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

It is normal for the alternator to become hot just like the rest of the parts in the engine bay area after the engine has been running for some time.

But, should the alternator feel warm or hot to the touch after the engine area has cooled down? Is this normal?

What dangers does this pose if not attended to?

Read on to find out what causes the alternator to feel warm or hot even after the engine bay area has cooled down sufficiently and tips to fix the underlying fault.

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 rectifier diodes that is allowing the flow back of battery power into the alternator.

Read on to find out why this happens and how you may fix it.

Should an Alternator be Hot when the Car Engine is Off?

There’s a problem with the alternator if it is warm or hot to the touch yet the rest of the engine bay is cool.

The alternator temperature should be at the ambient temperature just like the rest of the components in the engine bay.

Warning: The engine bay can be extremely hot and cause severe burns and injury. Take care when working in the engine bay area.

Make sure it has sufficiently cooled down. Contact a qualified auto mechanic.

What Causes the Car Alternator to Become Warm or Hot?

If the alternator is warm or hot when you touch it after the engine has sufficiently cooled then it is probably because the current is flowing back into the alternator from the battery because the rectifier diodes are faulty.

The windings in the alternator heat up and therefore you then have a warm alternator.

Faulty rectifier diodes. The diodes should only allow power from the alternator to the car battery and not the other way around!

If damaged, the reverse flow of current can happen which explains the warm or hot alternator.

The diodes in the alternator may be damaged by:

  • accidentally reversing the battery terminals when connecting the battery cables to the battery posts
  • when you disconnect the battery cables when the engine is running.

Or, the rectifier diodes can fail on their own too.

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

Besides a Warm or Hot Alternator – What Else Can Happen

Note: These signs are not exclusive to only faulty rectifier diodes in the alternator.

Drained Car Battery

If the diodes are damaged then the car battery can be drained flat. This may be the reason why there are those unexplained cases of the battery being drained overnight.

Car Battery Failure

In severe cases, the car battery may be damaged and require to be replaced from being deeply discharged repeatedly.

You may also be interested in this post. Why the car alternator keeps burning out

Tips to Fix a Hot Alternator

Replacing the rectifier or alternator

Take the alternator for testing at a reputable auto repair shop.

If it is confirmed that the rectifier is faulty, you may have the rectifier replaced (cheaper option) or choose to replace the alternator with a new one.

If reliability is important and the alternator is showing other signs of impending damage then you should probably replace it.

Related Questions

What Causes the Alternator to Become Warm when the Engine is Off?

If the car alternator feels warm while the nearby engine bay parts are cold and the engine is not running then it is highly probable that the alternator rectifier diodes are faulty and need to be replaced.

Contact a qualified mechanic unless you have the skills to repair it safely.

Final Thoughts

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

You can arrange to have the alternator tested at an auto repair shop.

Depending on the nature of the fault, you may be able to fix the alternator by replacing the rectifier (a more affordable solution) or fitting a new alternator.

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