A starter solenoid is a component of the ignition system on your vehicle.
In order to get your engine running, a small but powerful electric motor - the starter - must engage with the engine. A gear on the starter articulates with a large gear on the engine flywheel. When you turn on the ignition by turning the key or pressing the “start” button, the starter gear rotates the flywheel gear, turning the crankshaft over at about two hundred revolutions per minute.
The starting process (and the starter) require a great deal of energy. This energy comes in the form of electricity from your vehicle’s battery. In most cases, the starter is connected directly (or nearly so) to the battery with a heavy duty wire or cable. Unfortunately, the wires inside your ignition switch are not nearly heavy enough to carry all that power. Instead, a solenoid is used.
The starter solenoid is essentially a heavy-duty electromagnetic switch that can be switched or activated by another, smaller power source. In this case, the high powered side of the solenoid switch is placed between the battery and the starter. The low side is connected to the ignition switch. When you turn the key or press the button, the small circuit inside the solenoid closes the large circuit, and the starter engages. On some vehicles, another (and similar) device, a starter relay, is used to trigger the solenoid. There are more details to the starting and charging system, including computer module control and more, but that is the basic purpose of the starter solenoid.
A starter solenoid and a starter relay serve similar functions: to allow a low-power electrical signal (such as from an ignition switch) to engage a larger, high-power circuit (for the starter). A starter solenoid, however, is larger and handles more current than a relay. Both components can exhibit similar symptoms if they fail. If your starter solenoid goes bad, you might notice one or more of the following:
The starter solenoid on most modern engines is mounted directly on the starter motor on the engine. While it is possible to replace the solenoid separately from the starter, in most cases the two are replaced as an assembly. Because the starter must be removed from the vehicle in order to replace the solenoid, the extra labor involved is usually not worth the cost. Instead, most repair shops will recommend a new starter and solenoid.
The starter (and solenoid) is mounted on the side of an engine, near the transmission. In order to replace the starter, a technician must be able to gain access to the component. This usually requires that the vehicle be safely lifted and supported in the air. The negative terminal on the battery must also be disconnected to cut power to the starter before disassembly begins.
From there, a technician will disconnect the wires connected to the starter, remove the bolts that hold it in place on the engine, and dislodge the old starter. It is common for the technician to also mark the flywheel on the engine and rotate it 360 degrees to check the teeth on the flywheel gear for damage. The new starter is then inserted into its mount and bolted in place. On some older vehicles, the starter might need to be shimmed to align properly, but most modern vehicles do not require this step. The wires are reconnected and the vehicle is placed on the ground before the starter is tested.
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