Scope hiding voltage drop??

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Scope hiding voltage drop??

Postby dieseljohnny » Sun Feb 12, 2017 12:30 am

Hi All.

I was at a crank and no start K12 1.2l Nissan Micra earlier.

Crank and cam signal present and correct(no chain issue), no immo issues but engine control unit not triggering injectors or coil

Basically it was a bad earth by the intake manifold(F15 in the diagram) but I could not volt drop it with the scope as every time I probed pin 2 (earth) on any of the ignition coils It would start and not show any notable voltage and if I removed the probe with it running it would stay running.
Negative was connected to battery negative

Same would happen with multi meter. and no problem in repeating the no start/start scenario by removing and fitting the test lead
Continuity/resistance indicated no resistance (I know the downfall of this testing method :twisted: :lol: )

I cant figure out or find a reason for this to happen?
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Re: Scope hiding voltage drop??

Postby STC » Sun Feb 12, 2017 1:50 am

and if I removed the probe with it running it would stay running.


that is interesting :shock:
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Re: Scope hiding voltage drop??

Postby Fat Freddy » Mon Feb 13, 2017 2:55 am

This does not come as a total surprise, I had heard something like this is possible but this is the first time I have heard of it actually having an effect.

I did a lot of my training using Tiepie scopes and the class was set up with about a dozen students each with an individual scope/lappy that could be daisy chained together including the tutors but his lap top screen was projected onto a white board.
This enabled all the scopes to be connected to a board that would emulate a 'working' EFI system (you could watch the immobilzer check the key, rev the engine, cam, crank, coils, etc, etc, etc).

Any way , IIRC (repeat IIRC, it was a good few years ago) he was discussing the use of 'bleed down resistors' in circuits and I'm sure he said the scopes use them. Any way he demonstrated this by connecting the scope daisy chain to the TPS?????? signal on the EFI circuit board. He moved the marker on his lap top so we could all see what the original voltage and then asked each of us to disconnect our scopes off the chain. Sure enough the voltage rose. Can't remember how much. But the point of the exercise was to demonstrate that the scope, no matter how minor, can effect the circuit.

So I'm guessing this is what has happened regarding the ignition coil circuit.

HTH
FF
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Re: Scope hiding voltage drop??

Postby STC » Mon Feb 13, 2017 4:59 am

FF
But the point of the exercise was to demonstrate that the scope, no matter how minor, can effect the circuit.


Yes agreed, adding anything, (scope, dmm) would have the potential to affect a circuit, albeit with negligible effect.

But here we have 4 ignition coils that can not operate because their path to Ground / Earth is paralysed by high resistance / Open circuit at a Common Ground Point "F15"

As you know your self, these coils will be pulling circa 5 amps ???

To probe a scope or Voltmeter between the coil Earth and Battery Negative which then completes that 12v / 5a circuit(55watt Headlamp Bulb territory) allowing the engine to start and run ??? I'm struggling with that :?

And then removing the Scope / Voltmeter as the path to ground and current is still flowing through the primary circuits keeping the sparks flying and the engine running ??? - that is hurting my head :?


Johnny

I wonder if the same results could be achieved using a jumper wire between Battery Negative and Coil pin 2 to start the engine and then remove it - will the engine continue to run ??

What happens if you make the "Bad Earth" at "F15" an open circuit, disconnect and insulate it completely.

There has to be another variable here ?
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Re: Scope hiding voltage drop??

Postby Fat Freddy » Mon Feb 13, 2017 12:14 pm

But here we have 4 ignition coils that can not operate because their path to Ground / Earth is paralysed by high resistance / Open circuit at a Common Ground Point "F15"

A coil may pull 5 amps but I very much doubt the problem is in the coil circuit (although I did incorrectly use that terminology in my first post, that should of read coil/transistor circuit), I'd more be looking at the transistor circuit.

There has to be another variable here ?

That's a given. How was that transistor/coil circuit effected by cranking voltage. Hows that transistor effected by negative coil oscillations once started?

Did that back probing effect the thresholds of that transistor? After all a lot happens at thresholds, or not.

Regarding this particular fault my theory is based a lot on conjecture along with some assumptions regarding this particular case that ties into an example that was given on a scope coarse which I may add included examples that the scope did have enough effect to catch you out, I am just unable to recall those examples.
So I don't agree it's always with "negligible effect".
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Re: Scope hiding voltage drop??

Postby STC » Mon Feb 13, 2017 8:42 pm

FF

You make some good points, food for thought ! & Indeed Yes! a lot happens at thresholds ...

Looking at that wiring diagram, I see that a Good Ground Connection at F15 is essential for those coils to Fire, (perhaps the Injectors too if they terminate there) For that reason I cannot fathom how a Scope or DMM can substitute that and enable it to start, then the fact that when the "bridge" is removed and it continues to run is confusing to say the least. I accept that my knowledge of Electronics is basic, although I did have some confidence in 12v Automotive Electrics

Perhaps there is more to that circuit than Nissan care to share with us on the schematic - even so I am fascinated as to how a ~5 amp actuator is being furnished with sufficient current flow to operate.

Are these coils bolted to the head and getting an earth that way ??
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Re: Scope hiding voltage drop??

Postby dieseljohnny » Mon Feb 13, 2017 9:14 pm

Yes the coils are bolted to down,

I should of got a video of this happening :roll:

The only other component sharing that earth point is the condenser and I cant think of any way this would cause this phenomenon

The only thing that I can come up with is the engine control unit triggers the coil with a 5v signal and their might be some form of monitoring on the circuit but I didn't see control signal during key on or cranking with the scope.
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Re: Scope hiding voltage drop??

Postby dieseljohnny » Mon Feb 13, 2017 9:25 pm

Another bit of diagram
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Re: Scope hiding voltage drop??

Postby STC » Mon Feb 13, 2017 11:01 pm

I should of got a video of this happening :roll:


I'm not doubting you :D :D

The only thing that I can come up with is the engine control unit triggers the coil with a 5v signal and their might be some form of monitoring on the circuit


It is very likely that it is triggered by a 5v signal which is monitored. None of that explains what you encountered.

I'm assuming you have fixed it and its gone ? Was the fix as simple as making good the "F15" Ground point on the manifold ?
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Re: Scope hiding voltage drop??

Postby dieseljohnny » Mon Feb 13, 2017 11:33 pm

First I literally undone the 10mm bolt a bit, gave the eyelet a few turns and tightened it back up and the car started without the test instruments inserted and it started .

I took it off the eyelet to inspect it and gave it a bit off a clean and the same on where it was contacting on the engine.

Heard back today the car has worked perfectly over the weekend.
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