Why your Car’s Alternator is Hot when Car not Running

As the engine runs, it is normal for the engine temperature to rise.

What is unusual and had to catch, is when the car alternator feels warm even when the engine/ ignition switch is off. In fact most car owners stumble upon it by accident.

Is this normal?

What impact does it have on the performance and longevity of the alternator ?

These are probably some of the several questions you may have going through your mind.

So, in this post, I shed light on the possible cause of your alternator heating up even when the car engine is off, how this affects the car and how you may be able to fix this issue.

Quick answer: If the car alternator feels hot when touched while the nearby engine bay parts are cold and the engine is not running then quite likely there is a problem with the alternator regulator or diodes that is allowing the flow back of battery power into the alternator.

Is It Normal for the Alternator to become Hot when Not Running?

If the rest of the engine bay is cool when you touch it and yet the car alternator is warm or hot, then most probably there is a problem with the alternator.

This is not normal – the alternator should be cool when you touch it like the rest of the components in the engine bay.

Why the Car Alternator May Be Hot with the Ignition Off

Faulty diodes

If the alternator has faulty diodes then there can be back flow of current from the battery to the alternator which builds up as heat in the alternator windings.

On the other hand, an alternator with good functioning rectifier diodes restricts the reverse flow of current into the alternator.

Note, if the diodes are faulty, it is still possible for the alternator to charge the battery only that there’ll be a back flow of current to the alternator.

The diodes in the alternator can be damaged when the battery terminals are accidentally reversed or if the battery cables are disconnected when the engine is running.

For more information on what can cause the diodes in the alternator to fail check this post.

Risk Faced with an Alternator that Heats up

Battery drain and potential battery failure

If the alternator is hot, then the battery power will be drained over time. It is possible for the battery to be drained flat and the car unable to start. In extreme cases, the car battery may be damaged and unable to retain its full capacity.

How to Fix an Alternator that gets Hot

Replacing the rectifier or alternator

If your alternator gets hot even when the rest of the engine bay is cool, then it is likely faulty.

You can have the alternator tested at an auto repair shop and then the faulty parts fixed or the alternator is replaced depending on the nature of the fault.

Final Thoughts

If your alternator feels warm or hot even when the car’s ignition off and the rest of the car is cool then there is a problem – likely with the alternator rectifier diodes.

If you have the skills, you can test each of them and purchase replacements otherwise you can have the car tested at an auto repair shop.

Note that even with bad diodes, it is possible for the car alternator to charge the battery.

So, do not expect it to totally fail to charge the battery.

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