Starter solenoid
A starter solenoid is the part of an automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 which relays a large electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 to the starter motor
Automobile self starter
A starter motor is an electric motor for rotating an internal-combustion engine so as to initiate the engine's operation under its own power.- History :...

, which in turn sets the engine in motion. In many vehicles the solenoid also engages the starter pinion with the ring gear
Starter ring gear
A starter ring gear, sometimes called a starter ring or ring gear, is a medium carbon steel ring with teeth that is fitted on the periphery of a flexplate or flywheel of an internal combustion engine, mostly for automotive applications....

 of the engine.


The starter solenoid receives a large electric current from the car battery
Car battery
An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery to power the starter motor, the lights, and the ignition system of a vehicle’s engine...

 and a small electric current from the ignition switch
Ignition switch
An Ignition switch is a switch in the control system of an internal combustion engined vehicle that activates the main electrical systems for the vehicle...

. When the ignition switch is turned on, a small electric current is sent to the starter solenoid. This causes the starter solenoid to close a pair of heavy contacts, thus relaying a large electric current to the starter motor, which in turn sets the engine in motion.

The starter motor is a series
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...

-wound direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

 with a solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

 (similar to a relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

) mounted on it. When low-current power from the starting battery is applied to the starter solenoid, usually through a key
Key (lock)
A key is an instrument that is used to operate a lock. A typical key consists of two parts: the blade, which slides into the keyway of the lock and distinguishes between different keys, and the bow, which is left protruding so that torque can be applied by the user. The blade is usually intended to...

-operated switch, the solenoid closes high-current contacts for the starter motor and it starts to run. Once the engine starts, the key-operated switch is opened and the solenoid opens the contacts to the starter motor.

Most modern starters also rely on the solenoid to engage the starter pinion
A pinion is a round gear used in several applications:*usually the smallest gear in a gear drive train, although in the case of John Blenkinsop's Salamanca, the pinion was rather large...

 with the ring gear of the flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...

. When the solenoid is energized, it operates a plunger or lever which forces the pinion into mesh with the ring gear. The pinion incorporates a mechanism such that when the engine starts and runs faster than the starter motor the pinion is forced to unmesh.

Some older starter designs, such as the Bendix drive
Bendix drive
A Bendix drive is a type of engagement mechanism used in starter motors of internal combustion engines. The device allows the pinion gear of the starter motor to engage or disengage the flywheel of the engine automatically when the starter is powered or when the engine fires, respectively...

, used the rotational inertia of the pinion to force it along a helical groove cut into the starter driveshaft, and thus no mechanical linkage with the solenoid was required.

The starter solenoid can be located under the hood of a car by following the positive (red) cable from the battery, which usually leads directly to the solenoid. Then, the solenoid will have another cable of similar or equal weight which will go down to the starter, which is normally accessed from the bottom of the vehicle. The solenoid will also have a third (smaller) wire, which comes in from the starter switch. Starter solenoids can also be built into the starter itself, often visible on the outside of the starter.


If a starter solenoid receives insufficient power from the battery, it will fail to start the motor
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

, and may produce a rapid clicking sound. The lack of power can be caused by a low battery, by corroded
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

or loose connections in the battery cable, or by a damaged positive (red) cable from the battery. Any of these problems will result in some, but not enough, power being sent to the solenoid, which means that the solenoid will only begin to push the engagement gear, making the metallic click sound.
Starter solenoid problems are best diagnosed by an experienced auto-electrician.
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