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Summary
  • A starter-mounted solenoid has three terminals with three connections一these are the battery (B), start (S), and motor (M) terminals.
  • A typical starter uses a solenoid to generate a specific amount of horsepower for a limited time.
  • As you turn the ignition to the “start” position, voltage is sent to the S terminal, energizing the solenoid’s electromagnetic windings.
  • The windings create a magnetic field that pushes a disc against the B and M contacts, producing an electrical connection that cranks the engine.

The starter motor cranks the engine to get it going—you know that much. But do you truly understand how the starter and the rest of the starting system work?

How Are Starters Wired?

The positive cable links the starter solenoid to the (+) battery terminal. Meanwhile, the negative or ground cable connects the transmission or engine cylinder block near the starter to the negative (-) battery terminal.

how car starters are wired
Learn how your system is wired and how it works. Newer vehicles will use a starter relay that is activated by the PCM/ECM. | Image Source: Richard McCuistian.

How a Starter Works

Nearly all internal combustion-powered vehicles (with the exception of some hybrids) use a starter motor to crank the engine.

The starter, which operates with the help of a solenoid, can generate a significant amount of horsepower for a limited time. On most vehicles, the solenoid is mounted on top of the starter.

diagram of a resting and spinning starter motor
Diagram of a resting and spinning starter motor | Image Source: Richard McCuistian.

There are two common types of starters—direct-drive and gear-reduction—used in automotive applications. Both designs operate in a similar manner:

  • When the driver turns the ignition key to the “start” position, the solenoid engages a plunger, which, in turn, acts on a lever fork inside of the starter.
  • The fork then pushes the starter’s pinion gear into mesh with the ring gear on the engine’s flywheel or flexplate.
  • The solenoid’s plunger pushes a disc against a set of contacts. Current flows from the battery to the starter once the contacts connect.
  • Current then passes through the starter’s insulated brushes, which ride on the commutator portion of the armature, before entering the field coils and the armature windings.
  • The current flowing through the field coils and armature creates a magnetic field that causes the armature to spin.
  • The commutator continuously switches the polarity of the circuit to keep the armature spinning in the same direction.
  • If the starter has a direct drive, the spinning armature turns the pinion gear directly. But if the starter has a gear drive, the armature drives a set of gears that turn the pinion gear.
  • In either scenario, the starter’s pinion gear rotates the engine’s flywheel. Because the flywheel is bolted to the engine’s crankshaft, the internal engine components (i.e., pistons, camshaft, etc.) are set into motion.
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Once an air-fuel mixture and spark are added to the equation, the engine begins to run.

The driver can then move the ignition key to the “run” position to disengage the starter motor. If the driver fails to do so, the starter has a one-way clutch that will allow it to freewheel. That way, the starter doesn’t get damaged by the spinning engine.

Check out this video to know more about how a starter works:

Direct-Drive Starter vs. Gear-Reduction Starter

With a direct-driver starter, the starter motor and gear turn at the same speed on the same shaft.

With a gear-reduction starter, the starter gear has more torque despite spinning slower than its counterpart in a direct-drive setup. This is because the starter lowers the RPMs from the starter motor before it reaches the gear.

Gear-reduction starters are smaller, lighter, and more efficient than their direct-drive counterparts, helping batteries last longer.

How a Typical Starting Circuit Works

a typical starter solenoid circuit
An example of a typical, non-computer controlled starter wiring diagram.

Now you know how a starter motor works. But what about the rest of the starting system?

Starting circuit operation is fairly straightforward. When the driver turns the key to the “start” position in a typical starting system, battery voltage flows from the ignition switch to an underhood relay.

As long as the neutral safety switch is in the park position (or the clutch safety switch is closed), the relay closes, allowing voltage to flow to the starter solenoid. The solenoid then engages the starter to crank the engine.

modern starter solenoid wiring diagram
An example of a modern computer-controlled starter solenoid wiring diagram.

It’s worth noting that, on many modern vehicles, the ignition switch and neutral safety switch (or clutch safety switch) are not wired directly into the starter circuit. Instead, these switches act as inputs to control modules—usually the body control module (BCM) and the transmission control module (TCM).

