Alternator Wiring Diagrams and Information

Typical externally regulated alternator

Wiring instructions for the early GM Delco Remy external regulated alternator. How to wire an external voltage regulator on a GM vehicle. The early GM alternator is the 10DN series alternator and was used on GM vehicles from about 1963-1970


Typical generator charging system

Typical internally regulated alternator


Typical Externally Regulated to Internally Regulated Alternator Conversion

Info on conversion


We’ll start our tour of the alternator where it all starts in the alternator itself – at the alternator rotor. The rotor consists of a coil of wire wrapped around an iron core. Current through the wire coil – called “field” current – produces a magnetic field around the core. The strength of the field current determines the strength of the magnetic field. The field current is D/C, or direct current. In other words, the current flows in one direction only, and is supplied to the wire coil by a set of brushes and slip rings. The magnetic field produced has, as any magnet, a north and a south pole. The rotor is driven by the alternator pulley, rotating as the engine runs, hence the name “rotor.”


Surrounding the rotor is another set of coils, three in number, called the stator. The stator is fixed to the shell of the alternator, and does not turn. As the rotor turns within the stator windings, the magnetic field of the rotor sweeps through the stator windings, producing an electrical current in the windings. Because of the rotation of the rotor, an alternating current is produced. As, for example, the north pole of the magnetic field approaches one of the stator windings, there is little coupling taking place, and a weak current is produced, As the rotation continues, the magnetic field moves to the center of the winding, where maximum coupling takes place, and the induced current is at its peak. As the rotation continues to the point that the magnetic field is leaving the stator winding, the induced current is small. By this time, the south pole is approaching the winding, producing a weak current in the opposite direction. As this continues, the current produced in each winding plotted against the angle of rotation of the rotor has the form shown in figure 2. The three stator windings are spaced inside the alternator 120 degrees apart, producing three separate sets, or “phases,” of output voltages, spaced 120 degrees apart, as shown in figure 3.



A/C voltage is of little use in a D/C system, such as used in an automobile, so it has to be converted to D/C before it can be used. This conversion to D/C takes place in the Bridge Rectifier .  Diodes have the property of allowing current to flow in only one direction, while blocking current flow in the other direction. The Bridge Rectifier consist of six diodes, one pair for each winding. One of the pair is for the negative half cycle, and the other for the positive half cycle. As a result of this diode rectification, the output of the alternator looks as shown in figure 4.

Figure 4: pulsed DC voltage

Surprisingly enough, the output of the alternator is not a pure D/C as one might expect, but a pulsating D/C. Because there are three windings, each with a positive and a negative half, by the time the voltage is passed through the diodes, there are six pulsations for each rotation of the rotor. This is close enough to D/C for most automotive components. Critical components, such as radios, have their own internal filtering circuits to further smooth out the waveform to a purer D/C.



The diode trio consists, as the name suggests, of three diodes, one per phase, which provides field current to the alternator regulator. This output will be discussed in more detail later in the “field current supply” section.



The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.



Field current supply is provided from two different sources – from the alternator itself, via the diode trio, and from the battery, via the alternator warning lamp. When you first get in the car and turn the key on, the engine is not running and the alternator is not spinning. At this time, the voltage/current source for the field current is from the battery, through the ignition switch, and through the warning lamp. After the engine is started, and the alternator is up to speed, the output of the diode trio is fed back to the regulator, and serves as a source of current for the field current. At this time, the alternator is self sustaining, and the battery is no longer needed to power the automobiles electrical system WARNING!!! This is theoretical only – in actual practice, the voltage surges resulting from disconnecting the battery can seriously damage the regulator circuitry. All alternator manufacturers strongly advise NOT doing this! This test will not prove the functionality of the alternator anyway, as the engine may still run with a weak alternator output.



This brings us back full circle to the starting point – the alternator warning lamp. As can be seen from figure 5, a schematic for an actual alternator, there is a path to ground from the field current supply input [1] to the regulator. As a result, when the key is turned on, current flows through the warning lamp, through the resisters, transistors, and field coil, and then to ground, causing the lamp to illuminate. Once the alternator is at full output, voltage from the diode trio, also applied to [1], equals the battery voltage. At this time, with 12 volts on both sides, the lamp is out.

