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Guest Message by DevFuse

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Alternator wiring?

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4 replies to this topic

#1 Flow


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Posted 24 April 2004 - 12:55 AM

Planning a GM Alt swap. Got a freebie off a 91 GMC 3/4 ton truck plus the wiring harness plug. Puts out 105 amps. Got the bracket built now I'm just trying to figure out how to wire it with out starting a fire. The GM Alt has the + battery post on back and one wire that plugs into it. In the subaru wiring diagram below, I read "A" on alt as the +battery post. "S" is the "exciter" wire. Which leaves "L" pointing to a big A. Does A = ammeter? Is this the wire for the dash guage? The GM Alt is self-exciting, so I'm told. So besides hooking up the + battery wire, the one wire should go to the guage. If anybody sees anything wrong with my logic, please speakup and save me from myself. I'm tired of smelling burnt wire in the wagon. Thanks.

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#2 canajun2eh


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Posted 24 April 2004 - 05:56 AM

In my XT6 manual, the wiring for the EA82 model is essentially the same as in your diagram.

It shows a heavy white wire going to the battery through a black fusible link, dedicated to this connection.

There's a black/white wire going to a red fusible link and then to the battery. This fusible link is also the main feed into the car's electrical system, providing a "hot" connection to the battery for various electrical systems that aren't switched by the ignition switch.

Lastly, there's a white/red wire that goes to the voltmeter and idiot lights.

The ER27 model (with the 90 Amp alternator) has an extra very heavy white wire in addition to what's described above. This one goes to a blue fusible link and from there connects to the power steering system.


If your GM alternator is "self exciting", the one wire that plugs into the unit is probably for the idiot light.

I would be a little concerned that the standard Subaru wiring isn't heavy enough to handle the full output of your 105 Amp GM alternator. You might want to replace the white wire with a heavier direct connection to the battery, protected by a 100 Amp fuse or two 50 Amp fuses in parallel. If you do this, pull the black fusible link to disconnect the white wire from the battery.

When you try to work up an unusual fuse rating, you connect fuses in parallel, and add the fuse ratings.

Fusible links are very slow-blow fuses, made of wire. They're basically a deliberate weak point in the car's electrical system, designed to melt at a pre-determined current depending on the diameter of the wire used. Fuses do the same thing, except that they are made of metal that will melt a lot faster.

#3 78TurboBrat


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Posted 24 April 2004 - 11:30 AM

Posted Image
Hope this can help you, the red wire the usually thicker one on the double plug goes to the back of the alternator, and the other wire, if you have it inthe plug, goes to your idiot light, or your gauge. this essentially makes you a one wire setup, taking your output of the alternator to the battery is the best way. and then just connecting the old output wire on the subie to the battery as well, as that it can draw its power for the rest of the car. i would recomend puting in a fusible link at the batter connection for the alternator output, the fusible link on the subie harness will take care of the old out put wire now going to the battery. hope this helps,
I know its not a subie motor in the pic, but it will give you an idea. I use these alternators all the time as they are the easiest ones to deal with on getting an alternator to work with motor swaps. like ford v6's in nissan pick up trucks. alot cleaner looking for the wiring also.

#4 oddcomp


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Posted 24 April 2004 - 03:22 PM

hmm for what its worth on my swap i did
i went wire for wire
small chevy wire to small suby wire
big chevy wire to big suby wire
suby bat wire to chevy bat post <plan on changin that to direct to battery soon>
no smoke works fine and has at east 500 miles on it now since i did it

#5 78TurboBrat


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Posted 24 April 2004 - 04:11 PM

Yeah you can do that too, just so many ways of doing it, and all work the same.

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