Solenoid Switch: What is it, How does it work and what are its failure symptoms

The definition of a solenoid switch could be as follows: an electromechanical valve that is commonly used to control the flow of liquid or gas. In a car, solenoids are present in many of the main parts, such as the injectors or the starter motor. It should be noted that the generic name is usually used to refer to the starter solenoid, also known by other names such as contactor, automatic, traction relay, traction solenoid or starter relay.

The solenoid switch that most of us will be referring to, is the starter motor. The screw terminal is located at one end where it connects to the battery via a highly reinforced cable. On the other side of the solenoid coil, a link inside the housing would engage the visible pinion gear to the starter motor itself. In order to understand it better, we are going to describe in more detail how it works, where it is used and what its symptoms are when it is damaged.

How does a solenoid work?

A solenoid switch is a device made up of a corkscrew-shaped coil of wire, the housing, and a movable plunger (armature). When an electrical current is introduced, a magnetic field forms around the coil which attracts the plunger. More simply, a solenoid converts electrical energy into mechanical work. Electromagnets have the advantage that they can be turned on and off by the application or removal of electrical current.

Broadly speaking, the function of a solenoid is to make the crankshaft start to rotate so that the engine block comes into operation when the ignition key is activated on the steering column, or by means of the button on the dashboard. The cylinder chamber receives fuel and air to run the four strokes of a traditional internal combustion mechanism (intake, compression, explosion and exhaust) and, when the key returns to the ignition position, the electricity stops passing through the solenoid and stops acting.

The coil is made of many turns of tightly wound copper wire. When an electrical current flows through this wire, a strong magnetic field/flux is created. On the other hand, the casing, usually made of iron or steel, surrounds the coil, concentrating the generated magnetic field. The plunger is attracted to the stops (in a continuous push and pull) through the concentration of the magnetic field that provides the mechanical force to do the work.

Its body is usually made of plastic, aluminum, brass or stainless steel. These elements are chosen because they are non-magnetic and can work with the fluids or gasses they regulate. Additionally, they can run on alternating current (AC) or direct current (DC) , making them versatile for many machines. We won’t go into the detailed equations, but we can roughly say that the smaller the solenoid, the less force they can operate.

Where are solenoids used?

Solenoid applications include a wide range of industrial settings, including general on-off control, process control loops and systems, and various original equipment manufacturer (OEM) applications, to name just a few. Solenoid valves can be found in many different industries , including everything from water supply and treatment (drinking, waste, black, and gray) to tanks and transportation facilities.

Gas lines also use solenoids to control the flow of natural gas. We know that a leak is no joke, and being able to turn off the gas quickly increases safety and reduces hazards. Dishwashers and washing machines also use solenoids to ensure there is enough water to clean dishes and clothes, for example. As far as we’re concerned, automotive applications for solenoids come down to turning on a switch , like the aforementioned car starter.

What symptoms can a damaged solenoid give?

If the starter motor solenoid is damaged or simply past its useful life, the engine will not start when you turn the key . It is likely that the electricity will not reach the solenoid and the electromagnetic field that will allow the motor to wake up will not be produced. If this happens, it is important to rule out that it is not due to a battery with a low state of charge . To rule out a malfunction of the solenoid we can carry out the following test.

First of all, it is important to mention that a solenoid is a fairly delicate element that must be treated with great care so as not to damage it. To check that it works, you have to use battery clamps , locate the starter motor and the solenoid itself, which is usually found on it. The negative clip should be placed on the starter housing while the positive clip should lightly touch the positive terminal of the solenoid. The idea is to simulate what would happen when you turn on the ignition key.

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