A starter solenoid (also referred to as a “solenoid switch” or a “solenoid”) is a magnetically actuated switch that is attached to the vehicle start motor, which spins the engine to start. It’s often something to do with the starter motor or the solenoid on a car whose engine can’t normally crank or start.

Starter Solenoid Operations

To better comprehend the solenoid switch operations, I am putting it under the context of engine starter working procedures.

A powerful electric motor used to crank an engine is known as an engine starter motor. The starter motor has a pinion that meshes with the teeth on the engine’s flywheel.


Figure 2. A pre-engaged starter motor structure

The pre-engaged starter motor of modern vehicles uses an electric motor to provide torque to the crankshaft. The power for this motor can be drawn from the battery. An ignition switch (key switch) is installed between the battery and the solenoid to turn the power on and off.

As the key switch is turned on, the solenoid gets energized, and a moving core known as a plunger slides along the coil, which connects two copper terminals of the contactor switch. The circuit from the battery to the motor is now complete, and the rotor starts spinning.

Meanwhile, the starter’s pinion gear is pushed onto the flywheel when the solenoid is de-energized automatically. After the engine starts, the core returns and disconnects the motor from the battery.


Figure 3. How a solenoid switch engages in starting

As the engine is started, the pinion needs to be disengaged from the flywheel to prevent the rear drive of the motor, which might damage it due to excessive speed. This is done by releasing the ignition switch, which de-energizes the solenoid and returns the pinion.

Why is a Solenoid Switch Needed?

You may wonder why the solenoid and the contactor are used instead of directly connecting the battery to the motor and reducing the complexity. The reason behind not using this circuit is the high current demand of the engine. The motor is quite powerful and requires a lot of current to run. The ignition switch must be exceptionally large to accommodate this current. It also requires very thick wires running from the battery to the button and then the motor, which is technically inconvenient. This is why solenoid is used, demanding a lower current for being energized.

Starter Solenoid Wiring Diagram

Solenoid Terminals and Functions

The solenoid switch has three terminals: the S terminal, B terminal, and the M terminal. Now let’s recap the starter operation in the prospect of the solenoid switch. When the ignition switch is on, battery power is sent through the starter relay to the solenoid terminal S. The plunger thrust to meet the other terminals: B and M. Terminal B and C are connected, which provides the current to turn over the starter motor. Thus, the three terminals functions are:


Figure 4. solenoid switch pin out with a yellow solenoid engagement signal wire (source: https://www.idmsvcs.com/)

S (Start) – Connected to the high current power cell to receive energy

B (Battery) – Directly connected to the positive of the battery

M (Motor) – Connected to the windings on the starter motor

How to Wire a Starter Solenoid?


Figure 5. Solenoid control relay wiring (source: carparts.com)

A typical wiring diagram of the solenoid switch is demonstrated above. In this circuit, current flows Battery – key switch – starter relay – starter solenoid.

The leading starter solenoid battery post powers the control relay. Power is transferred to the main starter solenoid switch post when the control relay receives a start signal from the ignition switch. The central solenoid switch and the control relay are grounded back to the starter ground post in the back cap.

The primary advantage of using a solenoid control relay is that the signal to the primary solenoid switch post is stronger. The relay is placed close to the battery source (main solenoid battery post) and then switched to feeding (main solenoid switch post), ensuring a slight power loss as possible due to wiring.

The standard switch circuit can be pretty long, routing from the battery through a fuse panel and the key switch and eventually reaching lengths of several feet. This distance, combined with the connections between, may result in a weak signal when it finally reaches the starter switch post. The central starter solenoid transfers power to the starter and physically engages the starter drive, which necessitates a robust and consistent signal. The use of a control relay ensures that the main starter solenoid receives a strong signal, and because the control relay does not physically move anything, it can function on a weaker signal. This increases the lifespan of the main starter solenoid.

