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Guest Message by DevFuse

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ECU Power

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5 replies to this topic

#1 BoxersOnly


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Posted 27 April 2004 - 07:15 AM

I am working on the wiring off an 1998 EJ25.

The ECU pinout gives me the following info:

Terminal 39 : Backup Power Supply : Ignition Switch
'Off": 10 -- 13
Terminal 1 and 2 : Control Unit Power Supply
Terminal 21 : Sensor Power Supply

Does this mean I need to run an always hot wire to Termnial 39 ?
Can I just grab a hot wire from the fuse box and use that ? Or is it better to
run it
straight from the battery ?

As to terminals 1,2 and 21, should I just run a switched hot wire to those ?


Subaru EJ25 Powered 1970 VW Beetle
2004 Subaru WRX Sportwagon

#2 Legacy777



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Posted 27 April 2004 - 10:20 AM

it sounds like you have the right idea. In my mind....back up power is that....power to keep the ECU memory when the ignition is off.......but you may want to try and find someone that can prob those ports on their car and see if that's how it is.

#3 oddcomp


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Posted 27 April 2004 - 12:02 PM

not having the ecu in ront of me
but from what i do know of how stuff works /is wired
backup is a constant voltage <fused direct to battery type thing>
and the ecu power i would guess to be turned on when the ky is in the run/start positions

as far as sensor power.. hmm that could depend on
if its supposed to be a regulated steady voltage or if it takes the voltage and tehn regulates it to transfer out to sensors and then based on the return voltage take its readings...

but i could be way wrong

#4 Nug


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Posted 27 April 2004 - 03:29 PM

1,2, and 21 should have switched 12 volts. From the ignition switch. You might not want them directly off the switch, but through relays. You should include fuses.

IIRC, "backup power supply" was simply another switched 12v wire from the ignition switch. There is one wire that goes to the battery for ecu power 24/7, but I don't think it was the "backup power supply" wire.

Basically, there are 2 or three wires that power the ecu, and 1 small diameter yellow wire that connected directly to the battery.

I'm also using a '91 legacy engine. I don't know how much they changed.

#5 BoxersOnly


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Posted 02 May 2004 - 10:26 AM

Thanks for the info.
How did you wire up the alternator ?
There's a plug with 2 wires and another white wire on the Sube one.

How did you link this into the VW wiring ?



#6 Nug


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Posted 02 May 2004 - 08:20 PM

Um, the big wire goes straight to the battery. The other two, well, I just twisted them together and hooked them up to ignition+. One of them is the tickler, the other goes to the warning light. I'm not using a warning light, i'm using a voltmeter. It seems to work fine. And it doesn't drain the battery.

After studying the schematic for a while, I figured that the tickler and the warning light wire both need vlotage at them. So I'm assuming I did it correctly.

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