Blog Car Care Advice The Ultimate Guide to the Starter Solenoid (2024)
Car Care Advice

The Ultimate Guide to the Starter Solenoid (2024)

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The starter motor needs a lot of power to crank a modern car.
This means that it needs large amounts of current from your car battery. 

However, large currents need a large switch, and the ignition switch (or starter switch) is too small to handle this. That’s where starter solenoids come in.

But what exactly is a starter solenoid?
And what does a solenoid do?

In this article, we’ll explore starter solenoids and how it functions with the starter motor. We’ll also cover some solenoid related FAQs, including signs of a solenoid going bad.

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Let’s begin.

What is a Starter Solenoid?

The starter solenoid is a powerful electromagnet switch, which is why it’s sometimes called a solenoid switch. It activates the starter motor of an internal combustion engine. 

You might also hear it referred to as a starter relay. However, most modern car models have that title reserved for a separate relay in the starter solenoid’s control circuit these days. 

Inside the starter solenoid are two coils of wire wrapped around a movable iron core and a set of heavy metal contacts. 

A starter solenoid usually has three or four terminals — one or two small connectors and two larger ones.
The small terminals are for the ignition coil and starter control wire that connects to the ignition or starter switch. One large terminal is for the battery cable from the positive battery terminal. And the other large terminal is for the wire that sends voltage to the starter motor itself. 

Next, let’s see how the solenoid works. 

What Does the Starter Solenoid Do?

The starter solenoid has two primary functions — it controls the starter circuit and engages the pinion or starter drive gear. 

Let’s take a closer look:

1. Energizes Starter Circuit

The starter circuit connects the battery to the starter motor. The solenoid acts as the on/off switch for the starter motor circuit — controlling the burst of electrical current from the battery.

The solenoid itself is controlled by a control circuit, which links it to the ignition switch.

When you activate the ignition switch, the car battery energizes the control circuit. A small current flows from the battery to the starter solenoid, creating a magnetic field around the solenoid coils. 

The magnetic field draws a plunger down the center of the coils, pushing the starter solenoid contacts together. This bridges the gap between the battery and the starter motor, allowing voltage to reach the starter motor.

Note: Here’s the typical flow of current for a control circuit wiring diagram: 
Battery ➜ Ignition Switch ➜ Starter Relay (connected to a neutral safety switch) ➜ Starter Solenoid

2. Engages Starter Pinion Gear

As the solenoid coils pull in the plunger, a lever fork attached to the end of the plunger pushes out the starter gear or pinion gear. This movement meshes the small pinion gear with the larger engine flywheel ring gear. 

The pinion gear is connected to the starter motor, which cranks the engine (via the flywheel) once it receives battery power. 

It’s important to note that the solenoid coils perform a specific task. 
Here’s a breakdown:

What Does Each Solenoid Coil Do? 

When the ignition switch is activated, battery power flows to a strong pull-in coil and a weaker holding coil.
The coil’s functions can be divided into three stages:

1. Pull-in

The pull-in windings generate the magnetic force to draw the plunger down the solenoid core. 

2. Holding

When the plunger reaches its end of travel, it pushes the heavy solenoid metal contacts together, letting battery current flow to the starter motor. 
This action also disconnects the pull-in windings. 

The electrical current flows through a shunt to the holding windings only — saving some power, delivering more to the starter motor, and reducing heat buildup.

3. Release

When the ignition switch is released, the magnetic force is reduced, and the holding coil releases the plunger. The starter solenoid contacts open, cutting battery power from the starter motor.

We’ve seen how each solenoid coil functions and know that the solenoid pushes out the pinion gear to crank the engine. 

But what happens during engine cranking? 

How Does a Solenoid Work with the Starter Motor?

The internal combustion engine requires external assistance to start, which is what the starter motor does. 

Here’s what happens you turn on the ignition key:

The starter solenoid receives a small current from the 12 Volt battery. It engages the starter pinion gear with the flywheel ring gear and closes the starter motor circuit — sending battery voltage to the starter motor.

