Why an Alternator wire becomes Hot (+How to Fix the Cause)

Your car’s alternator can provide many years of faultless service thanks to the ever-improving reliability of the alternator and the available helpful articles on car electrical system maintenance.

Even with that though, there are always challenges.

For instance, you may be one of the several car owners who may be faced with the unusual and puzzling alternator problem, one where the wire from the alternator becomes extremely hot.

And in some cases, the insulation melts.

In such a case, it is important to find the underlying cause – why the alternator wire overheats to prevent damage and risk of fire to the car.

Not sure how to fix this?

In this post, I explore why your alternator wire may become hot, and how you may be able to find and fix the underlying causes.

Short answer: Should your car alternator wire feel hot when you touch it, it could be that the wire is undersized (if you have upgraded the alternator to a larger size) or there could be a short between the alternator output and the ground.

Read on to learn about other possible causes and how you can fix them.

4 Reasons Why the Car’s Alternator may be Hot

#1. High alternator voltage

If your alternator output voltage is higher than 14.5V then your alternator will push out a higher current.

If the gauge of the size of wire used is small compared to the current handled, the alternator wire will heat up.

Use a digital voltmeter to measure the output voltage of the alternator when the engine is running.

If it is higher than 13.6 – 14.5 V, this can point to a malfunctioning alternator. Arrange to have the alternator tested at an auto repair shop.

#2. Weak, corroded or loose connections at one of the car electrical charging systems

A weak or loose cable connection at the alternator or battery terminals can also cause the alternator output wire to heat up.

With a loose connection, the high resistance presented reduces the current flow to the battery.

Check if the battery ground is clean and firmly connected to the chassis?

Could the ground wire be broken under the insulation?

Is the contact clean – no rust? You may need to sand them to remove the corrosion. You can measure the resistance across the cable using a good quality meter across the ends of the ground wire.

It should be steady at zero ohms.

An open circuit or a fluctuating resistance reading assuming the meter usually indicates a break in the cable or bad contact.

This in turn necessitates the alternator to run longer than usual in order to charge the battery resulting in heating up of the alternator.

#3. Undersized alternator wire

Have you upgraded to a higher capacity alternator that generates more current at full load?

If so, there is a good chance that you may need to increase the gauge of the wire to handle the high currents.

Consult the alternator manual for the recommended wire gauge size to use and adjust the wire accordingly.

#4. A short-circuit in electrical system

A short in the electrical system can also result in a high current draw and overheating in the alternator.

This short be along the cable length, in the wire harness, or it could even be wired with melted insulation touching each other.

To check if there is a short circuit, disconnect the battery cables from the battery and measure the resistance across both cables.

A zero resistance points to a short-circuit in the wiring that needs to be investigated. You can work backwards from the battery cables towards the alternator to locate the short circuit.

Do not hesitate to consult a qualified auto mechanic for assistance.

How Hot should the Alternator wire get?

The alternator wire should be slightly warm to the touch when charging the battery. It should not too hot to cause burns.

Related questions

Why the Alternator’s Ground wire may Melt?

If the alternator ground is hot, it could be because the ground connection at the alternator is bad. It may be loosely connected or even corroded.

An alternator may be grounded via the alternator case or via the grounding strap. You can check the quality of the connection by conducting a voltage drop test on the ground circuit.

Connect one end of the voltmeter on the alternator case and the other on the battery negative terminal.

Run the engine at about 2,000 rpm while switching on the lights, radio, and blower.

Take the voltage reading. If the ground connection is good, the voltage drop should be less than 0.2V.

This may be because the alternator ground wire is undersized and loosely connected.

You may have to run an extra zero gauge wire from the alternator ground to the battery negative to improve the connection.

Final Word

An alternator wire should be warm when you touch it and not hot.

If it is hot, then there might be several possible causes that you need to work through – a loose, corroded connection, an undersized cable, or a failing alternator with a high voltage out[ut.

Additionally, if the grounding is bad, you can have an alternator grounding wire overheating and melting.

Consider running an additional zero gauge wire between the alternator ground bolt and the battery negative terminal.

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