3 wire gm alternator wiring diagram

We’ve covered a number of topics regarding alternators on VehicleFreak, from identifying alternator-related issues to finding the best replacement and explaining how to install it yourself.

The question that came up frequently is how to connect a 3 wire alternator where it’s not a factory option, so in this article, we’ll give you a complete answer so you can go out and rewire your car!

How to Connect a 3 Wire Alternator

An alternator takes the mechanical energy generated by the rotation of the engine and the movement of the drive belt and converts it into electrical energy. The alternator recharges the battery and acts as the main power supply for the entire vehicle when the engine is running.

The 3 wire alternator, as the name implies, needs three wires to function. When you’re dealing with a direct replacement, the process is extremely simple – memorize the position of the plugs, disconnect them, and then reconnect the new alternator.

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We’ve already covered the process of how to fix an alternator, so check that out if you need the step-by-step guide on how to remove and install a new alternator correctly. 

To connect a 3-wire alternator without a previous reference to go by or compatible plugs, you’ll have to follow these steps:

1. Identify the three connection points on the alternator. The bottom connection is a bolt for the o-ring wire connector to sit on. On the top, terminal 1 and terminal 2 are positioned next to each other and accept small plugs.
2. Place the o-ring of the positive battery wire on the bolt. Every car should have one by default regardless of the alternator type.
3. Get a short piece of wire with an adequate terminal plug on one end, and an o-ring on the other. Plug the wire into terminal 2, and place the o-ring side over the bolt, after the red wire. 
4. Tighten the bolt with a nut to connect the two o-rings with the alternator.
5. Terminal 1 should be connected to the gen, battery, or alt dashboard light. On the cars that don’t have it, you’ll wire a dummy light between terminal 1 and the On position of the ignition switch.

Now, let’s talk about what each of the steps does. The identification process is important, as you don’t want to get them confused.

The positive lead is the thick red wire with the o-ring. It goes to the battery, with or without passing through a junction block (an intersection for cables), and delivers power.

Terminal 2 connects to the positive lead and forms a small circuit. I’m not sure what exactly it does, but you can read the 1-letter code marking the terminal and check online. 

Terminal 1 connects to a dashboard light, a diode, or a LED light. It’s often called the dummy light as it doesn’t have a practical purpose in the sense of illumination. The wire goes through the diode and connects to the On position of the ignition switch.

When you turn the ignition to the On position, the diode will light up and the alternator field gets excited – hence the name. When you flip it back off, the diode will turn off and not leech battery power.

The diode itself prevents two-way current flow – once the key is released the alternator will run, and it will be able to shut off when the key goes to the off position. 

FAQs

Why does the alternator have 3 wires?

The 3 wire alternator detects the voltage at the fuse block and the ignition, so it can regulate the charge and adjust it when necessary. It’s an improvement over the 1-wire alternator and can be retrofitted into older vehicles through the steps explained in the previous section. 

What are the three wires for my alternator?

The first wire is the largest in diameter and painted red – the positive battery wire, which leads to the battery through a wire junction or directly.

The second and third wires are next to each other and are called terminal wires. The terminal 2 wire is the voltage sensing wire, while the terminal 1 wire is the exciter/ignition input wire. 

What is a 3 pin alternator?

The 3-pin alternator is a synonym for the 3-wire alternator we’ve been discussing in this article. Even though it’s called a 3-pin alternator, the battery positive wire is screwed on the alternator wire post, rather than plugged in. 

What brand of the alternator is the best?

Considering how affordable, reliable, and widely spread they are, we consider the ACDelco alternators as the best brand in the business. We’ve recently covered the top 6 alternator options, so take a look if you’d like to see what the other great options are. 

How many wires go to an alternator?

The 3-wire alternator technically has two wires going to and from it – the exciter wire, and the positive battery wire. The sensing wire is looped from the alternator’s terminal 2 to the positive battery wire post on the alternator.

Can I run a wire from the alternator to the battery?

You’re supposed to run the positive battery wire from the alternator to the battery so that the alternator can supply the battery, and the engine with electric power while the battery recharges.

What happens when the alternator goes bad?

If the alternator stops working, the car won’t be able to run, at least not for long. The car can’t run with a bad alternator because the alternator is responsible for recharging the battery and supplying the system with power.

Without it, the car is running solely on the power stored in the battery, which is enough for an hour of a drive at most.

What is the small wire on an alternator?

The small wire that loops from one part of the alternator to another is the sensing wire, responsible for determining the output of the alternator. It’s an important piece of the wiring system and makes sure the electrical systems and the engine itself operate correctly.

How do you manually excite an alternator?

If you don’t have the exciter wire, you can briefly run a wire from the “R” terminal to the battery wire terminal. Make sure not to touch the exposed parts of the wire and quickly remove the piece before the wire heats up from the current.

Where does the alternator exciter wire go?

The exciter wire should connect to the ignition and ‘excite’ the alternator field while the starter cranks the engine. After that, the exciter wire is not in function, with the positive battery terminal and the sensing wire taking over.

Does the alternator need a ground wire?

No, the alternator doesn’t need a ground wire because it’s grounded to the engine block.

Are 13.8 volts good for alternators?

13.8 volts is on the edge of what’s considered the minimum amount of voltage for the alternator to properly recharge the battery and supply the system.

Measure the voltage over your battery regularly to make sure it doesn’t drop below 13.8 volts when charging, or you’ll potentially run out of power while driving.

Why is the alternator connected to the engine?

The alternator and starter are connected directly, through a wire junction, or not at all. I cannot tell you exactly why this is the case, but the two modules are completely independent of each other.

The starter uses battery power to crank the engine, while the alternator uses the power of the engine to generate electricity that charges the car’s electricals and the battery.

Are 14.8 volts too high for an alternator?

14.8 volts is right on the edge of okay, with 14.6 – 14.7 volts being the upper limit. We’ve talked about the causes for alternator overcharge, so check that out if you want to make sure your alternator is not failing.

Summary

The 3 wire alternator follows a simple diagram, but unless you know exactly what you’re doing, you can cause some serious electrical issues.

I suggest doing the job with an assistance of a friend, just to make sure you get everything connected correctly. The job itself isn’t particularly difficult, you just need to route the wires correctly, and you’re set!

Get more tips from us by checking out these posts:

5 Reasons An Alternator Belt May Squeal After Replacement – [Solutions]

How to Tell Alternator Amperage by VIN Number

Can You Jumpstart A Car With A Bad Alternator?

What are the 3 wires on a GM alternator?

Does a 3 wire alternator need a regulator?

What is the L terminal on a GM alternator?