Section 16 Toyota Sienna

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5 years 2 months ago #25849 by JeepLife
Once fuel pressure is found to be low and fuel pump current readings are near spec. There is no need to test power and ground at the pump? Would that be a waste of time and how so? Can current readings with a probe also be done accurately without a scope, but just a probe and DVOM?

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5 years 2 months ago #25854 by Tyler
Replied by Tyler on topic Section 16 Toyota Sienna

BillyE88 wrote: Is it a waste of time?


Certainly not! But it MAY be excessively time consuming or impractical, depending on the vehicle and it's condition.

For example, just about every car I touch goes on the lift for an inspection, including no starts. Since I already have it in the air, I'll absolutely back up the current measurement with a power/ground check if there's an easily accessible connector. Why not? :silly:

But, if lifting the car isn't possible, or accessing the wiring means tearing open the harness or fighting rusty bolts, then it's no longer worth my time or the shops time. Pull the pump and verify power/ground while you're there, and diag more as needed.

Can current readings with a probe also be done accurately without a scope, but just a probe and DVOM?


Absolutely! I do this all the time. You just have to understand the circuit you're testing, and how the meter will behave when looking at different faults.

For example, this is the current waveform from a failed fuel pump motor. Several of the commutator segments are clearly open.



There's no power/ground issue, since we can see the pump drawing 12A at times. But, the 'Live' reading of 3.5A is likely what a DVOM would show you in the same situation. The scope will allow you to skip power/ground testing, but the DVOM may leave you with questions.
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5 years 2 months ago #25927 by Wood
Replied by Wood on topic Section 16 Toyota Sienna
is it possible for fuel pressure to be low and current readings to be in spec? In what scenario could this be the case?

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5 years 2 months ago - 5 years 2 months ago #25935 by Noah
Replied by Noah on topic Section 16 Toyota Sienna
I'd like to just add to this that I struggled with this concept myself when I first started studying the material. After seeing several case studies where Paul confirms circuit integrity with just an amp clamp, I asked the man myself.
The way he justified it to me was that the inrush current, or the initial voltage spike when you first turn on the pump is an indicator of the circuits ability to carry current.
Were the fault due to high resistance, you would not see the high spike at the beginning of the waveform, like the spikes in Tyler's example .
So, as far as using just a meter, like Tyler already advised, I'd say it depends on your equipment and if you know where to look.

To answer the question posted by Wood: hypothetically a leak from the pump internal to the tank could result in the condition you describe.

There was a case study where Paul's students wired a fuel pump reverse polarity, causing it to run backwards which would also probably be hard to find with just a meter and amp clamp.

"Ground cannot be checked with a 10mm socket"
Last edit: 5 years 2 months ago by Noah.

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