Testing Alternator Amp Output With Multimeter: How To Do?

Testing alternator amp output with multimeter is important to ensure that your alternator amperage is working properly. A multimeter is the most common tool to conduct this test.

It can support you in measuring the voltage and reading the amp results of the current supplied to the automotive.

This test is also useful to diagnose the failures or potential problems you can be experiencing to help your car perform at its best. What should you do to complete this task? 

Keep reading this article for further understanding.

Testing Alternator Amp Output With Multimeter: Detailed Guide

Testing alternator amp output with multimeter

How to test alternator amps with multimeter? First, you must check the alternator output of the amps to determine if it is running properly.

Besides, examining your battery is critical to identifying the bad alternator output and finding a thorough fix.

After checking the battery performance and amps, inspect the alternator voltage and amperage.

To support these main steps, locate the power amp output wire and set the multimeter to read the result and complete the test.

Examine Amps 

You need to examine amps to determine whether the alternator is fully functioning. Also, check your vehicle’s manual specifications to learn more about your alternator.

This can tell you the max or min amperage on the auto alternator.

You can understand that the amp maximum is the time you enable something to consume the power in your vehicle, while the amp minimum is when you turn on things like the automotive stereo.

Besides, wear gloves to get closer to the automotive battery to probe the amp output through the car’s alternator.

Then, open the vehicle’s hood, start your engine, and let it idle for a while.

You’ll also need to check the minimum ampere rating of your alternator to ensure you don’t have anything consuming energy, such as air conditioners or automotive lights.

Inspect the Battery

Besides examining the amps, checking what’s going on with your car’s battery is necessary.

After you turn the engine off, connect the voltmeter or multimeter’s positive terminal to the positive battery terminal and the voltmeter’s negative terminal to the battery’s negative terminal. Tighten the battery terminal if needed.

The last thing is checking the reading, which should be approximately 12.7 volts. You must replace it if it is less than 12.4 volts.

Examine the Alternator Voltage

If you don’t know how to check voltage on alternator, follow these simple steps. Start the engine but do not step on the gas. The alternator must power the battery when the engine is running.

Check the multimeter or digital voltmeter again to read the automotive battery voltage. With the alternator running, the voltage reading should be from 13.8-14.2 volts on the voltmeter.

Inspect the Alternator Amperage

If you are not using a multimeter, you can connect an amperage meter to the alternator battery after disconnecting the basic voltmeter.

Continue turning the electrical devices on in your vehicle, including the stereo, lights, windshield wipers, and anything else you can do.

You need to allow the engine to idle at about 1200 rpm. Double-check your car manual to check the alternator’s maximum amperage.

The meter’s amp reading should be close to the maximum output. If the amp is lower than the standard, you must change it with a new basic alternator.

Locate the Power Amp Output Wire

Locate the power supply output wire to examine the amperage output alternator with a multimeter. A typical alternator in a car includes two sets of wires.

The control side typically consists of a few small wires using the electrical tape to wrap them from the engine wire harness, and the larger wire goes directly to the battery’s positive terminal.

Locating the output wire is a must if you want to know how to check alternator amperage with a multimeter.

Set the Multimeter to Read Amp

If you don’t know how to measure alternator output, you may use your multimeter to check and read it. First, insert the metal on the black wire’s end.

Next, extend the multimeter to the battery’s negative terminal. Yet, you must note that the negative terminal of this battery will come with the marked black wire.

Continue placing the other metal piece on the red wire end on the battery’s positive terminal that can protrude.

With a red wire marked POS, look at your multimeter and read the minimum amp output you noted earlier. It doesn’t matter when it’s 10% higher.

Yet, if it is lower than the minimum specification, you need to take the alternator to a professional mechanic to check and replace it.

Also, contact the expert mechanic when there is no power to the electrical components.

Why Should You Test Alternator Output With Multimeter?

alternator output test

An alternator output test can ensure the engine and other accessories work properly since this part “fuel” the whole car, even the smallest components, by supplying power to the battery.

Take the car amplifier, for example.

As you know, the automotive sound system must amplify the audio signals to transmit via the speaker.

Thus, a malfunctioning amplifier can damage your speakers and reduce the sound transmission performance.

So, checking the amp output alternator with a multimeter that can measure the voltage, resistance, or current is critical. This will help it supply complete and clear sound signals.

Plus, an amp alternator output test with a multimeter will help you identify potential problems through the reading results. So before you cranking up the car, you may prevent damage to the whole engine.

What Are Mistakes You Can Make While Testing Alternator Output With A Multimeter?

Below are five popular mistakes you can make while testing alternator output with a multimeter.

1. You can make an improper connection of multimeter leads. Ensure that you connect the red end to the positive (+) terminal and the black one to the negative (-) terminal to have an accurate reading.

2. You are not using the correct multimeter settings.

To solve this mistake, you must set the multimeter to the accurate range for testing amplifier output more precisely.

3. Ignore the polarity. You should observe the polarity when connecting the amp multimeter cable to the battery terminals.

4. Not paying attention to the impedance mismatch is one of the mistakes you must notice while conducting the alternator test. This can impact reading accuracy.

5. Skip DC offset. This may influence the alternator output’s reading accuracy. So, you must note this mistake to solve it before checking the results.


The above is the ultimate guide on testing alternator amp output with multimeter. This test significantly ensures the alternator works in good condition and checks for any problems.

Based on the reading result of the multimeter, you can find the proper methods to maintain and improve the alternator amperage’s quality and performance.

Yet, you must note that some mistakes you make while testing and various factor can affect the measurement’s results.

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