  1. With battery power available, the starter motor armature rotates the starter drive shaft and the pinion gear attached to its end. The pinion spins the flywheel, cranking the engine.
  2. As the flywheel speed picks up, the engine starts, and the pinion gear disengages. The pinion usually has a one-way sprag clutch that allows it to spin independently of the starter drive shaft once the flywheel moves faster, preventing backdrive. 

Note: Backdrive is when the flywheel ‘drives’ the pinion instead of the other way around and can cause damage to the pinion gear and starter motor.

Now that we’ve covered the starter solenoid basics, let’s review some FAQs. 

9 Starter Solenoid FAQs

Here are the answers to some frequently asked questions on the solenoid:

1. Where is the Starter Solenoid Located?

If you pop open your engine compartment, you can follow the positive battery cable to the starter motor. The starter motor is often bolted to the engine or transmission, and the solenoid is almost always attached to it.

2. What Parts Does a Starter Solenoid Contain?

A typical starter solenoid consists of the following parts:

3. What are Common Starter Solenoid Problems?

Here are a few common ways the starter solenoid fails:

4. What are the Signs of a Faulty Starter Solenoid?

A faulty starter solenoid can exhibit several symptoms, including these:

Note: A repetitive clicking noise coming from the engine compartment during startup, instead of a single one, can indicate a dead battery or electrical system problem. It’s unlikely to be a starter problem.

5. What Symptoms Can Mimic a Bad Starter Solenoid?

Some symptoms from other issues can mimic those of a bad starter solenoid. If there’s no problem with the solenoid, you could be looking at these problems instead:

6. What are the Terminals on a Starter Solenoid?

A typical starter solenoid can have 3 or 4 terminals on its insulating cover — two large ones and one or two smaller ones.

The two larger terminals are usually copper bolts:

The smaller terminals are typically iron bolts:

The solenoid shell also acts as an invisible grounding terminal.  

7. How Can I Bypass the Starter Solenoid?

One way to check if you have a solenoid or starter motor problem is by bypassing the solenoid using an insulated screwdriver. 
Here’s what to do:

1. Locate the Control and Starter Motor Terminals

Find these two metal terminals on the starter solenoid:

  • A small one that connects a wire to the ignition switch (terminal S) 
  • A large one that connects the solenoid to the starter motor (terminal M)

2. Short the Terminals With the Screwdriver

Place the metal blade of the insulated screwdriver across both metal terminals. Doing this bypasses the solenoid, creating a direct connection between the ignition switch and the starter motor.

3. Turn on Ignition

Get a second pair of hands to turn on the ignition key
Because the solenoid is bypassed, the engine won’t start, but the starter motor gets some power to run at lower speeds.

4. Listen to the Starter Motor

Listen for starter motor sounds. 
If there’s steady humming, the motor is fine, and the solenoid is likely defective. If the motor doesn’t start or sounds choppy, then the motor likely has problems.

If this is too much of a hassle, it’s probably simpler to call a mobile mechanic to troubleshoot your starting system. 

8. How is a Failing Starter Solenoid Checked?

To check if the solenoid appears faulty, your mechanic will generally:

9. How Can I Get My Solenoid Fixed?

Bad solenoid symptoms often seem similar to starter motor or battery issues. 
To get your solenoid fixed and ensure your starting system is healthy, it’s always best to let a professional deal with it. A mobile mechanic is an even better option as they can come to you

With that in mind, the easiest thing to do is contact AutoNation Mobile Service!

AutoNation Mobile Service is a convenient mobile auto maintenance and repair solution. 
Here’s how you’ll benefit:

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Final Thoughts

The starter solenoid is a small but powerful relay switch that’s essential to your vehicle’s startup. It delivers up to 200A (and sometimes even more!) each time the ignition is turned on. Though the solenoid is a resilient component, you can expect it to fail at some point with that kind of frequent loading. 

And if your solenoid does fail, don’t worry.
AutoNation Mobile Service is readily available to help you deal with any solenoid and starter motor problems.

Just contact us, and our expert mechanics will be at your driveway in no time!