Scope hiding voltage drop??

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Re: Scope hiding voltage drop??

Postby steevegt » Tue Feb 14, 2017 1:13 am

I don't believe that the measurement instrument, scope or DMM, could be the ground path for the coils... No way...

The Picoscope has a 1 M ohm input impedance.
Usually a good multimeter, like a Fluke one, is even more, like 10 M ohm of input impedance.
In 12 volt systems with the Picoscope we are talking about 12 micro Amps or 0.012 mili Amps of current right!?

That said, that very low current in low current circuits can affect the signal voltage. Usually it is only enough to affects bias voltage type of signals.
I had experience in the past this effect, in an O2 bias voltage signal with a complete dead O2. The bias voltage was 450 mV, but with the scope connected to the wire, that voltage dropped some mV and fouled the ECM in thinking that the O2 was starting to respond. With scan data that was easily spotted...

In this case, the simplest explanation that I can find, is that the damaged ground path was sometimes making good connection.
Maybe it was just a coincidence? How many time have you tried that to confirm that the scope connected was the cause?
When you were inserting and removing the probe, you were also, consequently wiggling the ground point and making the connection or interrupting it?

With the car running, the high current going through the bad connection can sometimes make it stay together. Like a solder point.
That may explain why the engine will stay running once it started. That I believe was the thing that happened.
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Re: Scope hiding voltage drop??

Postby STC » Tue Feb 14, 2017 4:11 am

Steeve
I don't believe that the measurement instrument, scope or DMM, could be the ground path for the coils... No way...


Perhaps a DMM set to AMPS would support that current flow, however, when removed the coils could not fire and the engine would have to die.

In 12 volt systems with the Picoscope we are talking about 12 micro Amps or 0.012 mili Amps of current right!?

Yes, Right !!! Not enough to light up a Small LED ??

That said, that very low current in low current circuits can affect the signal voltage. Usually it is only enough to affects bias voltage type of signals

And this is an area where Volt Drop is not the technique, a simple continuity, or load test will suffice.
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Re: Scope hiding voltage drop??

Postby dieseljohnny » Tue Feb 14, 2017 10:33 am

I tried it four times, twice with the scope and twice with the meter

Test point was pin two of cylinder one coil. Earth point was on opposite side of cylinder head close to cylinder four intake so definetly didn't disturb that.

Again continuity check on the wire showed no more resistance than the meter leads.

I'm just putting it down to strange things happen sometimes
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