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post #7 of (permalink) Old 07-02-2019, 05:01 PM
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Originally Posted by Nexus9 View Post
Oh, awesome - that is very helpful. I did not think the ECU provided that level of voltage, and I was laboring under the delusion that current flowed from the voltage rail through the chassis of the device to ground, and somehow the ECU enabled that.

So if I understand you, the solenoid is held "neutral" by an equal and opposing voltage that prevents current flow. The ECU controls the solenoid by lowering voltage and allowing current to flow through the solenoid, activating it, using the ECU as a ground. I'm surprised that that kind of current can flow through the ECU!

Okay, now I know what I'm looking for, I just need to set up the right device to capture the data. Can't tell you how much I appreciate your input, thank you again.
Just a little more info to help with your understanding...

The ECU is not really providing an opposing voltage to the solenoid, it is providing a path to ground via the switching device. This switching device will either be a relay or most likely a FET (Field Effect Transistor). Both of these devices have a low "On" resistance when energized; typically milli-ohms. So when the FET is turned on it basically shorts the solenoid to ground with very little voltage drop across itself due to the low "On" resistance. And when the device is "OFF" it has a very high resistance; Lets say infinite for this discussion.

So what is happening when you are monitoring the signal line from the ECU to the solenoid is you are seeing the voltage drop across the switching device for the open and shorted state. When in the "ON" state, the resistance of the switch device is very low compared to the solenoid coil resistance and the voltage drop measure is near zero. When in the "OFF" state the resistance of the switch device is near infinite compared to the solenoid resistance and therefore the voltage drop across the device is at the battery voltage.

This solenoid in series with the switching device form a basic voltage divider with two states and follow ohms law.

I hope this electrical theory discussion helps.

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