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ECU still gets power after key OFF. Main Relay sticking?
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Old 09-08-2019, 03:11 PM   #1
SoCoNoHa
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Default ECU still gets power after key OFF. Main Relay sticking?

Ok so this is a little weird...
(97 LGT)
Starting from a freshly disconnected and reconnected battery, everything is normal. Zero voltage at the ecu, except for the backup power supply, which is normal from what I gather.
Once the key is turned to on/ign, everything has power, still normal.
Car will crank but will not start.
If the key is returned to off, everything visible shuts off; gauges/accessories. However, the ECU still receives 12v, and therefore so do a bunch of sensors and the fuel pump and whatnot. The only way to kill the power at this point is to disconnect the battery.
I assume the main relay is the culprit, as it supplies power to the ECU, but... The main relay is triggered by the ECU. The 'self shutoff control' is what energizes the main relay.
"Must be the ignition switch sticking." Well...
Without power, the switch is fine, but with power it's the same as the ECU: no power until it's turned on, and then the power just stays on no matter the keys position.
Tried a spare ECU, no change.

Sorry for the jumble, anyone got any ideas? I've been tracing the diagrams and checking voltages and nothing's sticking out except for EVERYTHING. Only thing to do is replace the main fuse, but I don't have time to take my dash out tonight.
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Old 09-09-2019, 03:12 AM   #2
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Do you have a security system or remote start? That'd be my first guess right there. After that, check voltage at the trigger for the main relay and probably work back from there. Something that gets powered by the main relay is managing to hold it open after the car is supposed to be off, but I can't guess what without diving into the wiring diagram.
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Old 09-09-2019, 02:29 PM   #3
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So the main relay is triggered by the Light green wire on pin 63 of the ECM. That's the 'Self Shutoff Control'. Like everything else on the ECM, once the key is turned on, the voltage remains at it's correct Key On Engine Off voltage.

Oh ya, no security system.
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Old 09-09-2019, 07:47 PM   #4
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Do be careful when you do think you're on to something with the wiring. I fried my head light switch because I took the FSM for fact when patching something. Of course a service manual correction had been issued, but that doesn't update what is already out there, or even the service manuals on STIS.
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Old 09-09-2019, 08:31 PM   #5
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Yup. I've done a lot of staring at a very similar wiring diagram, except for an Impreza instead of Legacy. What I'm thinking is that:
1) There's power getting onto the wire between the ECU and the trigger for the main relay -or-
2) The ECU has a short somewhere or some other internal gremlins (or for some reason is thinking it should still be powering the main relay because it's receiving odd inputs (?)) -or-
3) Something else is happening that I don't have a theory for yet. Rule out (1) and (2) since they're not too hard, then start looking for exotic explanations like noisy grounds, UFOs and vengeful spirits.

You should be able to rule out (2) by just yanking the plug from the ECU and seeing if everything turns off. If it's (1) then you'd expect power to stay on. If it is (2) then maybe swap a junkyard or ebay ECU while you have the carpet pulled up. If it still happens with a different ECU (or you don't have one at hand to swap), then start looking to see if the key-on signal to the ECU is somehow persisting after the key is physically off. Probing that while the ECU is plugged in doesn't seem super fun though. :/
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Old 09-09-2019, 10:45 PM   #6
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Already tried another ECU, same exact story. Once I'm back in town I'll have to pull the dash and make sure the trigger wire isn't picking up voltage somewhere else like you said, J. I'll also replace the main relay anyway. I know the fsm's aren't perfect, but I'm gonna hope the one I'm using is right, and leave tearing apart my harness to check every power and ground as a last resort.

Thanks for the help guys, I'll probly be back for more advice
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Old 09-11-2019, 09:43 AM   #7
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Before pulling the dash, first just lift up the carpet and try disconnecting the ECU while the problem is happening. That should answer whether your phantom power is on the trigger for the "main relay". Check the easy stuff first, even if it's not the most likely.
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Old 10-19-2019, 10:53 AM   #8
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Damn grounds... Moved The intake manifold harness ground from intake to pitch stop mount ground point. Problem solved, main relay turns off when the ignition is turned off.

CHECK YOUR GROUNDS PEOPLE!

Grounding kit just jumped to the top of the list, right behind brakes holy crap. Thanks for the help y'all!

Edit:
Quote:
Originally Posted by johnxx View Post
Before pulling the dash
Yea... I definitely already pulled it before trying the grounds. Shoulda listened.
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