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What Are The Terminals on an Alternator: Parts of an Alternator

Learn what are the terminals on an alternator to understand the principle behind the self-charging property of your car battery. 

There are fewer nagging issues than a car battery that keeps running out whenever you want to start your car. 

But did you know that sometimes the battery is never the problem? 

Alternator issues will render even the best batteries useless. 

Therefore, we’ll discuss the alternator in detail by fundamentally looking at its terminals. 

Read on to learn more. 

Table of Contents

Alternator Basics

A Mechanic with an Alternator. 

A Mechanic with an Alternator. 

An alternator is an electronic component that generates power (power source) in a car’s electric system. 

It indeed has complex wiring, but the basics, even in vintage vehicles, are the same, so by identifying the key parts, we can learn more about its operation. 

So what are some of the basics of this system? 

Pulley System

The pulleys are responsible for initiating the rotation of the magnet in an enclosure with a copper coil to generate AC current. 


It’s the rotation of the magnet around a copper coil that leads to AC power generation. 

Copper Coil

It is at the core of electricity generation when the pulleys rotate a magnet around it.

 Its primary role is creating electromagnetic force, which produces electricity upon interaction with a magnetic field. 


It’s typically made of several diodes that facilitate electric power conversion from AC to DC.

 A car’s battery uses DC; hence the rectifier is critical in realizing this. 

Voltage Regulator

Finally, this device controls the voltage from the alternator to the battery. 

It manages the current flowing into the rectifier diode and the battery. 

The above parts are the cardinal components that facilitate the operation of a car alternator. 

Their function happens thanks to the input of the terminals we’ll look at below in detail, so check them out. 

What Are the Terminals on an Alternator?

The W Terminal

The Car Alternator. 

The Car Alternator. 

The W terminal is among the most critical parts of the alternator fundamentally due to its role in producing an AC voltage via electromagnetism.

 The frequency of this voltage is directly dependent on the engine’s speed. 

Also, the W Terminal feeds the tachometer with the voltage output signal. The role of a tachometer is to detect the speed of an automobile. 

Hence, it is noteworthy that an alternator helps determine a car’s speed and charging the battery.  

A and B Terminal

These are battery-positive connections of an alternator; thus, we use them to connect to the alternator’s battery-positive TTerminal.

 You can tell this battery terminal by checking for the + sign. 

The B+ and D+ Terminal

Alternator coils of wire. 

Alternator coils of wire. 

Both terminals represent positive battery connections. 

The D+ terminal is sometimes called a battery connector and is synonymous with European alternators. 

The primary indicator light circuit is responsible for indicating the alternator is charging appropriately. 

 Also noteworthy is that there’s also a D- Terminal for the negative battery terminal. 

The DFM Terminal

Showcasing Coil Windings in an Alternator. 

Showcasing Coil Windings in an Alternator. 

This TTerminal transmits a field current from a car’s electronic control unit (ECU) to its alternator. 

So what’s the essence of this field currently? It controls the volume of electricity the alternator produces and its voltage. 

Notably, a car’s voltage can get too high and lead to components damage, but again, if the battery voltage is too low, it won’t charge.

 Hence, the DFM terminal regulates the voltage to avoid these extremes. 

R Terminal

It’s the relay terminal of your car’s alternator, whose primary function is to connect to the relay/key switch. 

A relay is an ignition switch that sets the alternator on and off. 

Hence the R-Terminal connects to the current relay switch of the alternator-battery system. 

EXC Terminal

Mechanic with a One-Wire Alternator. 

Mechanic with a One-Wire Alternator. 

Also called an excitation terminal, it’s responsible for connecting a warning light that indicates the alternator’s charging mode.

 So the light shows when the device is working correctly or when there’s a hitch hindering the charging process. 

Also, you can tell the charging rate from this warning light. 

I Terminal

It provides Direct Current (DC power) to your car’s ignition system. 

N/C Terminal

A PCR-Controlled Alternator. 

A PCR-Controlled Alternator. 

Like other N/C terminals in electronic systems, this is the no-connection port of the alternator.

 Hence, it doesn’t connect to any other part of your car. 

S Terminal

The battery sense connection terminal connects the car’s alternator voltage regulator (AVR). 

Hence its role is to regulate the voltage levels from the generator windings to the battery. In its absence, the car will not stably charge. 


This full-field bypass terminal connects to the regulator, common in externally regulated devices.


Cross section of a modern Alternator. 

Cross section of a modern Alternator. 

It feeds the electronic control unit (ECU) with the alternator’s charge rate. 


It’s the TTerminal responsible for connecting to the warning lamp. 


A Typical Alternator.  

A Typical Alternator. 

The car’s ICU communicates with the G terminal for voltage/electric load control since it’s essentially a chassis-negative connection. 

The Use of Identifying Alternator Terminals

A Typical Alternator. 

A mechanic checks the Alternator wiring. 

An alternator is one critical component/device that features a host of many other internal modules in addition to those we’ve just covered.

 There are core and support systems, such as the fuse box, cutout relay, and others. 


Now you have it all about the alternator basics and the various wiring terminals that facilitate the device’s functioning. 

Among the reasons we have prepared this guide is to help you understand your car’s electrical system. 

Why? Because a prevalent car problem is an inability to start it primarily because of a low battery resulting from an alternator failure.

 Now you know what to check in case this problem happens.

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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