Need help with a mysterious electric issue that doesn't make sense

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This one is really strange. I think there are some extraterrestrial powers in my engine wiring.

Last night I unplugged the 3-prong connector between the engine and firewall. This is the one that connects the water temp, oil pressure and ignition wires (#16). As soon as I disconnected it, the EFI unit turned on :huh: . I noticed it because the fuel pump primed and confirmed that it is on by looking at the LCD unit. I connected it again and the EFI unit turned off. Disconnected it again, and the EFI unit turned on. And BTW, my ignition key was not turned and all the ignition powered components were off. As a background, in my setup, the ignition wire that goes to the engine (#16) is only connected to the EFI. This is used as a signal to turn the EFI on since it has a direct connection to the battery. My ignition wire does not connect to the coil since I have a CD ignition unit that gets the ignition signal from the alternator wire. This setup has worked for more than two years without issues. It is still working but something doesn't make sense. I measured voltage on the engine side of the ignition wire and indeed I get about 11 volts with the 3-prong connector disconnected :chin: . I went as far as disconnecting the EFI and still get 11 volts on #16. Where is this power coming from? I even disconnected the positive of all my electronics, including the stereo, subwoofer, alarm, EFI and CD ignition, and it still showed 11 volts. The only time the voltage dropped is when I disconnect the big fat yellow wire from the solenoid side. And to cap it off, if I disconnect the ground cable from the battery I still get about 0.5 volts. Just in case, I checked the voltmeter and it goes to zero if the prongs are removed.

:huh: :chin: :huh: :chin:

 
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I would check the ignition switch, to make sure that it is adjusted correctly. It seems to me like one of the contacts in the ignition switch is making contact even though the key is off. I would connect your voltmeter to the #16 wire (with everything connected except the 3-prong connector) and see if the voltage goes away when you turn the ignition key back and forth.

 
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I would check the ignition switch, to make sure that it is adjusted correctly. It seems to me like one of the contacts in the ignition switch is making contact even though the key is off. I would connect your voltmeter to the #16 wire (with everything connected except the 3-prong connector) and see if the voltage goes away when you turn the ignition key back and forth.
Don, I will try that, but what is really strange to me is that I am getting voltage on the engine side of the wire with the 3-prong connector disconnected. Shouldn't this in "theory" disconnect #16 from the ignition switch? or, could there be some other connection between the ignition and #16?

 

midlife

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Coil (+) is back fed from the starter solenoid "I" post. Try disconnecting that boot. If it's already off, then ignition switch is  suspect as Don suggested.
That's what I surmised as well: the starter solenoid is bad.  I hope our hunch is right!

 

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I agree, most likely the solenoid. I should have thought of that :( . I'll blame it on an old fart's brain fart.
How dare you blame me for this!!!

 
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So I have good news and bad news. The good news is that I figured it out. The bad news is that ET is no living in my Mustang!

I truly spent 3 hours the night before dealing with this issue and trying to think of all the possibilities. If the plug is disconnected there shouldn't be any feed to the ignition wire. Remember that the wire to the solenoid is on the cabin side of the plug. While I was going over the different suggestions posted here I kept thinking that the only way I get voltage in the ignition wire when the plug is disconnected is that I have something else connected into this wire that is causing an issue.  Here is where it gets fun. I have a relay between the ignition wire and the EFI. The signal side of the relay is connected to the ignition (+) and ground (-). However, the ground wire is shared with another gadget that was leaking power. The ground connection is not active when the ignition is off. So the power leaked from this gadget was enough to send voltage back through the relay coil and through the ignition wire. The coil/relay must have acted as a capacitor of sorts because even when the battery was disconnected, I still got a trickle of voltage. When the plug was connected, the power leak must have been used by whatever is powered on the cabin side of the ignition wire so it was not enough to power the EFI, but when disconnected it was eventually large enough to signal the EFI to turn on. :chin:

Thank you to the OF Power of this site that made me think harder about my gadgetry setups!

 
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It's amazing what voltages can do to other components when they are seeking a ground. You should see what they can do to a two-way radio system (including the building it is enclosed in) that is missing a ground, not only to that radio, but others nearby that aren't even connected.

 
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It's amazing what voltages can do to other components when they are seeking a ground. You should see what they can do to a two-way radio system (including the building it is enclosed in) that is missing a ground, not only to that radio, but others nearby that aren't even connected.
They are like ants finding the path of least resistant!

 
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Gotta love ground loops.
Interesting. I didn't know this term. So does this mean that to reduce radio interference we should ground all components to the same point in the "chassis"? Or as long as we are grounding to the chassis it is okay no matter the specific location in the chassis?

 
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They don't have to be grounded to the same point, just grounded. All systems using the same voltage source need the same reliable ground reference. An exception would be for high end audio systems and other high power uses that need "quiet" power sources, which will likely function better when grounded directly to the battery with separate copper wiring, rather than relying on sheet metal connections.

 
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