advice, you seem well versed in scope usage and i am beginning to use it more and more in my diagnostics.

Have the typical 01 f150 tow in, bang tank, vehicle sputters, fires up, drive into shop. Unfortunately now it starts every time.

I have dealt with this several times with gm's and been able to show customer the low amp 20 waveform with the typical dead spot.

This is the 1st ford i have actually stuck my neck out and said "lets diag this damn thing" vs. throw a pump in it, which is what most tank-bangers get.

I watched greasybob's video from another thread below, and i am curious... when i low amp 20 and 20/50/100ms sweep the pattern looks decent, no abnormal dead spots. drawing 8-10amps. There are a few spots on the down-side of the curves where it glitches, but its not screaming "bad pump" at me.

Question 1). When i switch to "ac coupling" like in the video, it looks horrible. You can see very uneven pattern, definately makes me want to throw this pump away. But... why is it different in AC coupling? What excatly does that mean? Why wouldnt it show on a standard waveform?

Question 2) How do YOU calculate RPM and what rpm is "tired". If i do a 8 hump cursor locate on 50ms sweep, multiply by 60, i came up with 3457rpm. I have heard on this forum that 6000 is healthy, but i have a pamphlet from a training seminar that states 3krpm is healthy. wtf.

Question 3) I notice a dramatic difference in the waveform when i change things like sweep, 40Avs 20A probe setting, or even the amperage range in my scope. It seems in this particular setting i get the "most condemning" waveform in Low amp 20 scale, with my trace set to 10A, and sweep set to 50ms. It looks questionable, but it really says "oh damn" if you enable "ac coupling".
I am just curious.... is there a "preferred setting" as a go-to when you are amp probing a fuel pump?

Question 4: As an experiment, and having in the past used a low-amp with my DVOM, i for the hell of it set the scope to 4channel lab scope with the low-amp on channel 1, then ran the truck, the waveform looked like it should when you encounter a bad fuel pump. Is there a situation when you use the amp probe in regular 4-channel scope setting or is it a "low-amp20/40" only situation

[geoff]

fuel pump

ref you fuel pump waveform look on web for pico technology in uk
look on automotive library &actuators for info on waveform

[woefman]

I think I can help you with the ac coupling part.

The Fuel Pump Starts out its voltage as DC volts
The Commutator type circuit your testing is switched to AC volts
That is just how electric motors are. (lamens terms)

By switching your meter to ac coupling your only viewing the ac voltage.

Basically the Signal you see without ac coupling is ac voltage over a DC (direct current) line.

Lamens terms again....

DC Volts at steady rate of say 12.6 volts would be a straight line
So now Picture your AC signal traveling along a shared DC line
As you know the motor in fuel pumps as well as say window motor
Spins and Creates that Commutator signal.

If you want to condemn the fuel pump, you switch your scope to AC coupling
to filter out any DC voltage for that particular test.

Hope this helps a little

[Lee Brown]

the easiest way to understand A/C coupling

The pattern gets averaged at Zero. easy enough right.

In practice how is it used.

You are looking at a fuel pump pattern that is at 6 amps so you need a 10 amp scale minimum to view this, correct.

Know you want to look at the pattern in a scale that gives the appearance of zooming in so you go to 2 amps on the scale.

Oh-no now the fuel pump pattern is off the screen at 6 amps.

No problem just select A/C coupling and the pattern is viewable because it is averaged at Zero. ( so move the trace to the center of the screen to view.

Reminder: this is for viewing the pattern, not for viewing the how many amps or volts or being measured. You will notice that you min and max values will not read. Its all about the pattern.