Delco 10-SI: The Delco 10-SI alternator was introduced in the early 1970’s and was the first internal regulator alternator widely used by GM. This alternator was the most common standard equipment alternator used on GM cars & light trucks. It was also used extensively in other applications such as industrial, marine, agricultural equipment. It is recognizable by the three air vent holes on the rear of the case and the old-style stamped steel fan. The 10-SI alternators are available in 37, 42, 55, 63, 70 and possibly 85-amp versions.

Delco 12-SI: The Delco 12-SI alternator, introduced around 1983, was a replacement for the 10-SI. The 12-SI alternator has the same outside dimensions as the 10-SI and will fit in place of a 10-SI without modification.

The 12-SI improvements over the older 10-SI model include:

6 large cooling holes at back for better cooling of rectifier, stator and other internal components ;
High volume axial turbo fan for greater air flow through the alternator;
Larger rectifier to handle more amps;
Wider stator and rotor for improved magnetism for better low-end output;
Increase in mass helps with heat dissipation.
The 12-SI alternators are available in 56, 66, 78-, and 94-amp versions.

Delco 12-SI

1-Wire or 3-Wire?

Look through the aftermarket catalogs like Summit Racing or Jegs for Delco alternators, and one of the most commonly touted phrases used when describing an SI-Series alternator is "simple, 1-wire hookup". Or, you may see "OEM-style 3-wire connection" in the description. What does this mean? What are the differences and advantages between these connection methods? Read on!

This is a 1-wire Delco 10-SI alternator installed on a farm tractor. That one wire is bolted to the BATT terminal (the charge output) and, in this case, connects directly to the battery. These units are commonly found on farm machinery.



A typical 3-wire Delco 10-SI is shown at right. Besides the "Bat" (Battery) terminal, there are also connectors for the field (Terminal #1) and the sensor (Terminal #2) wires. Continue reading for more info on this...



3-Wire Explained

Let's examine the 3-wire setup first. The 3 wires in this type of GM alternator are identified as: [1] Field Excite/Warning Light Indicator Input, [2] Remote Voltage Sense Input, and [3] BATT Output.

Wire 1: Field Excite/Warning Light Indicator Input

This terminal is fed by the switched 12V source (from the ignition switch and through the ignition warning light indicator or "idiot light"). It is used to energize or excite the field windings inside the alternator. The alternator cannot start charging until there is voltage on the field windings and a magnetic force is created. In addition, this terminal will cause the ignition warning light to illuminate if the ignition is turned on and the alternator is not supplying enough voltage.

Wire 2: Remote Voltage Sense Input

The sense line is used by the internal regulator to determine how much voltage is present in your charging system.

This sense wire can be connected (theoretically) to any point in your system, but it is should be connected to a main distribution point such as the power terminal post near the battery for best performance.

The photo at right shows an example where the sense wire is looped directly over to the BATT terminal. This is a common configuration used when retrofitting the Delco SI alternator and does not require any in-depth understanding of the car’s wiring system.



While this is the simplest connection location for the sense line, it is also the least desirable: the alternator will sample the output voltage at the alternator itself instead of at the load connections further downstream. A voltage drop downstream of the alternator due to the internal resistance of the wires and components is not accounted for in this scenario, and the voltage at the battery may be lower than it should be.

This approach will work, but it's not the best solution.

It is better to run the sense wire away from the alternator and over to the power terminal post near the battery. This will ensure that the system voltage is properly maintained.



For illustration, let’s say the regulator is set to maintain 14.4 Volts. The alternator will do it's best to maintain 14.4V at the point where the sense line is connected. Let's also say that we have a battery feed cable that exhibits a 1V voltage drop across its length (see illustrations below).

If the sense line was connected directly to the rear of the alternator to the BATT terminal, then, with the 1V drop in the cable, the far end of the cable would measure 13.4V. At this voltage, the battery would not be receiving a full charge and the car's accessories wouldn't be getting optimum voltage:

On the other hand, if the sense line is connected at the far end of the battery feed cable like in the figure below, then the voltage at the terminal post will be held at 14.4 volts. And, the voltage at the BATT terminal at the alternator would read 15.4V. Why? Because the alternator must boost its output voltage by 1V to overcome the voltage drop in the main output feed on its way to the power terminal post. This is better!



NOTE: When using the remote voltage sensor, you should not make additional electrical connections at the BATT terminal of the alternator since the voltage there will always be somewhat higher than the regulated value. Instead, make all additional connections at the point where the sense line is attached.


Wire 3: BATT Output

This terminal is for the alternator output and is usually a threaded stud located on the rear of the alternator. A heavy-gauge cable is connected to this stud and usually connects to the positive terminal of the battery, or to the power terminal post near the battery on the E-Type.

Note: the gauge of the cable connected to this post must be large enough to handle the alternator’s maximum current rating. If replacing the OEM alternator with one of a higher amp rating, it is imperative that this output cable be replaced with a fatter one.



1-Wire Explained

A 1-wire alternator, also known as a self-exciting alternator, is commonly used on custom cars & trucks, tractors and other non standard applications when simplified wiring is a factor. As the name implies, these alternators provide only one wire connection: the BATT terminal. A heavy cable is typically run directly from the BATT terminal to the positive terminal of the battery.

1-wire alternators are essentially 3-wire setups that have been rewired internally for a simplified final installation:

No Remote Voltage Sense Input:
The regulator's sense line is still used, but instead of connecting to a remote location in your car, it is simply routed directly to the output feed of the regulator. This connection may be inside the alternator where it would be completely hidden from view, or the sense line may actually have an external terminal attached to the output feed stud on the back side of the alternator (see photo above).
A common complaint among custom street rod owners using the 1-wire setup is that their headlights are dim, even with a huge 100-amp alternator wired directly to the battery. This is due to the lack of remote voltage sensing at the power distribution point.

No Field Excite/Warning Light Indicator Input:
The ignition warning light indicator function is omitted with a 1-wire alternator, so your idiot light will not be operational with this setup.
For the 1-wire, self-exciting design, the field windings are not energized via the ignition switch; instead, a special circuit is built into the internal voltage regulator that senses the rotation of the alternator’s rotor. The rotor must turn at sufficient speed to trip the circuit which excites the field windings and starts the charging process. This “cut-in” speed is affected by several things and is typically higher with certain high amperage alternators. Prior to reaching the cut-in speed, the charging system is not activated and the battery will be discharging. However, once the cut-in circuit is tripped, the alternator will charge at all speeds, even very low ones, until the alternator’s rotor comes to a complete stop. At that point, the circuit will shut off and wait for the process to be repeated.

Typically, after starting the engine, the engine must be revved above 1200-2000 RPM to turn-on the 1-wire alternator.


The advantage of a 1-wire system over the 3-wire system is that the installation is simplified and provides for a very clean installation. All that is required is a heavy gauge cable running between the output feed stud on the rear of the alternator and the battery.

Some of the disadvantages associated with the 1-wire system are:

Less than optimal voltage levels at points downstream from the alternator;
The ignition warning light is omitted;
At startup, a slow-running engine will not be spinning fast enough to turn-on the alternator and the system will be discharging until the throttle is blipped and the engine revs higher than the alternator cut-in speed.
Higher cost than 3-wire equivalent.