If the alternator should fail, voltage from the diode trio would drop, and once again the lamp would light from the battery voltage. If the alternator output is only a little low, the lamp will be dimly lit. If the alternator fails completely, and the output voltage goes to zero, the lamp will be lit at full brilliance. Conversely, if the battery should fail, and the battery voltage drops, with the output voltage of the alternator on one side and the low battery voltage on the other, the lamp will also light.

As stated earlier, if the light grows dimmer as the engine is revved up, it is because the alternator voltage is rising with the RPM, producing more voltage on the alternator side of the lamp. The closer the output voltage gets to the battery voltage, the dimmer the bulb becomes. By the same way, if the light gets brighter with increasing RPM, it is because as the alternator voltage increases, it is getting higher than the battery voltage. The higher the voltage with respect to the battery voltage, the greater the voltage difference across the lamp, and the brighter it gets.



In summary, then, we can say that field current through the rotor coils produces a magnetic field, which is coupled over to the stator coils, producing an AC voltage. This AC voltage is converted by the output diodes into pulsating DC voltage, which charges the battery.

The field current is supplied from either the battery, via the warning lamp, or from the diode trio. The amount of field current allowed to pass through the regulator to the rotor, or field coil, is controlled by the voltage feedback from the battery.

And there you have it – the complete operation of an alternator in a nutshell. The next time you see the little red light, you will know exactly what it is trying to tell you.

The above article has been provided courtesy of

Dan Masters



One Wire Self-Exciting Alternators

The Self-Exciting alternator is an alternator that has a special voltage regulator that doesn’t need an ignition wire  to activate it. This is usually based on a chevy alternator type and only requires a battery wire  connected to the battery terminal. The voltage regulator which controls alternator output contains  circuitry that uses the residual magnetism in the alternators rotor fields to determine when  to turn the alternator on and off, a standard Delco alternator would not do this without an ignition  activation wire. The regulator does this by sensing the RPM the alternator is turning,  when it gets to a certain rpm the voltage regulator “turns on”. Typically you start the vehicle, rev the motor slightly with the si series and alternator starts charging. With the CS  series delco alternator with the self exciting voltage regulator you do not need to rev the engine.

The CS series alternator with self exciting voltage regulator will give you faster alternator voltage. This type alternator is  commonly used on custom cars & trucks, tractors and other non-standard applications when ease of alternator wiring is a factor. In choosing this type alternator you must consider, do you want  to rev your motor slightly to get the alternator to turn on, if not the choose the CS series type alternator. Also when using the self-exciting  alternator on tractors or other slow turning motors does the engine  have enough RPM’s to start the alternator charging. This can be overcome by using  a smaller pulley or by adding an ignition wire or momentary push button.


What is the difference between a one wire alternator, two wire alternator and three wire alternator?

One-wire Alternator: 
Is another name for the Self-Exciting Alternator mentioned above. You only need to connect the battery wire (one-wire).
The alternator turns on the voltage regulator when the engine starts turning the alternator.

Two-wire Alternator:
Most standard and all self-exciting regulator alternators will work using the two wire setup.
Two wire means that you use the main battery wire to the back of the alternator and also ignition wire to the
#1 terminal to activate the alternator. With this setup the alternator starts charging as soon as the engine is running

Three-wire Alternator:
This setup uses a battery wire, ignition/warning light wire and voltage sensing wire, Three wires. Voltage sensing is used when you want
the alternator to read voltage at some other point than the battery. Or the battery wiring is such that the battery is a long distance from the alternator. 



Wiring 10Si, 12Si, 15Si, 17Si, and 27Si Series Alternators

General Motors has only had 4 different series alternators since it first replaced generators with alternators in the 1960’s. The very first alternator was the 10DN externally regulated alternator. The first internal regulated alternator was the 10Si series starting in the early 70’s and used till the mid 80’s. These alternators were quite popular for auto, truck, industrial, marine, farm and adaptive applications. Since the introduction of the 10Si, GM’s Delco-Remy line has offered the 12Si, 15Si, 17Si, 27Si then the CS and AD series alternators for cars and light trucks.

