Starter Motor 101

The Purpose of the Starter

Under normal operation, an engine keeps itself running. It's not quite a perpetual motion machine, since it needs the input of energy in the form of gas, air, and a spark, but combustion in one chamber drives compression in an adjacent chamber via the crank shaft. This is what keeps the combustion cycle cycling and keeps the engine running.

The cycle has to start somewhere, though. As Thomas Aquinas pointed out, a chain of cause and effect cannot stretch infinitely backward. So, there must be some prime mover in your engine. That's the starter.

The starter is a small electric motor with an armature ending in a gear. The engine's flywheel has a ring gear that the starter gear, called the drive gear, engages with. When the starter motor runs, it spins the drive gear, which spins the ring gear and the flywheel. That cranks the engine and starts the combustion cycle.

As a side note, since the starter turns the engine, albeit at very low speed, it would be possible, in a dire emergency, to move a manual transmission car or truck with the starter. You could put the transmission into first, and crank the starter until it slowly, painfully drags the vehicle to safety. This will almost certainly wear out the starter, though. Now, with that being said, if you happen to find yourself stranded on the railroad tracks with the train coming, we would advise getting out of the automobile, rather than trying some trick you read about on the internet once.

A Brief History of Starters

The use of electric starters, much like your engine on a cold day, took a little while to get going. At first, most cars were started by being cranked by hand. This was a difficult and occasionally dangerous process. The cranks used had an overrun mechanism which prevented them from continuing to turn once the engine started - you wouldn't want a crank to start spinning at hundreds of revolutions per minute (RPM) in your hand. The problem was that sometimes these engines would kick back, and rotate the opposite way. Cranks weren't designed for this eventuality and would spin backwards with the engine. Automakers recommended that drivers hold the crank palm up with a cupped hand so that a rebellious crank would slip out of the driver's hand. This grip was fairly unnatural, though, so many drivers used a more familiar overhand, closed fist grip. A runaway crank in your hand could lead to a broken thumb or a broken wrist, and it often did.

Some early cars used other mechanical starting techniques. Some used pull-cords. Kickback in these systems could pull the driver in toward the engine or whip a loose cord around wildly. Some early engines had to be started with a small gunpowder explosion.

Even when they didn't result in danger, these methods of starting were often inconvenient. While a mechanical start, like kick starting a motorcycle or pull starting a snowmobile, can still make sense for small engines today, they quickly became ill suited to rapidly growing automotive engines. Imagine if you had to pull start your four-liter pickup every morning before work.

It was necessary then to invent some way to more easily start a car's engine. The engineers at Cadillac were the first to develop an electric starter motor. It debuted in 1912. The project was pushed along by Cadillac President Henry Leland who had recently lost a friend in a fatal car-starting mishap. Their invention made motoring safer and more accessible to more people, but it still took a while to catch on. The Ford Model T continued to use hand cranks until 1919.

Types of Starters

A couple different types of electric starters have been used throughout automotive history. Their essential operation is always basically the same, but they differ slightly in terms of how they engage the ring gear. These different designs all have to compensate for the high gear ratio between the ring gear and the starter's drive gear.

For the starter to stay compact, the drive gear must be fairly small. It has far fewer teeth than the ring gear. It takes more revolutions of the drive gear to spin the ring gear once. So, it gets the engine cranking at a relatively low RPM level at first. Eventually, though, the engine gets up to speed. If the drive gear stayed engaged to the ring gear it would get spun at very high speed. For each revolution of the engine, the drive gear would spin several times. Just as human engine crankers risked getting their arms broken, a starter would risk getting broken if it didn't have some way to disengage from the ring gear.

There are two methods of keeping this from occurring. In the older style, called inertia starters, the pinion is threaded onto the motor shaft like a nut on a bolt. As the shaft spins, the pinion threads out. There is a stop that holds it once it reaches the end and engages with the ring gear. Once the engine starts turning, the much faster movement of the ring gear essentially spins the pinion back and threads it back down the shaft.

These types of starters require the pinion to start spinning before it engages with the ring gear. When the spinning pinion engages with the stationary ring gear, some wear of the gears can occur. To reduce this wear, pre-engaged starters were invented. In these, current from the battery activates a solenoid that pushes the pinion out, then another current starts the motor. The pinion on these has a one-way clutch, much like the free-wheel mechanism that lets you coast on a bicycle.

Some more recent starters use a process called gear reduction. In these, the pinion is not directly attached to the armature. Instead, the armature ends in a gear, which drives an intermediate gear, which drives a gear on the back of the pinion assembly. The intermediate gear allows for a more favorable gear ratio, which means the pinion spins faster with less drain on the battery. This is used primarily in high-end performance applications.

