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Catching car's ignition signal

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cmcraeslo:
Hey guys.

I have 12V and 2 signal wires coming out of the ECU into the ignition coil pack. To my understanding, they are active low signals, which means they get activated by the ECU with low signal (ground). I'd like to catch that signal (individually) and I have problem reading them.

If I just connect the scope to a signal wire, it looks like this:

Spark is active for 3-ish milliseconds and then it bounces back to 12V and repeats the ignition again.

If I cut the wire and connect the wire coming out of the ECU to 12V using a pullup, I get 12V like I should (I get 12V without the pull up as well) and it does not change when I crank. I tried different resistor values, even pull down - no change. Just as if ECU is not triggering it. What could be the issue?

Benta:
ECUs are very often smart enough to detect a wrong load, eg, your resistor.

cmcraeslo:
Good point - but i guess ECU would try at least once?

Benta:

--- Quote from: cmcraeslo on August 03, 2020, 01:09:48 pm ---Good point - but i guess ECU would try at least once?

--- End quote ---

Not necessarily. It'd normally do a system self-check when starting. If it finds something wrong, odds are that the ignition won't be released.

David Hess:
If it is driving the ignition coil directly, it could be looking for the series resistance of the primary; use that for the value of your pull-up resistor.

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