This first section will cover wiring information for the SI series only, we will go over the CS and AD series wiring later. Although they may vary in size and output, the wiring is the same for all the Si series. Wiring these alternators is quite simple. All the Si alternators can be considered both 2 and 3 wire systems. To activate these alternators you are only required to supply the main battery wire to the (BAT) terminal which must have power on and an ignition wire to the #1 terminal. Most all the Si series alternators should have two spade terminals, but some that have three terminals, the third is for a tachiometer connection and senses the alternator RPM.

Near the spade terminals, the rear housing should be marked #1 and #2. Some aftermarket housings are not marked and others may be worn off. So if your alternator housing is unmarked, look from the rear of the alternator: the #1 terminal is on the left and the #2 on the right. You only need an ignition wire to the #1 terminal to make an Si series alternator work. The #2 terminal is for voltage sensing, and is optional. The #2 voltage sensing terminal allows the voltage regulator to sense the battery voltage so it knows when to turn the alternator on and off. The #2 terminal, if it is not used, causes the regulator to revert to internal sensing and pick up the battery voltage at the main battery wire on the back of the alternator. Some alternators are wired with a jumper from the #2 terminal directly to the battery connection at the back of the alternators, but this is not needed.

The ignition wire to the #1 terminal can be supplied in two ways, either a direct ignition wire from the key switch or through a light bulb. Running the ignition wire through a light bulb before connection to the alternator will give you a warning if your alternator fails to start charging or if it quits working while the engine is running, this is commonly known as the idiot light. Another thing that should be mentioned is the #1 terminal must be ignition switched. I have heard of situations where a wire was run to the #1 terminal that had power on at all times. What happens is when the engine is turned off the #1 terminal switches to ground which over time can burn up the voltage regulator or the wire if power is not cut to the #1 terminal. Another note for special installations of the Si series alternator is that in some cases when you try to shut the vehicle off, power to the #1 terminal from ground to positive power, the positive power feeds up to the coil and keeps the vehicle running. To cure this you must install a diode in the ignition wire that activates the #1 terminal. The diode will stop the power from feeding back up the ignition line.



My engine management computer on my Dodge/Chrysler vehicle is no longer making my alternator work, Is there a way to wire or connect an external voltage regulator to my Dodge or Chrysler vehicle?  Chrysler Dodge Alternator Computer Regulator Kit

erck diagram

Yes you can use an early (late 70’s early 80’s) Dodge/Chrysler electronic external voltage regulator kit like our part # ERCK . To make it work you just need an ignition wire run to both the voltage regulator “I” connection and to one of field connections (brushes) on the alternator. The “field” connection on the voltage regulator is run to the other brush (field) connection.

This is an “A” circuit type voltage regulator which means full power is sent to one brush while the voltage regulator controls the output by varying the ground to the other brush. It does not matter which wire you run to the regulator field connection and which one you ground, you can swap them around. The external regulator will make your alternator work.

ECU troubleshooting: If the battery voltage is between 12.8V and 14.7V, your ECU Power Module should be fine. If you alternator is not working or your alternator voltage is above 15 volts you have a problem with your ECU. If your voltage is low you may have an ECU problem or your bridge rectifier in the alternator is failing.



What is the difference between an “A” circuit voltage regulator and “B” circuit Voltage Regulator in an automotive alternator.

When you have an “A” circuit type voltage regulator means full power is sent to one brush while the voltage regulator controls the output by varying the ground to the other brush.
It does not matter which wire you run to the regulator field connection and which one you ground, you can swap them around. Early GM internal regulated alternators such as the 10si used the “A” circuit voltage regulators

The “B” circuit type voltage regulator grounds one brush inside the alternator. Then the voltage regulator controls the positive power to the other brush which runs to the rotor coil inside the alternator. Control of this positive power into the rotor coil turns the alternator on and off.
Examples: Ford used “B” circuit type voltage regulators in early Motorcraft external regulated 1G alternators . Later Ford units such as the 3G, 4G, and 6G alternators use “A” circuit voltage regulators.




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