Control wire voltage = 0 unplugged?

If you determine that you have a ground side switched circuit then unplug the connector to find the control wire, why does the control wire measure 0 volts? I mean there is no ground on the wire so you're going from ground, through your DVOM and reaching an open. Why would that open read 0 volts?
Thanks

Hey rolomoto! Are you thinking of this page from Engine Performance Diagnostics?



The control wire ends up with 0V after unplugging because there's no longer any electrical potential in the circuit. There's no ground from the PCM (or whatever controls the output), but there's also no voltage coming through the load. The exception would be if there's a bias voltage present, as is sometimes the case.

rolomoto wrote: If you determine that you have a ground side switched circuit then unplug the connector to find the control wire, why does the control wire measure 0 volts? I mean there is no ground on the wire so you're going from ground, through your DVOM and reaching an open. Why would that open read 0 volts?
Thanks

A ground side switched circuit, with the connector unplugged from the load source will always read 0v regardless if its ON or OFF when you connect your DVOM to the control wire (ground switched) and the other end of your DVOM to battery ground. What you want to do with your meter is POS lead to BATTERY + and the NEG lead to the control wire. If it reads 0v, the circuit is inactive (OFF), when it is active (ON), you will read battery voltage. Don't confuse this thinking you have battery voltage on that control wire. You are just using the ground control wire to complete the circuit through your DVOM.

The other way to test a ground side switch circuit is usually with the KEY ON ENGINE OFF, if the circuit is NOT commanded ON, then with the solenoid plugged in, DVOM NEG lead to BATT NEG (GROUND) and the POS lead to either side of the coil, you will see battery voltage (12v) on BOTH side of the solenoid. This is because BATTERY + will travel from the BATT + feed wire, through the coil of the solenoid and out the other side when the ground side control circuit is OFF (OPEN). When the circuit is commanded ON (CLOSED) you will see 0v or near 0v on your DVOM on ONE of the wires and 12v (battery voltage) on the other wire.

A test light will give you similar results. The light bulb in your test light replacing the DVOM and using either side of the test light as your NEG or POS test lead positions.

This is all assuming the circuit is a good circuit.

Don't forget about the bias voltage variable (circuit unplugged)

rolomoto wrote: If you determine that you have a ground side switched circuit then unplug the connector to find the control wire, why does the control wire measure 0 volts? I mean there is no ground on the wire so you're going from ground, through your DVOM and reaching an open. Why would that open read 0 volts?
Thanks

Technically it would be the same as you connecting your positive voltmeter lead to nothing, the meter still reads 0 volts and actually shows you a small fluctuation of voltage (electrical noise).
Remember that a voltmeter reads difference in potential between your test leads. So what is the difference in potential when your black lead is connected to ground (a 0v source) and then your red lead is connected to nothing or to an open circuit which is the same thing as nothing.
In fact, an open circuit may at times act as a microphone and will increase the amount of electrical noise you see.
Hope that makes sense.

Thanks for the replies. My confusion arose from watching Dan Sullivan's videos and him saying that an open would read ghost voltage, while a short would read 0V.

He is not wrong. I just called it by a different name "electrical noise"