The BCM and TCM communicate the position of the switches to the engine control module (ECM) over a data network. If the conditions are correct, the ECM then operates the starter relay to supply power to the starter solenoid.

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What are the Different Starter Solenoid Terminals?

starter solenoid terminals with labels
Starter solenoid terminals: B = battery terminal, S = start terminal, M = motor terminal

Many DIYers wonder: What wires go to the starter solenoid? Typically, a starter-mounted solenoid has three terminals with three connections:

  • The “B” or “battery” terminal: The terminal that connects the solenoid directly to the positive battery cable.
  • The “S” or “start” terminal: The terminal that receives power from the ignition switch.
  • The “M” or “motor” terminal: The terminal that attaches to the cable that connects to the starter motor.

When the driver turns the ignition key to the “start” position, voltage is sent to the starter’s “S” terminal. That voltage energizes the solenoid’s electromagnetic windings.

The windings then create a magnetic field that pulls the starter’s plunger, forcing it to push a disc against a set of contacts. Those contacts attach to the solenoid’s “B” and “M” terminals. Once the two terminals have an electrical connection, current can travel directly from the battery to the starter motor to crank the engine.

FAQs About Wiring a Starter

A licensed mechanic is your best source of information about wiring a starter. If you want to learn more but don’t have time to stop by your auto repair shop for a consultation, you might find the answers to these questions helpful.

Where Does the Starter Get Power?

The starter draws power from the battery through thick cables that can channel high electric current.

What Wire Goes Where on a Starter Solenoid?

Two cables usually connect the battery to the starter: a red wire and a black or greenish-yellow one.

The red wire connects the battery’s positive terminal to the solenoid, while the other cable connects the battery’s negative terminal to the motor.

The colors vary depending on the vehicle’s year, make, and model.

Can You Connect a Starter Directly to the Battery?

Yes, you can. It’s one of the ways mechanics check starters. They bypass the car’s electrical system using jumper cables.

First, they turn off the ignition and put the transmission in park mode. Then, they connect one end of the positive cable to the battery’s positive terminal and the other end to the starter motor’s positive terminal.

Mechanics determine issues based on how the parts react. For example, if the starter cranks the engine, there might be something wrong with the cables or relay.

What Happens If You Wire a Starter Solenoid Wrong?

A high electric current might surge through the solenoid due to faulty control or starter circuit connections. The heat could merge crucial parts and lead to starter solenoid problems.

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What Happens If a Starter Is Not Grounded?

Your car will struggle to start if there’s loose ground to your starter.

Where Does the Starter Ground Wire Go?

A starter ground cable often goes from the starter housing to the transmission case and the engine block. In some vehicles, it skips the transmission case, running from the housing to the engine block.

What Are the Signs of a Bad Starter?

Your vehicle might not be firing up due to a bad starter if you notice these telltale signs:

  • Engine won’t start
  • Engine cranks slowly.
  • Engine intermittently fails to start.
  • Whirring or grinding noises when starting the engine.

Check out this guide to learn what causes each bad starter symptom.

Note that a faulty starter solenoid can exhibit similar signs, so it’s best to have a mechanic check your vehicle to pinpoint what’s causing the issue.

Check out these videos on how to replace your starter:

Get a Starter and Starter Solenoid That Fits Your Car

Knowing the wiring arrangement for the starter and starter solenoid is invaluable if you have to replace those parts by yourself. Of course, you have to get new parts that fit your vehicle. CarParts.com understands this, so we make it easy to find replacement starters and starter solenoids that fit your car.

CarParts.com offers high-quality replacement parts like starters and starter solenoids. Plug your car’s details into our website’s built-in vehicle selector, and you’ll see our products that meet your requirements. After finding the part that suits your needs and fits your budget, place your order with only a few taps on your phone. If you live in the continental US and order before noon ET, you can expect your new starter and starter solenoid to arrive in as fast as two business days.

So what are you waiting for? Check out our selection of starters and starter solenoids and order from CarParts.com today!

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About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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Jeffrey

My car has two wires connected on the silonoid plug. On the starter silonoid as well as the positive and the ground. When I try to start it I hear a click coming from the boxes under the dashboard, passenger side interior

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