Starter relay: teardown to see what causes it to go bad

It's interesting to know how a kick starter relay of a car works by tearing it down to see the internal components.

This blog post is meant to satisfy the curiosity of people who may be wondering what is in the kickstarter solenoid. The solenoid is often a sealed unit such that once it develops a fault, it is simply replaced.

We have cut open one kickstarter solenoid with a hand saw as shown in the image below.

Kick starter teardown

The part that is often problematic which is the high current relay contact which is our area of interest.

Sometimes, when the solenoid fails, we hear only a click sound when we attempt to start the car. This is because the relay contact has failed. The senoid is actuating but at the contacts, there is no closed circuit which is supposed to pass current to the starter motor.

Why is this starter relay necessary?

This relay is there because the kickstarter is meant to work only during engine cranking period. The kickstarter is fed with electrical energy and this energy is then converted to mechanical energy (rotational motion) by the electric motor. The kickstarter turns with a torque (rotational force) that is sufficient to turn the engine and make it to start.

When the engine has started and the key is returned from the "start position", the kickstarter is de-energised and is in a rest state.

The relay feeds power to the armature of the motor through the carbon brushes and slip rings. The power is directly tapped from the battery and it is "hot" (meaning power is always there), hence the need for a relay. If this relay is not there, the kickstarter will continue to turn even after the engine has started.

How does the car starter relay fail?

The internal contacts can burn out causing a very high resistance to current flow or there may be no contact at all causing no passage of current to the starter motor.

The plate which makes contact with the input and output terminals can bend causing lack of contact. This happens especially when there are multiple failed start attempts and it has heated up, hitting the contacts now makes it easier to bend. Remember that metals bend easily when they are hot.


Kick starter solenoid shaft

On the terminals inside the solenoid, there is a softer coating (which looks like a carbon coating). This is there to prevent iron to iron contact that can cause large spark, heat and melting together of the terminals with the plate that short-circuit both terminals. This coating can either detach from the terminal or burn-out.

Kick starter teardown contact

Another way that there can be failure is when the solenoid itself is not working. The solenoid has coil of conductor which when energized, creates a strong magnetic field that is used to move a shaft in the middle in a linear motion. When the coil goes bad, the contact plate which is attached to the shaft cannot move to bridge the two terminals.

Another simple cause of no-start is when there is loose connection at any of the terminals of the starter.

In a state of emergency, a quick way to start the car is to get a wire that can conduct the enough current (not a soft wire) and bridge the two terminals while starting the car. Be careful as this wire can heat up easily while doing this. It can hurt your hand. This wire should be removed immediately the engine starts.

Another temporary solution is to get a contactor (a high current relay) and connect it to function in place of the faulty kickstarter relay until the car is safely taken to a mechanic workshop for proper repair.

Here is what causes starter (the whole starter and not solenoid alone) to go bad.

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