7 Common Symptoms of a Short in the Alternator (+fixes)

Is your battery draining flat even with a good battery?

Do you suspect that it may be a short in the car alternator that is causing the drain and loss of electrical power in the car?

In this post, I cover symptoms for a short in the alternator, how to check for a short in the alternator, common causes of short-circuits in the alternator and how you can fix the shorts.

Can an Alternator have a Short?

Yes, you can have a short-circuit in the car alternator.

This affects the alternator’s power output with your alternator supplying less than the expected voltage or current.

The parts in the alternator that short-circuit usually are :

  • shorted rectifier diodes responsible for changing the AC power generated by the alternator to the DC power which is then used to charge the car battery and supply power the car electrical accessories, or
  • shorted stator windings.

For more information on causes of short-circuits in car alternators, check here.

Symptoms that a Alternator has a Short-Circuit

The signs below point to a car alternator possibly having a short-circuit.

Note, these are not conclusive and in fact, similar signs¹ may be attributed to other faults as well.

So, even with these signs, arrange to have additional tests done to confirm that the alternator has a short.

#1. Lower than expected alternator output voltage and current

If the alternator output voltage is lower than 13.8 -14.5V, then you may have a case of a short in the alternator.

Also, in case of a short in the alternator, the output current will be lower than expected for a given rpm.

This can be checked at an auto shop with testing equipment.

#2. Battery drain

If an alternator has a short circuit, it can drain the car battery power.

So, if you have a case of a car battery draining flat, it could be a case of a bad diode.

One way you can quickly check for this is to observe the alternator temperature when the car engine is switched off and the engine has cooled.

If the alternator is draining the car battery, the car alternator will be warm when you touch it.

You can also check for a short-circuit by comparing battery drain overnight, first with the alternator disconnected and with it connected (car engine switched off).

If the fault is with the alternator, then your battery will drain considerably faster.

Note, though, that there can be other causes of the parasitic drain of the car battery’s power.

You may want to check out this article: What can drain a car battery with the ignition switched off?

#3. Interference in the sound system

The car’s sound system may pick up interference when the car engine running if the alternator has broken diodes.

With shorted diodes, the power output from the alternator is not as clean and produces interference that can be picked up in the sound system.

#4. Flickering lights

The car headlamps may also flicker when the car engine is running if the alternator has broken diodes with them lighting steadily when you turn off the engine and switch them on.

¹Other faults with similar symptoms include bad wiring in the charging circuit, drive belt not properly fixed.

How you Can Check for a Short-Circuit in an Alternator

There are a couple of tests you can perform to check if the car alternator has a short, particularly, a shorted diode

#1. Using a good quality multi-meter, you can measure AC voltage on the car battery terminal (connected to the alternator) with the car engine running.

With bad diode(s), the alternator is not able to properly convert the AC output to DC, so some AC voltage will filter through and be detected at the alternator output or positive battery terminal.

#2. You can also use a portable oscilloscope to check the wave pattern from the alternator.

If an alternator’s diodes and windings are still intact, you should see a smooth, regular wave pattern on the oscilloscope’s display.

With a bad diode, the waveform is irregular, with low amplitude in some places.

Common Causes of Shorts in Alternators

These are some of the causes of short-circuits in car alternators

  • accidentally grounding the alternator output.

You can have a short in the alternator when you accidentally connect the alternator output’s positive terminal to the ground.

  • reversing the alternator terminals – accidentally connecting the alternator positive terminal to the battery negative and the alternator negative to the battery positive can also lead to a short in the alternator.

  • Disconnecting the car battery when the engine is running can also cause voltage spikes that create shorts in the alternator.

  • Alternator may also fail after jump starting another vehicle with a dead battery.

Will a Shorted Alternator Work?

A shorted alternator may still generate some power.

You can still have an output voltage and current but this will usually be lower than designed or inconsistent and therefore cause problems such as battery drain and intermittent charging of the battery.

For more information on problems caused by a shorted alternator, check here.

How to Fix an Alternator with A Short?

It may be possible to easily replace the shorted diodes or have the alternator windings rewound depending on the type of alternator, or you can simply buy a new alternator.

Buying a new alternator has several benefits. A new alternator usually comes with a warranty – some have lifetime warranties!

With a reputable brand, you can be confident of many years of service.

Related questions

Does accidentally grounding the alternator output damage it?

Grounding the alternator output is bad for the alternator and can damage it.

You need to check for the symptoms of a short-circuit or arrange for the alternator to be tested at a professional auto shop.

Final Thoughts

Alternators can have shorted diodes or windings.

These shorts lead to a lower than expected power output from the alternator which causes battery charging problems.

You may be able to check for shorted diodes for checking for an AC voltage at the alternator’s output.

You may be able to fix the shorts by replacing the shorted diodes, rewinding the shorted windings, or replacing the alternator.

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