Bad Starter Symptoms and Causes

The starter systems are co-functioned by the switch solenoid, starter motor, starter relay, battery, ignition key, etc. So don’t just blame the starter solenoid or starter motor if you have trouble with the start engine.

Bad Starter SymptomsPossible Causes
A clicking noise is continuously transferred from the solenoidThe solenoid switch’s holding coil keeps holding
The contacts are sealing-off
After being switched on, the starter solenoid makes a clicking noise and moves slightly but won’t spinThe connection between the solenoid and starter is defective
The starter solenoid is faulty
The ignition switch/start button is off; the starter continues to pivot despite the returned pinion gearThe contacts of the solenoid switch hare burned out
The ignition   switch/start button is off; the pinion gear cannot retrieve to the original positionThe solenoid switch has contact burnt
The movable plunger disc and return spring cannot be pulled back
The starter motor doesn’t make any noisesThe starter brushes are worn out
Start motor is running noisilyThe front and rear armature bushings have worn out
The dash lights are dim when you turn the ignition key to the START positionThe battery is low on charge, which may lead to the lack of power for the starter motor to   spin the engine

How to Test a Starter Solenoid?

Before testing the starter solenoid, make sure you have tested the battery and the circuit continuity from the battery to the solenoid. If they work well, then the problem might fall to the starter solenoid itself.

Test with a Screwdriver

Using a screwdriver is a simple method to test a solenoid switch for the clicking noise problem.

If the starter does not rotate after connecting the two terminal posts of the solenoid switch with a screwdriver, the problem is in the starter. If the starter works properly, the problem is in the solenoid switch, which is frequently caused by the corrosion of the starter solenoid contacts.

You can also try shaking the connecting line between the car battery and the starter; if the starter works appropriately, the starting circuit may be in poor contact. If this is the case, you should investigate the high temperature or the sparking parts in the starting system circuit.

Use a Digital Multimeter


Figure 6. you can also dismount the solenoid and test by pushing the plunger to make contacts; it’s of the same testing theory

Make sure the ignition switch on your vehicle is turned off.

Set the ohms setting on your digital multimeter (DMM).

Disconnect the starter cable from the “M” terminal of the solenoid (the terminal that attaches to the cable going to the starter motor).

Connect a one-meter lead to the “S” terminal of the solenoid (the terminal that receives power from the ignition switch).

Connect the other meter lead to the “M” terminal on the solenoid.

The solenoid in your meter is faulty and should be replaced if it reads out of limits.

How to Jump a Starter Solenoid?

You want to jump/bypass a starter solenoid when you get stuck midway because of the start engine and need to temporarily bypass a broken solenoid and drive to the nearest maintenance store. This is not what you do all the time and might cause hazardous conditions to yourself so think twice before you go.


Figure 7. jump a solenoid switch(source: https://mechanics.stackexchange.com/)

Have your park brake engaged.

Get an insulated screwdriver.

Locate the starter solenoid.

Locate the S terminal (to the ignition) and the B (to the battery) terminal.

Place the metal part of the screwdriver on the two terminals simultaneously (create a short circuit between the primary contacts).

Ask a friend to turn the ignition on.

Hopefully, you’ll see sparks, and the motor starts to spin, and the engine starts.

Remove the screwdriver and continue your journey.

Tip: On a non-pre-engaged starter with a Bendix gear (similar to the pinion gear), you can also jump the solenoid by bridging terminal “B” to terminal “M”. This is especially useful if the solenoid fails.


In this blog, we’ve discovered how a starter solenoid works, symptoms of solenoid switch gone defective, and how to test or jump it to start an engine. A typical starter solenoid has three terminals that connect to the battery, the ignition switch/starter relay, and the starter motor. It works with the electric motor to provide the spin trigger for the engine. Testing or jumping a starter solenoid requires specific safety preparations, so beware. I hope this guide helps you someway. If you have any ideas about this blog, please let me know.

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