GM Alternator In A Ford
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While working on my 1964 Ranchero, I decided I needed a better alternator and wondered if I could put a GM internally regulated alternator on the car to help improve the charging system performance. They're good solid units, they are readily available at any parts store in up to 94A outputs, and I'm very familiar with them. I surfed around a bit and found out that swapping a GM 10SI or 12SI series alternator in place of the Ford unit is a pretty straightforward swap, so I decided to do it. In preparation for the swap, I put all the info I learned into this tech page - the theory being that if I can't explain it to someone else, I don't really understand it yet.

I already have a page for upgrading a GM charging system with lots of good details and links to other tech resources - both on my site and on other sites. If this swap interests you, I would suggest you read my other pages as well as this page.


Mounting Considerations

Here's the two alternators side by side for comparison. Apologies for the focus in a couple of the pictures, but you get the idea.

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 Front this quick look, we can glean a good number of good things.

  • They are roughly the same size overall and will fit into roughly the same mounting space.
  • The pulley location relative to the front mounting face is the same.
  • The pulley sizes are roughly the same and use the same type of belt.

We can also learn what we need to worry about for this swap.

  • The main mounting/pivot point on the Ford alternator is deeper front to back than the GM unit.
  • The mounting hole on the Ford alternator is a slightly larger size.
  • The Ford alternator has slightly larger distance between the pivot point and the adjustment mount.
  • The "clocking" of the GM alternator could be important.

The difference in depth on the main mounting/pivot point means the original Ford spacer (shown below the alternators in the last photo) will not work, so a new and longer one has to be made. Start with a 1" aluminum bar from your local metal supply store - heat treating does not matter for this, so any decent aluminum bar will do. Avoid steel because it will be much harder to work with. Look in your local yellow pages if you don't know where to find buy metal pieces - they're around pretty much any place where any amount of industrial work gets done. You can also go online and order it there, but then you have to pay for shipping. Anyway, the process is basically cutting a section off the bar for the length you need and then drilling out the center on a drill press to the ~7/16" size of the Ford mounting bolt. Make sure the ends are cut square and that you drill the hole in the center of the bar. There is a small amount of play acceptable here - the important part is that the spacer solidly support the alternator without allowing it to twist or slide "in" towards the engine. Most importantly, you need to make sure the spacer you cut is right for your engine. Depending on the details of your original Ford alternator mounting, the length of the spacer you need could be different than what I needed. To get the new spacer length, just measure the total depth of the mounting section + the spacer for your Ford alternator, and subtract the depth of the mounting section on your new GM alternator. On my car, the original Ford pieces had a total  of 4 5/8" deep with 1 5/8" being the spacer size and the remaining 3" being the size of the mounting section on the Ford alternator. The mounting section on the GM alternator is only 2" deep, meaning the new spacer needs to be 2 5/8" long so the total size is still 4 5/8". This is clearly shown in the second photo.

For the mounting bolt, you simply need to drill out the GM alternator to accommodate the ~7/16" bolt the Ford uses. Be sure to cover any openings in the alternator when drilling to keep metal shavings out, keep the drill steady and in line with the hole as you drill, and be sure to use the right size bit so the hole is the right size. It's a pretty simple operation to do with the alternator clamped down to your favorite work surface. I'd suggest using some plastic bags and masking tape to cover everything on the alternator, except the mounting section you need to drill out, before you start drilling.

The small difference in the distance between the pivot point and the adjustment mount on each alternator should be easily handled with the movement already available on the factory adjustment arm. I dug around on the internet for the size data, and the typical Ford alternator will be 6.875" between the mounting points while the GM 10SI and 12SI series alternators will be are 6.600" - a mere .275" smaller. Converting to fractional sizes that most of us are more familiar with, this works out to a bit over 1/4" - easily accommodated by shifting the adjustment arm slightly on it's mount.

The "clocking" of a GM alternator is the way the front and back case halves are put together. The front case half has the two mounting points, and the back case half has all of the electrical connections. The two are held together with four equally spaced bolts running from back to front and the two case halves can be assembled in four different positions to adjust where the wiring is at relative to the mounting points. MAD enterprises has a great page on this with pictures, so go see them for more details if I'm not making sense to you. For reference, the alternator I show in the pictures happens to be a "12 o'clock" unit. This is important because you want the wiring to come off the alternator at a reasonable location that does not come too close to moving parts or, especially in the case of the main power output, things that they could ground out to and cause serious problems later on. Because the alternator in my case mounts "upside down" on the lower passenger's side of the engine, the 12 o'clock alternator I have places the main output terminal away from the engine on the lower part of the unit and the smaller connection is exactly on the bottom of the alternator. A 9 o'clock alternator may be preferable here - the output terminal would be at the top away from the engine, and the smaller connection would be on the side where the wiring needs to exit at anyway. A 6 o'clock unit might work depending on how close the main output terminal came to the block and other bracketry - some test fitting would be in order. A 3 o'clock unit would be a particularly bad choice here because that would place the main output terminal very close to the engine block, with the 3 o'clock being the worst because it would place the main output terminal very close to the block and the smaller wiring connection would be tucked up against the brackets in a very hard to reach location for testing/troubleshooting if later problems arise. If you bought one with the wrong clocking, the good news is that you can almost always re-clock the alternator quite easily by removing all four bolts, rotating the two case halves relative to each other without spreading them apart, and then bolting the two case halves back together again. The key is to not let the case halves separate - if you do, the brushes could fall out of their holders, snap off, or other "not good" stuff. The bad news is that re-clocking the alternator will likely void any warranty on the unit you bought, so it's better to buy the right unit from the store - if they don't stock it, they can usually get you the right one in a day or two.


