Over the years, I have had to replace many alternators in a variety of vehicles I have owned. Something I learned to do early on was to always use a common alternator, regardless of what the vehicle originally had. Why pay $300 for a factory replacement alternator for a Toyota truck, for example, when a common GM Delco can be had for $20 and does a better job to boot? That is a real example, by the way; not just a made-up scenario. You may have to weld up a bracket out of junk-box parts to make it work (I did), but you can buy a cheap Harbor Freight 110-volt buzz-box welder and the GM alternator, and still save over $100 over the cost of the Toyota part.
I used to always use internally regulated Delco alternators for this because the only connections you need, other than the main battery connection, is a voltage sense wire to the battery and a switched 12 volt source from the ignition switch. You can also get one-wire marine versions of these alternators, that need only the battery connection. The marine versions cost quite a bit more though, and they also tend to run at a higher voltage than optimal, if my experience is any indication. The great thing about them, though, is that they work even if the original charging system on the vehicle used an external (or worse, part of the ECM) regulator, and that regulator and/or its wiring is hosed.
Nowadays, I have a better way of doing it. I have come to prefer external regulators, initially because they make it easier to switch an alternator between standard battery charging mode and welding/power tool operation mode. Eventually I realized that, because all regulators do pretty much the same thing, the most commonly available extrenal regulator in the world is capable of controlling any automotive alternator you may have, including internally-regulated alternators with the internal regulator bypassed or removed (I have done that, too).
What made me start thinking about this was working part-time in a small auto repair shop where I often had to figure out unorthodox ways of solving a problem that normally would cost a lot of money to fix. One commonly-encountered problem of that sort is as follows:
Most if not all modern vehicles, those built since the early '90s, do not have stand-alone regulators, either external or internal to the alternator. Rather, most of them have the regulator built into the central ECM; the computer that controls... well... pretty much everything in the vehicle. Various other functions are often connected to this circuit to give, for example, the break-in alarm the ability to stop the car within a few miles by not allowing the charging system to work. Yeah, you would be amazed at how much electrical current modern cars absolutely require to even continue down the road. That control aspect is one reason that the voltage regulator is built into the ECM, but I suspect the larger reason is simply that it is much cheaper to do so than to build a separate regulator.
Anyway, I discovered that it is quite common for either that circuit, or another circuit which has the ability to interrupt it, to develop some kind of problem that halts all electrical power generation, while leaving the higher functions of the ECM unaffected, save of course for that low voltage problem. According to the factory service manual, you are supposed to troubleshoot all systems external to the ECM and, if any of those are causing the problem, fix or replace. That stands to reason. But if it turns out to be the actual regulator circuit, you are supposed to pull and replace the ECM. That could cost a thousand bucks, easily. So the first time I was confronted with that problem, I thought about it.
Most of the time, what control circuits within ECMs do is activate a power transistor to pull a line to ground, either on-off style or in a linear fashion. I know this because I used to actually break down, troubleshoot, and repair ECMs, then re-pot them to be sold as refurbs. One of the many jobs I have had.
So, back to the circuit in question. Because this is a regulator circuit, it has the same problem most regulator circuits share: that transistor operates in a linear mode, so it wastes a lot of energy as heat. That heat tends to greatly reduce the life of the transistor, compared to a similar transistor that operates as a simple switch. Knowing this, and poking around the alternator wiring with my meter, I figured out that of the two field terminals on the alternator, one was connected to switched 12 volts, while the other was pulled somewhere between 12 volts and ground by that transistor (or more likely, just a junction) deep within the ECM, and that disconnecting those connections and replacing them with a ground connection to one terminal and a common external regulator ($9.95 from the local discount auto parts store) to the other would make all systems go again.
Before doing that, though, I decided to search Google to see if my hunch could be verified. What I found, via several online forums, was that many Mopar vehicles are out there plugging away, years after such a low-buck fix has been effected.
So here's how it's done. Find the one or two field connections to your alternator. If there is only one, check its continuity to ground with your VOM on ohms setting. If there is no connection, check it on the diode check setting; if it indicates a diode, you have an internally regulated alternator. All is not lost, but you will have to open up the alternator case and bypass that regulator, or preferably eliminate it altogether.
So now you have either one field connection available, with the other end internally grounded; or more likely you have two open-ended field connections. If that is the case, connect a ground wire to either of the two field connections, then connect a wire to the other connection, and hook it up to the "Field" connection of your cheap external regulator. Find a switched 12 volt line, perhaps the one that may have been connected to one of the field terminals of the alternator (depending on how it was wired on your specific vehicle), and connect that to the "Ignition" terminal of your regulator. Last, or better yet first, before you hook anything else up but in any case before you power the thing up, connect a ground wire from the metal case of the regulator directly to the negative terminal of the battery. At this point, you are done. While you are at it, carry a spare regulator; they are cheap insurance.
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