5 Ways To Test An Alternator

Is your car battery dying overnight? Do the lights dim whenever you turn something on? These could be signs that your alternator is failing. This part of your caris responsible for charging your battery while the engine is running, so if it stops working properly, your electrical system will quickly be in trouble.

Luckily, there are a few simple tests you can do to identify weak alternator symptoms, and you can even do them in your own driveway.

Follow these steps to get started.

Steps to Identify Weak Alternator Symptoms

Visual Inspection

Pop the hood and take a close look at the alternator itself. Check for any loose wires or connections. Also look for cracks or damage to the alternator housing that could allow dirt and debris inside. Examine the belts connected to the alternator – are they excessively worn or slip while the engine is running? Any of these visible signs could indicate potential alternator problems.

Voltage Testing

Use a multimeter to test the voltage coming directly from the alternator with the engine off and running. To do this, attach the red probe to the output terminal on the back of the alternator and the black probe to the ground. With the engine off, you should get a reading around 12 volts. Now start the engine – the voltage reading should increase to between 13.5-15 volts. If it stays around 12 volts, that indicates your alternator is not charging properly.

Battery Testing

A weak alternator will fail to charge the battery while the engine is running properly. To test for this, use a multimeter to check the battery voltage with the engine off. Then start the engine and test again. 

The voltage should increase when the engine is on – if it stays the same or drops, that’s a sign your alternator may be having problems. You can also turn on the headlights and check if the voltage drops below 12 volts.

Battery Disconnect

This is a simple way to test alternator output. With the engine idling, disconnect the negative battery cable. If the engine stalls out, it means the alternator can’t generate enough electricity to run the engine alone and is likely failing. If the engine keeps running, the alternator is still charging properly. Just be sure to reconnect the battery cable promptly.

Check Warning Lights

Many cars have built-in voltage meters or warning lights related to the charging system. The battery/charging warning light should come on when the ignition is first turned on, then go out once the engine starts. If the light stays on or comes on while driving, that could indicate an issue with the alternator.

Final Note

Watching for common symptoms like dimming headlights and dying batteries can help alert you to potential alternator problems. If you notice these warning signs, use the simple at-home tests above to check whether your alternator may be failing before paying for a full diagnostic. 

However, if your tests are inconclusive, having a certified mechanic properly test your charging system is best. Catching and replacing a bad alternator promptly can save you from getting stranded with a dead car battery.

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