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Alternator Voltage Regulation Explained with Wiring Diagrams

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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.

How The Alternator and Charging System Work

automotive alternator
The alternator charges the battery whenever the engine is running while also supplying electricity to the car’s electronics.

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.

Components of a car's alternator
The alternator’s stator (background), rotor (center), and rectifier (foreground).

The primary components found within the alternator are the rotor, stator, and rectifier bridge.

  • The rotor, which is the rotating portion of the alternator, consists of coils of wire (known as a field coil) behind opposing magnetic poles.
  • The stator consists of three sets of stationary coil windings and a laminated core.
  • The rectifier bridge contains diodes that act as one-way electrical check valves.

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.

Electromechanical, Electronic, and Computer-Controlled Voltage Regulation (Diagrams)

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.

Electromechanical Voltage Regulator

a vehicle's electromechanical voltage regulator
Some vintage vehicles use an electromechanical voltage regulator that’s external from the alternator which contains three electromagnetic switches: the cutout relay, the regulator, and the current regulator.

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.

  • The cutout relay closes to connect the alternator to the battery, thereby allowing the battery to be charged. The cutout relay will also open as needed to prevent the battery from discharging into the alternator.
  • The regulator opens and closes to control the alternator’s field circuit, thereby regulating the alternator’s voltage output.
  • The current regulator opens and closes to control the alternator’s field circuit, thereby regulating the alternator’s current output.
automotive alternator wiring diagram with external voltage regulator
A typical alternator wiring diagram with an external electromechanical voltage regulator.

Electromagnetic voltage regulators are no longer found in production vehicles. All modern charging systems use some form of regulation that’s purely electronic.

Electronic Voltage Regulator

car relay voltage regulator
Unlike electromechanical regulators, electronic regulators are solid-state without any moving parts.

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).

complete 3-wire alternator wiring diagram with internal voltage regulator
A typical 3-wire alternator wiring diagram with an internal voltage regulator.

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:

  • Battery terminal: Connects to the battery for charging.
  • Voltage sense terminal: Allows the regulator to sense the battery’s voltage.
  • Field or ignition terminal: Allows battery voltage from the ignition to flow to the alternator’s field coil during startup.

Electronic voltage regulators have been used on many cars since the mid 1970s.

Computer-Controlled Voltage Regulation

engine control unit
Many late-model vehicles use the PCM to control alternator output.

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.

detailed PCM-controlled alternator wiring diagram with alternator connections
A typical PCM-controlled alternator wiring diagram with alternator connections going to the battery and PCM

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.

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