Hitachi to GM 100A Alternator Swap by Raven (raventbk@yahoo)

Number of Hits to this page: since 02/05/05.

Last updated: 01/03/2007 21:20:23 -0500
This page has been revised: 11 times since its beginning.. why?  Cause I think one thing and type another... or I notice a grammar error.. or I just felt like fixing something.

I've been meaning to do this write up for some time now.  For the last week or so, I've been working on one.  However, after re-reading my thread again, I realized that just about everything that I was typing into my write up was already contained in the thread.  So, being the lazy one that I am, I've decided to just copy and paste the relevant parts of my thread and put them in proper order to create a useful write up.  Has all the useful information, but in one place to make life easy.  Makes my life easy by not having to retype each and every character. :D  As life goes on with this swap in place, I'll continue to edit this to reflect the most current status/knowledge when it comes to this swap.  My commentary is in italics.. like this paragraph is.  Enjoy.


Well, I've been doing an absolute ton of research on this. I've been searching for an alternator of at least 100A that would bolt up without having to do any fab work to make custom brackets, pulleys or whatnot, as is necessary to do the famed XT6 alternator swap. I was also looking for a one wire alternator.. but my research has shown me that going to a true one wire setup isn't really the best idea.. two wire alts are the way to go. Besides.. it isn't that hard to wire one up the right way. Why do it halfassed?

Well, I haven't found the holy grail, but I have found one that is damned close. Its a large case 100A off of an early 80s GM product.  I've forgotten exactly which GM product it belongs to, but I do have the part number that you can cross reference.

Only work that needs to be done is file off 1/16" of metal off one of the ears, flip the adjustment bracket left to right, and attach the larger belt. With not having to cut, weld, for fab up custom bracketry, this should be a great swap for just about everyone. And.. with only having to shave that tiny bit of aluminum off the ear, its something that can easily be done on the side of the road, or in the parts store parking lot if you ever need to replace a bad alternator. Oh yeah.. I've found that most parts stores carry the stator plug in the electric connectors section. No need to wait for a special order T plug or having to yank one off a parts car.

I'm well aware that a similar GM alternator swap has been done numerous times in the past with resounding success. Same with the XT6 alt swap.

The issues I came up with with regards to both of those is there is some decent fabrication required for both swaps. With all of the GM swaps I've read up on, there was a need to fab up a custom bracket or two to get it to mount correctly. Since I don't have the time, materials, or the tools to do it right, this route was out of the question.

With the XT6 alt swap, there is not as much fab work, but more grunt work with swapping pulleys and such. With me living in the southeast, finding an XT in the JYs, let alone an XT6 is a challenge. Yall would probably die if you saw how much they want for a reman one at the local parts place with them not being "popular" here. They also don't stand up to the elements as well as the EA series alts....which is well documented. The XT6 alternator doesn't like mud.  With the BushBrat and me living in the Apalachicola National Forest with all those trails to play on... well.. mud, sand, water, and all those goodies are on the menu on a semi-regular basis. After such, the parts store comes back into play - having to special order these items. So I should pay out the rear, only to have to wait two weeks for it to arrive from the warehouse each time I blow it up? Nope. Although I'm a diehard Ford guy (and prefer a 200A Ford G3 alt), the GM swap is cheap, and easily had down in these parts.

The goal of this was to find a high amp GM swap that would drop in with even less effort than the XT6 swap. No fabbing of spacers, custom brackets, cutting any of the Subaru stock harnesses, or having to swap pulleys. With only a tiny big of filing, its literally plug and play.

Still interested?  Read on....

Many folks believe that the stock harness is unable to handle more than 70 amperes.  This is true, but here's my answer.  It makes sense, I swear!

With the issue of the stock harness not being able to handle more than 70A, well, that doesn't apply all too much. If you need the output, then logically you'd enlarge the "tube" the extra power must go through. Thats just common sense. However, the power curve of this alternator puts 70A at 4200 crank rpm (assuming a 2.5:1 ratio, stock GM). How often will you really be needing a full 70A of output? For those rare nights that you're on the trail, with your system cranked up full blast, highbeams and all the driving lights on, heater fan cranked cause its cold, while you're running the winch trying to get yourself unstuck? And even then, if that is your argument, for how long of a duration will you be needing the full 70A? 45secs at the most? Please. Odds are you will never need the full power except for those short bursts, or jumpstarting another vehicle.

This is a cheap, easily had swap that can deliver the power when its needed, rather than overtaxing the alternator, pulling reserves from the battery, and leaving you with a fried alternator, and dead battery.

If 70A at 4200rpm is not enough for you, places like JC Whitney and such sell upgrade kits that replace the windings in this common GM alternator that would give you more amps at a lower RPM... all on the cheap.

My reasons for the swap is when I kick my lights on high beam, I'm already pulling 490 watts, or 40.8 amps, which leaves a mere 15A of possible output from the stock EA series alternator ....but wait, that extra 15A only exists at full throttle. At cruise RPMs, the max output from the stock alternator *is* 40A, leaving 0. Thats before you figure in the draw from the radiator fans, blower motor, stereo, and the rest of the system.. (taillights, markers, turn signals, etc).

