Ready, set, go

Tips for getting off to a good start

The key to a good ride is … a good start, of course. You’ll find everything you need to know about the starter motor right here.

The starter motor—how it works

To get the starter motor running, a solenoid is supplied with electrical current from the ignition switch. The solenoid attracts an iron core (plunger), which mechanically engages the starter pinion with the ring gear on the engine flywheel. At the end of its stroke movement, the iron core simultaneously closes the contact bridge, which switches the starter motor on. In most types of starter motors, the starter motor uses a reduction gear to crank the engine.

The starting process is complete as soon as the combustion engine reaches its cranking speed and starts. At this point, the ignition switch stops feeding current to the solenoid. A spring returns the iron core to its starting position. This opens the contact bridge, cuts off the supply of electrical current to the starter motor, and returns the starter pinion to its starting position. Because the engine speed increases to idle speed as the starting process ends, but the pinion is still engaged with the flywheel, a free-wheel clutch prevents the starter motor from overspeeding.

To see how this works, click on this link:


Out of whack:

too much force ruins the starter motor, of course

When the engine is switched off, it continues to spin in the running direction for several seconds before coming to a stop—how long depends on its size, weight, and add-on parts (alternator, air conditioning compressor, power-steering pump, etc.). Just before the engine stops, however, the compression in the respective cylinders causes the engine to spin not only in the running direction, but also briefly in both directions.

If the engine is started again at this point, the force of the starter motor is amplified by the force of the engine spinning in the opposite direction, applying significantly higher forces to the components than during a normal starting process. Depending on the engine and the type of starter motor, this mechanical overload causes damage ranging from deformation to fracture of components such as the pinion, shaft, flange, and free-wheel clutch.

Broken components:

>> This phenomenon is called mechanical overload due to starting when the engine is coming to a stop.

And this is what it looks like on the test bench:


Back on track:

how to fix a damaged starter motor

The starter motor is supplied with working current via the battery cable (B+) directly from the battery at terminal 30 (continuous plus). The starter motor also receives control current from the battery at terminal 50 via the ignition switch. Because there is no protective mechanism for the starter motor built into this switch, it is possible for the starter motor to be started in the engine both when the engine is running and when it is coming to a stop. In addition, even when the starting process is successful, the starter motor can be energized for too long, causing the starter motor to pick up speed along with the engine and spin out of control.

Perfect start thanks to a blocking relay

This can be remedied with a starting/blocking relay as shown above. This relay is installed between the ignition switch and the starter motor’s terminal 50. It is also connected to the alternator at W. When the driver turns the ignition switch, working current is fed to the starting/blocking relay. This switches the relay, which supplies the starter motor with control current via terminal 50. As soon as the engine reaches the point of self-sustained operation, the alternator also starts to generate current and sends a signal via W to the starting/blocking relay. This disconnects the control current connection at terminal 50 and prevents the starter motor from being energized for too long.

The relay also receives a continuous signal from the alternator while the engine is running, making it impossible to start the running engine. If the engine is switched off at this point, this signal from the alternator to the relay is interrupted. The relay, however, has an integrated time delay (which can also be adjusted manually, depending on the model), by means of which it interrupts the connection to terminal 50 for several seconds after the engine has stopped. This ensures that the engine cannot be inadvertently restarted before coming to a complete stop.

More information about thermal load.... can find in our Technical Messenger "Starter motor failure due to overload"


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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