Ultimately, these different types of starters are largely a historical curiosity. The overwhelming odds are that your vehicle uses a pre-engaged starter. You won't need to know the different types of starters if you need a new starter for your car or truck; you'll simply have to find the starter that is designed for your particular model.

The Starter Motor

At this point, you might be wondering what makes the armature and the ring gear spin. The starter works the same as basically any other electric motor. There are fixed magnets with opposite poles on either side of the armature. The armature acts as an electromagnet. It has two plates, called commutators, one on either side. The commutators receive a charge from the battery via fixed copper or steel brushes that touch them. This turns the armature into a two pole magnet. Each pole of the armature is repelled by its identical-pole fixed magnet and attracted by the opposite pole magnet. Positive charge corresponds to a south pole and negative charge to a north pole. That is to say, the side of the armature with the positive charged commutator will spin toward the north magnet and the side of the armature with the negatively charged commutator will spin toward the south magnet. You may expect that once it got there it would stick, and you would be right, if not for the brushes carrying charge to the commutators. Once the armature has flipped over, the commutators will have switched position, putting them in contact with the opposite brushes. That means they now have the opposite charge and are repelled by and attracted to the opposite pole from what they were before, so they have to flip over again. It continues spinning (like your head may be at this point), until the starter stops receiving electricity from the battery.

Common Starter Problems

Starters may fail to work due to either damage to the pinion (or the internal gears in the case of gear-reduction starters) or malfunctioning of the electrical parts. Generally, you can expect a starter to last for a long time. A starter replacement on a low mileage vehicle would be unusual.

If the pinion gear has worn or broken teeth, it may not mesh properly with the ring gear and won't spin the flywheel fast enough to get the engine started. Worn or broken teeth on the ring gear may also cause this problem.

In order to keep the starter small, the electrical components inside are not designed to be very resistant to heat. As such, the starter can only run for a brief period of time. Cranking the engine for too long (perhaps because you read somewhere that you can move the car with the starter), or not resting the starter before cranking it again can overheat the brushes that carry current to the armature or the wiring in the starter. If this occurs, the armature may not spin fast enough to get the engine started. The electrical components can also simply wear out over time. This is more likely in vehicles that are frequently turned off and restarted, like delivery vehicles.

Problems with the ignition timing could also cause the engine to kick back. Just as kickback injured people on the old hand-crank cars, kickback can damage the starter.

A good sign that your starter isn't working is that your car won't start, but the dash lights turn on when you turn the key to the on position. This tells you that your battery is working, which makes the starter the next likely culprit. You may find that the engine cranks slowly or not at all when you try to start it. It may also slip or make chattering noises. If you can hear the starter motor spinning, but the engine doesn't crank then something is keeping the pinion from engaging properly. This could be broken teeth or, in the case of pre-engaged starters, it could be a problem with the solenoid that engages the pinion.

Do It Yourself Starter Replacement

It will take some effort, but it is certainly possible to replace the starter yourself. The most difficult part of replacing the starter may be accessing it. The starter is generally located underneath the engine near where the transmission engages with the engine. In some cases, the starter may be found on top of the engine near the intake manifold. In most cases, you will have to raise and secure the vehicle and access the starter from underneath.

Be sure to disconnect the battery before working with electrical components, and be sure that the engine has cooled before trying to remove the starter. With all that in mind, you'll simply have to disconnect the wiring to the starter, unbolt it and connect the new one. Be sure you have the correct starter for your vehicle. These may vary depending on the particular transmission and flywheel that your vehicle has.

Need a Starter Replacement?

If you are in need of a replacement starter, you have come to the right place. 1A Auto not only knows everything about automotive starters, but we stock the highest quality, most dependable aftermarket starters available. We carry new starters for many cars, trucks, vans and SUVs, and all at great prices, so that you can get your vehicle back in good working order again!

At 1A Auto, we're here to help you get the right starter replacement for your vehicle. You can browse our large selection of aftermarket starters and shop right here on or, if you have any questions about the product, warranty, compatibility, or simply prefer to order via phone, you can call our customer service toll free at 888-844-3393.


Same Day Shipping

In stock orders ship same day when ordered by 4pm ET. Need your parts faster? Select expedited shipping at checkout.


Guaranteed to Fit

Our team of auto experts work hard to provide you with the highest quality, direct fit replacement auto parts. We know not all auto parts are created equal. This is why we enforce a strict level of product standards and sourcing with every part.


USA Customer Support

Our goal is to exceed our customers' expectations every day. We are a team of passionate auto enthusiasts who are here to help. If you have a question about our parts or a repair contact us.


Instructional Video Library

Our 1A Auto Video Library has thousands of how-to auto repair installation videos specific to year, make and model. Each video guides you with step-by-step instructions to empower you to tackle your own repairs on your car, truck, SUV, or van.