Alternator conversions for older GM's
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This page attempts to document some of the common details and facts you need to know about when converting the existing charging system on your older GM vehicle to something newer and (hopefully) better. This information comes out of my research and work on upgrading my 1958 Buick Special from a generator to an alternator and from my previous work on the High Amp Alternators for older GM's article written while working on my 1973 Buick Electra. It is not a step-by-step guide, but rather a roadmap of important things to consider and be aware of. You should read my article on Alternator and Generator Theory before delving into this as it has lots of good details and basic cautions that will help you understand the details in this article. It's a tad heavy on theory, but once you understand why the system does what it does, then the wiring makes perfect sense and it becomes very easy to figure out what you need to do on your vehicle.

The information here can be of use to someone who wanted to convert the electrical system on a non-GM vehicle, but how much of this is accurate is up to you to decide. Ford and Chrysler and even alternators on imported cars operate on the same basic electrical theory but their specific wiring differs and may have unique quirks to it. In particular, I believe both the Ford and Chrysler alternator systems were externally regulated until well into the '80s, and neither has the remote voltage sensing feature. There are unique issues to be aware of on each one, so I'd suggest that you go read up on them elsewhere before you attempt a non-GM swap. Or, just be like me and stick a GM alternator in it even if it's not a GM. :-)

Before actually tackling any physical wiring, you should take the time to read the suggestions outlined in my Reusing Plastic Wiring Plugs article for ways to get a cleaner appearance to your wiring efforts.  Also, if you own an older Buick with the "push gas to start" feature, check out my page on preserving this feature after you convert from a generator to an alternator.

Lastly, there are many other great resources out there - don't rely only on my site. Read up, and learn as much as you can. Car Craft has an excellent page about wiring work, and MAD Enterprises has some great technical details on why a 3 wire alternator is better than a 1-wire alternator as well as a great technical description of the remote voltage sensing feature of a GM 3-wire alternator and details on the various models of the 10SI and 12SI series of GM alternators.

Choices, Choices, Choices...

There are three kinds of upgrades you can do. You can go from a generator to an alternator, from an externally regulated alternator to an internally regulated alternator, or you can take the monster leap and jump from a generator all the way to an internally regulated alternator. I detail all three separately, see below. Before tackling them, you should give my GM Alternator Identification page a quick look to see what the visual differences are.

A conversion like this has two basic pieces to it - mechanical and electrical. You need to mount the new alternator so that it's secure and you can put a belt around it and then you need to wire it up so it actually does something when you spin it.

This is a non-trivial upgrade to your vehicle. It's not that hard, but it could take some time to get right. Don't expect to have the car back on the road the same day you take it all apart. This is not an afternoon job for most home "shade tree" mechanics, although some of the easier conversions can be done fairly quickly. There are also commonly available kits to convert from an externally regulated alternator to an internally regulated alternator from places like MAD Enterprises - shop around to see if you can find one as it will make your life much, much easier.

Mechanical Details

If you are swapping out a generator, your hardest challenge will be finding brackets to mount your new alternator. Unless you have access to custom built brackets (or are good enough to make them yourself), you need to get ready-made units from somewhere. Since GM has produced lots of motors with generators, I can't even begin to list aftermarket suppliers for them - start hunting around and see what you can find. My other suggestion is to find out if your motor was produced in some form after 1963 (that was the first year for alternators in GM vehicles) and try to scavenge brackets from a later motor. Whatever you do for brackets, you'll need to make sure it's all secure, that you can adjust the alternator belt properly, and that all the pulleys line up. You may need to put new pulleys on your crankshaft and water pump in some cases - check carefully and get this parts right. Other than that, there's not much I can say here - this is the ugliest part of the problem to solve.

If you're running a Buick Nailhead engine, see my page on Nailhead Alternator Brackets for more specific details and suggestions. If you're running something else in your ride either convert to a Nailhead engine or you're on your own for brackets. You can also try aftermarket sources if you're running a popular motor - check the ads in a magazine like Rod and Custom to find suppliers for some really odd parts. You can find places that will sell you the right brackets to run a GM alternator on everything from an early Ford flathead to an '50s Chrysler "Baby Hemi" engine - most of them are done in chrome or polished aluminum too.

One thing of note to look out for is that later model GM alternators switched to metric threads and a metric sized bolt for the top mounting adjustment bolt. If you try to thread in a standard size bolt, it will act like it's been cross threaded, but it might just hold once. Or even twice. But it will strip out and cause problems. Either use the right metric bolt and always remember to use the proper metric wrench on that one bolt - or just get an earlier model alternator with the standard bolt threads on it.

If you are converting from an externally regulated alternator to an internally regulated one, well, your life will be much easier. Snag any first-generation internally regulated alternator (from around '73 to '85) and it will bolt right on in place of your existing unit. You can even swap your original pulley over to the new one if you need to. For the fans, be aware that the fan design changes on the later 12SI series alternator are there to keep it cool in higher output models, so the fan that goes with the alternator should always be used. Basically, this is as close to a bolt-in as it gets.

At this point, I'm assuming you have a way to mount and adjust your new alternator. Put it in, but leave the adjustment bolts loose so you can move it around and get to the wiring while you work on it. After the wiring is done you can snug up the fan belt, tighten everything down, and fire it all up for a test drive.

Electrical Details

This is the part that most people freak out about. Many people think electricity is a mysterious and evil creation that must be feared. It's not that bad. Really! Just take your time, think about what you're doing, and it won't bite you (much). Plan out where all of the wires will go, make sure the battery is disconnected while you work on the wiring, and connect all of the wires to the right places. There really isn't much else besides that. Except all those little details, that is...

The first thing you need to do is figure out what you're going to do with all those wires. You sketch it all out on paper and make sure it at least looks like it ought to work before you dig in and start clipping and crimping.

