How To Test an Alternator

By Jay Cork

Updated: Jul. 12, 2024

Save money and save your battery by testing your alternator with one simple tool.

Time

10 minutes

Complexity

Beginner

Introduction

A faulty alternator can leave you stranded in rush hour traffic and damage your battery. If the alternator stops charging — or charges intermittently — the voltage the battery supplies the vehicle drops, and other systems may stop working or be adversely affected. If you've noticed your vehicle running low on juice, it's time to test the alternator and see if it's giving the battery what it needs.

Follow these simple steps to test your alternator while the car is running without any special equipment.

Tools Required

Multimeter

Project step-by-step (4)

Step 1

Test the Resting Battery Voltage

If you don’t already own one, get a cheap multimeter.
Set it to read volts.
With the engine off, touch the red probe to the battery’s positive terminal and the black probe to the negative terminal.
The voltage should read between 12.3 and 12.8 volts. If it’s below that, charge the battery with a battery charger before you conduct the test again.

Test the Resting Battery Voltage TMB Studio

Step 2

Test the Alternator

Start the engine. Keep all lights and accessories off.
Touch the black lead to the negative terminal and the red lead to the positive terminal.
If the voltage reading is in the 13.8 to 15.3 volt range, the alternator is working properly.
More sophisticated testing equipment is needed to detect an open or shorted alternator diode. Bring your vehicle to your mechanic or dealership to have these tests done.

Step 3

If Your Alternator Passes

If the alternator is functioning correctly, but your battery is still dying, it’s time to test the battery.
There is also a chance that a parasitic drain caused your dead battery. These are caused by a short circuit or an electrical device that remains “on” when it should be off. Get your vehicle into a shop and pay a professional mechanic to find and correct the misbehaving module.

Step 4

One Outdated Method to Avoid

In the pre-computer days, we would do a quick field test by pulling off the negative battery lead while the engine was running. If the engine kept running, we figured the alternator was healthy. And while a good alternator may indeed keep the engine running without the negative cable attached to the battery, this was never a good test.

If you use this technique with modern vehicles (think post-2000), you risk frying every electrical device in the system. If the battery is disconnected while the engine is running, the voltage regulator commands the alternator to release maximum power. With no battery in the circuit to act as a buffer, the alternator can put out up to 150 volts, depending on engine rpm. When the smoke clears, that “easy test” could cost you several thousand dollars for new electronics. Don’t do it.