Solenoids are small electric motors or actuators that use magnetic fields to convert electric energy into mechanical power. Modern cars have several types of solenoids, with the most well-known being the starter solenoid.

Your vehicle is composed of many small and large parts that must work together to keep the car running. When one component fails, it can cause a ripple effect that results in further problems or even prevents your vehicle from moving at all. Because of this, it’s important to understand how some of the more essential parts function and learn the signs they display when malfunctioning.

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What is a car solenoid, and what does it do?

A solenoid is an electromagnet that turns an electrical current into mechanical power. While there are many types of solenoids in your car, the most well-known and commonly encountered are the starter and transmission solenoids.

The starter solenoid, also known as the starter relay, is an essential part of your car’s ignition system that allows you to start your vehicle by turning the ignition key. The ignition switch alone does not have enough power to start your engine. Instead, the small electric current that it sends to the starter solenoid is used to open the starter circuit between your starter motor and the car battery, turning on the engine in the process.

On the other hand, the transmission solenoid—sometimes called the shift solenoid—controls the flow of fluid that moves through automatic transmissions. It is usually located in the transmission valve body or the transmission control module. And it uses the signals it receives from the transmission computer to direct fluid to the proper clutch packs and servo valves to control shifting and meet the driving conditions.

Other types of solenoids found on modern vehicles control processes ranging from opening and closing doors to engaging and disengaging four-wheel drive. In fact, fuel injectors—the devices used to deliver a precise amount of fuel to your engine’s cylinders—are themselves a type of solenoid valve. They use electric signals from the engine control unit (ECU) to send pulses of fuel to the cylinders.

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How does a car solenoid work?

In basic terms, solenoids are electromagnetic switches, which is why they are often referred to as solenoid switches. They are composed of coiled metal wires wrapped around a metal rod or plunger and a pair of heavy metal contacts. These parts are usually enclosed in an iron or steel case.

When the starter solenoid receives an electric signal from the ignition switch, the current flows through the coiled wires, and the metal contacts close, opening the starter circuit. The contacts then send a larger electrical current through themselves to the starter motor. The starter motor then takes that signal and begins to start the engine.

When the solenoid is engaged, its plunger forces a pinion into contact with another part called the ring gear or flywheel gear. These two parts lock together, preventing the starter motor from spinning up to unsafe speeds.

The solenoid coils perform specific jobs in three stages:

During the pull-in phase, the ignition switch sends an electric current that generates the magnetic force necessary to pull the plunger down the solenoid core.

When the plunger reaches the opposite end of the solenoid, the two metal contacts close, opening the starter circuit in what’s known as the holding phase.

As the ignition switch is released, the magnetic force diminishes, and the plunger is released. This opens the metal contacts and closes the starter circuit. This is known as the release phase.

Key Takeaway Solenoids use small electric currents to generate a magnetic field. The magnetic field then pulls a plunger down the solenoid and closes two heavy metal contacts. When these contacts close, they open a circuit generating the electricity necessary to perform a specific task—in the case of the starter solenoid, that job is engaging the starter motor and ensuring that the car engine starts.

What are the symptoms of a faulty solenoid?

A bad solenoid can cause a lot of problems for your car. While there are many solenoid valves in your car, the one that most frequently causes problems for drivers is the starter solenoid. Let’s take a look at some of the most common symptoms of a bad starter solenoid:

The engine won’t start. If the engine doesn’t crank, it can be a sign that the solenoid is not sending power to the starter motor.

No clicking sound. A clicking sound is often a sign of a dead battery or a bad starter motor. If your car won’t start and there is no clicking sound, it’s usually a sign of solenoid problems.

The starter motor spins without engaging the flywheel. If you can hear the starter motor spinning, but the engine won’t crank, it could be because the solenoid is faulty and does not have adequate power to engage the starter gear.

The engine starts slowly. Sometimes, high resistance can burn out the metal contacts in the solenoid. Excessive resistance is the result of this problem, and it can lead to slow cranking.

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What happens when a solenoid fails?

Typically, solenoids fail when they don’t receive enough power from the battery. Things like damaged battery cables and a worn-out battery can prevent the solenoid from getting the necessary power.

If your solenoid fails, the best thing to do is take your car to a mechanic. Modern starter solenoids are usually built onto the starter motor, and these two parts are typically replaced together. However, a professional mechanic will have the tools and expertise to test your electric components and any other areas that can cause the problems you’re facing.

If the starter solenoid fails and you absolutely must start your car, it is possible to bypass the solenoid. To do this, you’ll need to locate the solenoid and identify the small terminal that connects to the ignition switch and the large one that connects to the battery. Then, have a friend turn the ignition key while you use an insulated screwdriver or a metal rod to touch both terminals simultaneously.

If your battery and starter motor both work, this process should start the engine. Be sure to remove the screwdriver quickly once the engine starts; otherwise, it could arc weld itself to the terminals.

This method should work to start your car in a pinch. But you should not rely on it to work always, and taking your car to a professional automotive mechanic remains the best and smartest way to resolve a solenoid problem.

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What is the best way to prevent a solenoid from failing?

Since solenoid problems typically are the result of insufficient power from the battery, the best way to prevent failures is to keep up with regular battery maintenance. This means ensuring that your battery terminals are free of corrosion and that the cables are not frayed or damaged.

One easy way to keep your battery components in good shape is to purchase a can of terminal protectant from your local auto parts store. Simply spray a light coat of this chemical on your terminals and cables every few months.

How much does it cost to replace a solenoid?

If your starter solenoid fails, you’ll most likely need to replace both the starter motor and the solenoid. The average cost for this part ranges from $80 to more than $350. When you factor in the cost of labor, the entire job might run you upwards of $500.

Transmission solenoid replacements typically cost between $15 and $100 for a single transmission shift solenoid or $50 to $300 for a pack. Depending on your car’s make and model and your location, the cost of labor will be an additional $100 to $200.

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How to find affordable insurance

Solenoids are subject to wear and tear and, as such, are not covered by insurance. However, if you have a faulty solenoid, the last thing you need is an overpriced insurance policy on top of your repair bill. And the best way to keep your insurance costs low is by comparing rates with the

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FAQs

What is the average lifespan of a solenoid?

Starter solenoids typically last between 100,000 and 200,000 miles.

How is a solenoid tested?

You can test a solenoid by listening for the characteristic clicking sound while a helper cranks the engine. Alternatively, you can use a multimeter to measure the voltage drop.

Can you drive with a faulty solenoid?

Depending on which solenoid fails, you can drive your car with a faulty solenoid. However, you should take your car to a mechanic as soon as possible to prevent the development of worse problems.

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