I highly recommend that you get a wiring diagram of the charging system for both your existing vehicle and the vehicle you are using as a reference, then make a copy or two of each one so you can write on them, takes notes, etc. You want to merge the two diagrams together so that the result contains the pieces of the newer one grafted into your existing wiring diagram. Typically, you will want to keep one or two reference points on the old diagram and build the merged one from there using the new one as a guide. These points will be any wires that go from the existing charging system to the rest of the car - the power to the car itself and the battery, and perhaps the charge warning light or other similar wiring if they exist already. If you need to add new wires, that's fine too - just get it all there with things hooked up to the same conceptual places. Below are three sample wiring diagrams I have copied from various manuals to use as a basic reference. Also, if you are upgrading your system from a low-power alternator or generator to a higher power unit, you must pay attention to the size of the main feed wire that connects the alternator to the larger battery cable - the higher maximum power output could be a source of trouble (aka, melting and a possible fire) if you don't take care of this.

Typical 1962 and Earlier Buick Generator Wiring
(Diagram is scanned from a 1958 Buick service manual)

1963 to 1972 Buick Externally Regulated Alternator Wiring Overview
(Diagram is scanned from a 1970 Buick service manual)

1973 to 1985 Buick Internally Regulated Alternator Wiring Overview
(Diagram is scanned from a 1973 Buick service manual)


About now, you'll need to start figuring out where all these wires will go to on the car itself. Some things are up under the dash, some are under the hood, some are over on the fender by the battery - but they all go someplace. The schematics you made copies of and marked up to merge things together probably don't look much like an actual wiring harness - and that's because they're not supposed to. They help you figure out the circuit electrically - laying out the physical wiring harness is something different entirely, and that's what you need to do now. You may have some existing wires to reuse, or perhaps you need to remove them entirely. If you need to get wires from under the dash, out through the firewall, and to the engine, now is the time to figure out how. If you will have wires running between an externally regulated alternator and the alternator itself, you can reuse the two old generator to regulator wires and add the extra ones you need. If using an external regulator, mount it in the same basic place that the old regulator was at - it's got most of the right wires there already for you to hook up to. Now you can re-work your merged diagram some more to show what existing wires will get hooked up to what new things, what new stuff needs to be added, and if needed, what wires need to be replaced with larger ones.

At this point you should know what parts you need (wire, terminals, connections, etc.) and be able to make a basic run to the parts store. If you've done wiring before, you should have lots of spare bits and pieces laying around to help you out. If you've never done wiring work before, expect to make a few trips to get the various little pieces you need to get the job done. Some of the parts are a bit odd and may require some hunting.

If you need to add a resistance wire (because you are converting from a generator) you will need to hunt down the right resistor, or pirate the full length of the special resistance wire from a donor car in the junkyard. If you opt to get it from a junkyard, you must get the entire length of the resistance wire - if you cut it too short, it will be rendered useless for your needs because it will not have enough resistance in it. For example, if 6 feet of that resistance wire has a total of 10 ohms of resistance, 3 feet of that wire will only have a total of 5 ohms of resistance, and that's not enough for what you need. You can verify this resistance with a special "multi-meter" electrical tester that is available from places like Radio Shack for around $20. (Ask for one that can test voltage and resistance.) While you're at Radio Shack, you might want to just buy a 10 ohm/1 watt ceramic resistor and use that in place of the factory resistance wire, but you'll have to figure out how to mount it and hook it up. You can also use an ignition resistor if it's about 10 ohms - they already come with nice mounting points and easy to hook up terminals. It's your choice - as long as you put about 10 ohms of resistance into that one wire in the diagram and the resistor you use is rated for enough power to not overheat and melt, it'll work just fine. I'm told by my readers that a Radio Shack 10 ohm 10 watt 10% wire wound ceramic resistor (part #271-132) has worked well for their GM vehicles. Your mileage may vary, but if your system is similar to the system described here, this should be a good starting point for your work.

Now all that remains is to do the work. Carefully extract the affected wiring harnesses from the vehicle, add or remove wires as needed, and hook it all back up again. This is where the details come in - all of those wires need to be hooked up to various kinds of connectors on the car - take the time to make secure and neat connections - it will work better in the long run and it just looks nicer that way. You may need some special connectors to do this - most auto parts stores sell the connectors that plug into the alternator and regulator with a short pigtail of wiring on them - you can use these as a nice base for your custom wiring work. Also, you will need wire of the proper size (gauge) and color. Don't go nuts trying to hunt for the exact color (good luck finding some colors in unusual sizes...), but getting it close and noting what color you used on your merged diagram will be a big help later on if you need to troubleshoot a problem. Observe basic colors - like red, pink, and yellow are usually "live" feed wires into something. Black is usually a ground, sometimes dark green too. Blue, purple, white, and sometimes light green go between things and are usually "live" if some switch is turned on or something like that. There are exceptions, but you get the basic idea. Don't use red for a ground wire and black for the feed wire - you'll get confused and end up burning something out someday if you do goofy things like that. Follow the examples already staring at you in the factory wiring harness.

This is all sounding suspiciously like a "to-do" list of things we need to get done, and it is. Tackle them one at a time and you'll be fine. You also need to think about what you will be cutting up and re-using. Is your car valuable enough to ever want to restore it to "original" condition? If so, avoid destroying anything - save factory wiring harnesses and make new "custom" ones for your conversion. If you never want to return the car to factory condition, go ahead and cut and splice your harness. Just remember that if you want to get it back the way it was, you were warned! Go back up and find the 3 links to the detailed pages I have on each conversion type, pick the ones you want, and dig into the gory details.

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 01/01/2010 03:20:57 PM