Wiring Considerations

The wiring needs to change to accommodate the new alternator. For starters, the external regulator is gone - you can remove it and pitch it in the trash, or see if some really desperate soul will buy it off you cheap to save a few bucks compared to a new one. It also matters if you decided to install a "one wire" or "three wire" alternator - MAD enterprises has a great page detailing the differences. For a street-driven vehicle, I recommend the three-wire units over the one-wire units virtually every time.

A one-wire unit is pretty simple - there is only one wire to hook up to get things working. Connect the alternator output terminal to the battery side of the starter solenoid with proper gauge wire for your alternator's rated output and your basically done. I have details on that below. You do need to make sure the battery output and the main vehicle feed at the original regulator location are tied together - I'd suggest an insulated junction post for this. Other wires may be "dead" in the harness and can be simply taped off or removed depending on how adventurous you are.

A three-wire unit is only marginally more complicated. You run the main power wire to the starter solenoid and hook up the battery output and vehicle feed wires to a junction block just like you would with the one-wire unit, but then you need to connect two more wires to complete the job. The easiest one is a the field connection - it can simply be connected to the field connection wire at the original regulator connector and you're done - you can even re-use one of the original regulator to alternator wires if you want. The final wire you need to connect is what is know as a "remote voltage sensing" wire. The place where you connect this wire to the main power wire in the harness is the place where the alternator will maintain the proper ~14V in the system. The junction block where you connected the battery and vehicle feed wires where the regulator used to be is a good choice for mostly stock type systems. In the original Ford harness on my 1964 Ranchero, this is at the front of the engine compartment on the driver's side. This arrangement will work at least as well as the original wiring setup did, the horn relay is right there already, and it's nice later on if you decide to add relays later on for various things because you have a ready-made connection point under the hood for their high-power wiring feeds. In cases where you have moved the battery and solenoid out from under the hood, you should run the remote voltage sensing wire out to the main junction location in the harness - typically back near the battery. The MAD Enterprises page on one wire vs. three wire alternators has lots of good details on this.

For the size of the main output wire, you need to put in a wire capable of safely handling the maximum possible output of your new alternator. The rule of thumb on wire sizes is that bigger is almost always better here. I would use 10 gauge for a 60-70A unit, 8 gauge for a 80A-100A unit, and even larger cable for higher amperage output units - 100A is a lot of power to be flowing through a piece of wire! Also, you need to keep in mind that the power doesn't just go to the battery, it also goes back into the vehicle and powers the rest of the electrical system. The original Ford wiring in most '60s, '70s, and even 80's vintage cars/trucks was a bit undersized to start with, and adding more power-hungry accessories onto the undersized wiring and already overloaded fuse panel along with a huge alternator out on the engine is a great recipe for a nasty electrical fire. You really need to run a new main feed wire - again, appropriately sized - inside the car and use that for your main power distribution. Yes, this can be a significant amount of work - but would you rather see your pride and joy burn to the ground, perhaps with you or someone you care about in it? Do the job right and stay safe - don't install a huge alternator unless you're willing to make all of the upgrades to the wiring to match the alternator. Besides potentially saving your own skin, we all want these great cars to be around for future generations to enjoy and they're certainly not building any more of them.

You'll need a simple wiring connector for the hookup of the field and remote voltage sensing wires. Most parts stores will carry these plugs with small wiring pigtails attached to them. If you're really cheap and want to go the "hack" route, you can just run standard female spade connectors on individual wires - they terminals on the alternator are standard 1/4" spade terminals. The right plug is only a few bucks more and you'll thank yourself the first time you have to service the alternator and need to plug the wires back in. If you want to be really sanitary about it, you can lookup my article on reusing plastic wiring connectors and remove the small wiring pigtails form the plastic connector you buy and make the connections using one long pieces of wire up into your harness, or perhaps even re-using wires from the original wiring. I'm re-doing a nice chunk of the wiring on my Ranchero, so I'm going with new wiring runs - at least back to the area where the original voltage regulator was.


Parts List

He's the crib sheet you can print out and take with you to the parts store. Note that most rebuild alternators will not come with any hardware, so be sure to open the box and check on yours. You will almost always need the mounting bolt, and usually the nut for the wiring connection as well.

  •  GM internally regulated alternator of your choice (metric hardware is needed for the later 12SI series alternators)
  •  1" aluminum round stock, minimum 2 5/8" long (for making new spacer)
  •  6mm nut, lock washer, and washer (for main alternator output connection - may be on the alternator already)
  •  8mm x 1.25" bolt, lock washer, and washer (for alternator mounting bolt)
  •  GM internally regulated alternator wiring plug (for hookup of field and remote voltage sensing wires)
  •  Rubber terminal cover for main alternator output (re-use existing, or find one from a junkyard - they're hard to find new)


New Spacer

Here's the raw bar stock I started with, the spacer cut to length and drilled out, the new spacer compared to the old alternator to be sure it's square, and finally the GM alternator mounted on the Ford engine with the new spacer. I neglected to get a picture of the alternator wrapped up in plastic while I was drilling out the mounting hole. You'll note that the hole in the new spacer is slightly off center and not completely round. The off center is a purely visual thing, and the out-of-round is because I initially drilled the hole a bit out of square and had to re-drill it to make it square to the ends. The spacer was not clamped square to the vise in my drill press initially, and then it moved on me in the middle of the drilling operation. It works and no one looking at the final install would ever know about it, so I'm happy.

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New Wiring

I'll try to post some wiring pictures and diagrams here at some point. Right now I am in the middle of other wiring work, and am running a very temporary wiring arrangement - just enough to be able to run the alternator, starter, and the ignition system, plus have an accessory lead for testing out various other wiring work I am doing like converting to delay wipers and installing gauges from a 1966 Mustang.

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

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Page last updated 01/02/2009 01:51:39 PM