How to get rid of dirt in the starter motor once and for all

Starter motors have a hard time in the day-to-day running of a vehicle: they have to ensure that the combustion engine goes from stop to self-sustained operation in a very short time and in all circumstances. And they even carry out this task under adverse environmental conditions—whether in winter, in the heat, or, depending on the application, with a lot of humidity or dust.

Different types of starter motor: Sealed noseless (left) und nosed starter motor (right)

In addition, starter motors are installed in such a way that their front part engages with the ring gear of the flywheel, meaning that there could also be contamination from abrasion and clutch dust here. All of this can cause problems for the starter motor, but only if it isn’t designed to cope with it! Starter motors can be constructed in different ways to deal with the entry of dirt, depending on their use and field of application. This is because the starter motor’s actual field of application determines the amount of dirt it is exposed to. For example, the starter motor of a wheeled loader used for mining operates under totally different conditions to the same model that is fitted in a truck for long-distance hauling.

There are therefore three different types of starter motor:

  • Nosed starter motor
  • Nosed starter motor with wiper seal/brush (dust protection)
  • Sealed noseless starter (dust proof)

Causes of defects

—why the starter motor goes on strike

What you need to know about the cause and effect of dirt deposits:

Badly clogged freewheel clutch

Dirt in the front part of the starter motor:

Dirt in the overrunning clutch causes the grease in the freewheel clutch to bind. This can make the overrunning clutch so hackly under certain conditions that the starting process causes the whole starter motor to rev up with the engine. Or the freewheel clutch has no power in either direction and slips through as a result. So, although the starter motor turns, it doesn’t drive the combustion engine.
If the grease binds the dirt to the shaft, then the pinion linked to the freewheel clutch can no longer engage or disengage smoothly in the ring gear. Depending on the contamination level, the solenoid no longer has enough power to push the pinion into the ring gear.

Chips und soil blocking the engaging lever (arrow)

Dirt in the engaging lever area:

Sometimes the soiling is so severe that the dirt penetrates right to the engaging lever and can even partially block it. As a result, the pinion cannot engage far enough.
The consequence is that the contacts in the solenoid no longer close, the starter motor doesn’t turn—and the engine doesn’t move a single millimeter.

Corrosion on the electric motor caused by water ingress in the starter motor

Dirt in the motor component of the starter motor:

Depending on the level of soiling in the motor component of the starter motor, a layer of dirt may form on the commutator. This can lead to increased contact resistance between the commutator and the carbon brushes, with the result that the starter motor no longer provides the full power output.

Fluids in the starter motor:

The starter motor is protected against water spray, depending on the protection class. However, the starter motor reacts badly to aggressive substances and those containing oil, such as engine oil, gear oil, hydraulic oil, or other consumables. Malfunctions can arise if larger quantities of these liquids penetrate the starter motor—right up to the complete failure of the starter motor.

Neat tips for a strong start

And finally, tips for preventive measures in your workshop:

When replacing the starter motor, make sure that you clean the surrounding area thoroughly. Check that connections and openings on the bell housing are closed to prevent dirt entering. Ideally, you should fix any leaks in or around the engine immediately. Depending on the field of application, you can also use a high-pressure cleaner to wash the motor occasionally—but please do it carefully and from an appropriate distance! You don’t want to end up causing water damage to the starter motor.


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


MPULSE - The magazine

The print issue of MPULSE will come out twice a year.