See, I told you it would make sense.  Even NorthWet agrees with me:
"RavenTBK, good points about max rating vs. reality. Nothing likes running at max rating for long, so it makes sense to have something that is "loafing" under everyday situations. Things will run at 60-70% all day long."

Right.. so heres the parts list and easy outline:

Here's my list:
Alternator part number: P7157 (re: P7157, / Advance Auto Parts, click part# to view)
Stock GM stator plug
Two male spade connectors
Belt 1.5" longer than stock (your mileage may vary.. I just happened to use 1.5")
Grinder/file to remove the bit of aluminum
About an hour's worth of time

Due to the large case design, those of you that have kept the stock aircleaner, or A/C might have fitment issues.

To wire it up was simple. On the EA T-plug, there is a large gauge wire, and a small gauge wire. What I did was connect spades to the wire ends of the GM plug, and connected the large to large, small to small. Easy huh?  I chose to run with spades so I will be able to switch back to the EA alternator easily if its necessary.  The stock harness connected to the B+ output. Eventually, I plan on rewiring the stock harness with some nice 4ga wire.. but that is not necessary at this time.

Easy foscheezy huh?  I know, I know.  You're thinking, "This cant be it, can it?"  Yes.. it is.  Simple, drop in swap.  Here's some pictures with descriptions so you can visualize what the heck I've been blabbing about.

Click any thumbnail to enlarge..

First, disconnect your battery.  No fires allowed!

Then, remove your Hitachi alternator and set it on the shop floor....

EA left, GM right.
Alternators side by side..
note the similar depth and
similar mounting points.

GM left, EA right.
Side by side again..
this is where you
might have fitment
issues with the GM
having such a large

EA left, GM right.
Here is a comparison
between the ears of the
two.  You can plainly
see the "small bit of
metal" I was talking
about having to file off.
No big deal, right?

Now that you see how closely related these two alternators are, I bet you're now having visions of how it just "drops right in" to your Subaru.  I know I was the first time I realized it myself.  I'd even bet that its so damn obvious that you don't even need to look at the rest of this write up do you?  I still think it is, but for those of you that would like to continue, lets do...

Since there really isn't that much work required to do the swap, I didn't take any pictures of me filing down the ears.  That should be self explanatory.  If it isn't.. look at picture #3 above once more.  Its extremely simple.. file the inside of the GM ears till they match and/or fits into the EA's mounting location.  Then play musical belts to see which size works best for you.  1.5" longer was my personal winner.

party time!



This is where the spades and GM stator plug mentioned above come into play.

GM stator plug.  Yes, I
know its dirty.  I didn't
snap a pic of this when I
did the original install.
I had to go take a fresh one
today.  It should be a
plastic plug that matches up
with the two pins on the GM
alternator.  $3 or so in
the electrical section of
your local parts house.  GM
had enough sense to make
the stator plug semi-universal
so finding one should be
a snap.

Crimp on the male spade
connectors to the pigtail ends
of the stator plug.  This is
optional, but I wanted to have
the ability to go back to stock
if I had the urge to.  Going with
spades gives me that ability. :)
Also, please note the difference
in wire sizes.  Red is a bit bigger
than the white pigtail. 
Remember this for later.

Grab ahold of your Subaru
T-plug from your stock

Yes, I know its blurry.  Sue
me.  Anywho, notice that there
is a large wire and a small wire
that make up the link.

Connect your spades to the T-
plug.  Large (red) to large (top
of the "T"), and small (white) to
small (bottom of "T").  For those
of you that are curious, the red
wire is the field signal wire,
which gets battery voltage when
the key is turned "on", and the
white is for the "alternator is dead"
idiot light on your dash.

<no pic>
Finally, connect your "hot wire" from
your stock harness to the output stud
on the alternator!  You're done!

Yaay... you're now finished.  Time to test!  Reconnect your battery, and grab a multimeter.

This is my battery voltage at
rest. Yours should be near
this reading.

Kick on a small load (blower
fan, headlights, whatnot) and
crank her up!  You should quickly
get a reading similar to this.  The
voltage should stay near 14v the
entire time.. thats a good alternator!
For fun, kick on every accessory
you've got.. you should still remain
above 12.9v at idle speed...unless
you've got 300 foglights or something.

Good job!  Go have a beer!

Take peace in the fact that if your GM alternator ever gives up the ghost and/or you have a desire to go back to your EA for whatever reason,, you wont have to scramble your brains trying to remove the fabbed up brackets and stuff to go back to your EA style alternator.  Just disconnect the hot wire/spades, pull out the GM, and drop your EA right back in like it used to be.


Hope you've enjoyed my cluttered writeup.  It should make enough sense to you now to be able to accomplish this task on your own.  Please, use common sense when doing this swap.  Thats the only know-how you need to succeed.  If you have any suggestions to make this easier or better, please feel free to drop me an IM on USMB.


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