There are many different charging system designs used in automotive applications. One primary factor that differentiates one system from another is the voltage regulator.
Some classic cars have electromechanical regulators, while modern production vehicles use either an electronic regulator or a computer (sometimes both). The design of the regulator affects the entire layout of the charging system.
Before jumping into the different types of regulators, it helps to have a basic understanding of how the charging system works.
As most people know, the alternator charges the battery whenever the engine is running while also supplying electricity to the car’s electronics. The alternator accomplishes these tasks through electromagnetic induction—a phenomenon that generates electrical current from a magnetic field.
The primary components found within the alternator are the rotor, stator, and rectifier bridge.
The rotor’s field coil receives electrical current through a set of slip rings and brushes (located on the rotor’s shaft). That current creates a magnetic field, which is enhanced by the rotor’s magnetic poles.
Whenever the engine is running, the vehicle’s drive belt turns the rotor via a pulley on the front of the alternator. As the rotor turns, it causes the stator to create an alternating current. The rectifier bridge converts the alternating current (AC) into direct current (DC) that the car’s electrical system can use.
But there’s one more piece to the puzzle. For the charging system to work properly, the alternator must produce enough voltage to charge the battery—but not so much voltage that the car’s electronics become damaged.
To address this issue, the alternator’s output is controlled by regulating the amount of current that flows through the rotor’s field coil. The voltage regulator is responsible for performing this task.
Until the mid 1970s, many cars used electromechanical voltage regulators. Modern production vehicles, however, control the alternator’s output with either an electronic regulator, a computer, or both.
Although each type of regulator operates differently, they all perform the same task: controlling the alternator’s output. The output is regulated by managing current flow through the field coil.
Let’s take a look at each type of regulator and its circuit.
Some vintage vehicles use an electromechanical voltage regulator that’s external from the alternator. Most of these regulators contain three electromagnetic switches referred to as the cutout relay, the regulator, and the current regulator. Each serves a distinct purpose.
Electromagnetic voltage regulators are no longer found in production vehicles. All modern charging systems use some form of regulation that’s purely electronic.
Refer to the simple diagram below for systems with an external electromechanical voltage regulator.
Electronic regulators use semiconductors (zener diodes and transistors) to control the alternator’s output. Typically, the regulator controls the alternator by opening and closing the ground side of the field circuit. Doing so permits or obstructs current flow.
Unlike electromechanical regulators, electronic regulators are solid-state without any moving parts. The solid-state design allows for quicker cycling and more precise control over the alternator.
Electronic regulators can be mounted internally (inside the alternator) or externally (somewhere else).
Exactly what wires go to an alternator will depend on the system design. But generally, an alternator (that’s not computer controlled) with an internal regulator will have three terminals. The terminals are as follows:
Electronic voltage regulators have been used on many cars since the mid 1970s.
Refer to the diagram below if you’re working on three-wire connections.
Many late-model vehicles use the engine computer, which is often referred to as the powertrain control module (PCM), to control alternator output. Most modules use an internal driver to turn the alternator’s field circuit on and off.
An example is the General Motors (GM) Electrical Power Management (EPM) system. With this setup, there’s an internal, non-serviceable regulator inside the alternator. But the PCM controls system output by changing the on-time of current flow through the field coil.
The PCM decides how much voltage the charging system needs by looking at data from the body control module (BCM). A data network allows the PCM, BCM, and other modules to communicate with one another. The BCM monitors a battery current sensor, which is located in one of the battery cables, to monitor the current going in and out of the battery.
If there’s a failure within the EPM system, a message will be sent over the data network, instructing the instrument cluster to turn on one or more warning lights.
GM’s EPM system is just one example of a computer-controlled charging system. There are many other system designs, some of which do not use a regulator inside the alternator. Certain Chrysler charging systems, for example, house all of the regulator electronics within the PCM.
Below is a simplified diagram showing connections to the vehicle’s PCM and battery.
You can also check this video to get an idea of how alternators work and how to replace one:
Given the importance of the alternator, driving while it’s malfunctioning is risky because it could damage electrical components. It could cause issues like stalling and a dead battery. Luckily, CarParts.com has a wide selection of high-quality alternator replacements for you to choose from!
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View Comments
This was a great site very informative. I have been having an issue with my vehicle and it has been very expensive and the still is not been corrected. With this information, I can troubleshoot more precisely. Thank you.
Hi Joshua,
Thank you for the feedback! We're glad you found the article to be helpful.
I've been trying to find an external regulator to replace my old "Filco VRF-330 HD.
It is an "A" circuit regulator. (externally grounded field)
All of the regulators I checked do not give any info about what circuit they have.
Where do I find that info?
Hello Klaus,
What is the year, make, and model of your vehicle? With that information, we'll be able to check whether we have a replacement regulator for your application.
I have a modified Denso alternator. I removed the built-in regulator and mounted a Filco VRF-330 regulator along with a parallel wired over-voltage relay. This is in a homebuilt airplane.
The alternator was off a Geo Metro. (lighter than aircraft alternators)
This system runs an "A" circuit for the field modulation.
None of the regulators listed on the Internet, and there are dozens, say what circuitry is used for the field.
Often, the product descriptions on websites are rather limited. If you're looking for more detailed information on a particular regulator, you might try searching for the manufacturer's product datasheet. Another option (if you want to go with the same Filco VRF-330 regulator) is to look for a used unit on eBay and similar outlets. Good luck!
Can I modify a 12v alternator to charge a 24v battery? thanks for any thoughts
Hi Bob,
You might be able to modify the charging system to produce more voltage, but the alternator would be operating at maximum capacity and would likely have a short service life. So, your best bet would be to find an alternator that's designed for a 24-volt system.
With a newer/modern day alternator, do I still need a voltage regulator for my 292 inline 6 motor?
Hi Alex,
Whether you need an external regulator will depend on the charging system design and whether the updated alternator has an internal regulator.
Try quick start high output alternators
I have a 1999 Blazer 4.3L ....the 100amp Alternator quit charging, tried a new 220amp it won't charge either. The battery is new and the small red wire to the alternator reads switched 12v battery voltage. Not the 1.5 - 5 volts I thought the PCM provided.
i have a 1996 blazer . chek grounds (at control module) , physically and with meter. any wire near exhaust could be damaged. that fuse box is worth an inspection.
Fuse Box Diagram Chevrolet Blazer (1996-2005) (fuse-box.info)
on my 96 the heat/fan has been removed along with the a/c .
A very well written article. It helped the layman understand the complexities of the charging system without being over the top complex. Great job. I ove the the affection of all-wheel drive Astro vans. One of my favorite vehicles also.
I want to connect the older internal regulator used years ago by GM to my one wire alternator on my 1998 Chevy pickup. Can you show me which terminal connects to the alternator windings and which one connects to battery voltage, I know that the bottom of it is used for the ground connection to the battery.