Avoid overheating your starter motor

As the weather warms, some owners' thoughts will turn to how to keep the cabin cool. Others of older cars especially may be concerned about engine overheating and, therefore, their attention, may be focussed on their cooling systems. Yet, what about the electrics?

What is a starter motor?

Unlike steam engines, modern internal combustion engines cannot start themselves without assistance. Virtually every petrol and diesel engine (mild-hybrids included) possesses a starter motor. On activating the ignition key, or push-button, to start the engine, the starter motor's main pinion teeth engage with those of the flywheel.

The main electrical contact bridge closes and power is sent to the main motor. Therefore, when the motor's armature starts to rotate, so does the flywheel and, therefore, the engine. Once the engine starts, power is cut to the motor, and the teeth disengage.

How do starter motors wear?

Being an electrical component, starter motors do not last forever. Eventually, it will need replacing but, in normal use and provided that it is not damaged by external factors, a starter motor should last for over a decade and 100,000 miles. Eventually, the internal brushes and bushes wear out, necessitating either repair, or replacement.

Inside the motor unit are several sets of coiled wire, which carry the main current required. As with any electrical coil, they heat up. Yet, as the starter motor is designed to be used for short periods, excessive heat-build tends not to be a problem.

However, should the battery be in a low state of charge, the slower starter motor speeds can increase the heat build within the motor, increasing the risk of failure. The low voltage spike can also cause problems with other electrical systems. More

details about this issue can be found in our blog: https://www.motoringassist.com/news/replace-your-battery-before-it-damages-your-car.

How to stop your starter motor from overheating?

However, the solenoid is also vulnerable to overheating and the cause is down to abuse. This small component is mounted on top of the main motor and ensures that power reaches the main motor, as well as enabling the motor pinion to engage with the flywheel. Electrical experts at Mahle confirm that this solenoid can be damaged by owner misuse but how?

Mahle confirms that, if the starter motor is operated repeatedly in quick succession over an extended period without sufficient breaks, the windings of the solenoid warm up. Repeated instances of use in this way can cause the solenoid to heat up so much that the winding burns out.

To avoid this, the starter motor should not be activated more than thirty seconds, prior to being given at least two minutes to cool. If the starter is activated more than three times in quick succession, Mahle advises that you give it longer between attempts so that the pent-up heat can dissipate.

Naturally, an engine that either fails, or has become difficult to start, is a typical cause of starter motor overheating but not the only one. DIYers and garages can also use the starter motor excessively during certain maintenance operations, such as bleeding diesel fuel systems after replacing the filter.

If your starter motor stops working, you can check if the solenoid's coils have failed, by checking for an open circuit situation, using a multimeter. Replacing the solenoid should be within the remit of most competent DIYers, so a garage should have no problem.

With the old coil removed, note the colour of the paper that is wrapped around the coil. The picture above shows a new unit, with its bright white paper, against the discoloured and charred wrapping from a pair of overheated solenoids.

Should you witness this, suspect that the main starter motor may also bear internal overheating damage.

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