How to Perform an Alternator Test

The alternator is a part of the electrical system on an automobile. While the battery provides voltage to start the car, the alternator recharges the battery, so that it maintains the voltage needed to spin the starter. In addition to recharging the battery, the alternator provides a constant flow of electricity to sustain the battery and power the electrical components, such as the radio, lights, and windshield wipers. An alternator has an internal regulator that keeps the voltage output in the proper range. If a regulator fails, it can cause the alternator to overcharge the battery and damage the cells inside the battery so the battery will no longer hold a charge. If the alternator fails completely and stops charging, the battery will expend its reserve of electricity and the vehicle will stop running. If symptoms indicate a problem with the vehicle’s electrical system, testing the alternator is a good place to start.



Open the Hood of the Vehicle

Access to the engine compartment of the vehicle will allow you to measure voltage at the battery and on some vehicles directly from the alternator. Make sure that access to the battery or alternator is not going to interfere with pulleys or belts that can catch items of clothing and cause injury.


Use a Voltmeter as the Testing Device

A simple voltmeter is sufficient to test the output of the alternator. If using a multi-meter select the 20 V measurement range to read the voltage of the battery and the alternator output. Typically, the voltmeter will have color-coding, such as red for bad and green for good. You can check your owner’s manual for the desired range that the voltmeter should reflect.


First Test the Battery Voltage

To get an accurate reading from the alternator the battery needs to be fully charged. If the battery is low, the alternator is going to produce higher voltage in an attempt to recharge the battery. Using the red lead of the voltmeter, touch it to the positive terminal of the battery. Then touch the black lead of the voltmeter to the negative terminal of the battery. The voltage should read approximately 11 to 13 volts for a normal battery. If the battery voltage is too low, charge the battery to bring the voltage reading up to 12 volts. Make sure the battery terminals are not corroded, which can cause poor connectivity and lower voltage readings.


Check the Alternator Belt for Excessive Wear or Slippage

Before starting the vehicle, check the belt that spins the alternator pulley to make sure it is not loose or slipping. When an alternator belt has the proper amount of tension, it should be hard, if not impossible, to turn the alternator pulley by hand. Check for a glazed or frayed alternator belt and make sure no oil is leaking on the belt, which can cause the belt to slip.


Locate the Electrical Output Terminal on the Rear of the Alternator

While the vehicle is not running, locate the output terminal on the backside of the alternator. The output terminal is often identified with a red collar and will be a stud sticking out of the alternator case, with a nut holding a cable end for the power lead that runs to the battery or a connection point in the electrical wiring system. Measuring the voltage of the regulator at this output terminal provides the best reading. Make sure you have safe access the terminal once the vehicle is running.


Test the Alternator Output

Start the automobile, allow a few minutes for the battery to return to normal after starting. During the warm up, check the alternator belt to make sure it is spinning the pulley properly. Place the red lead of the voltmeter on the alternator output terminal and the black lead on an engine ground of solid steel. Do not place the black lead on the case of the alternator, to avoid the potential of damage to the electrical system. The reading from the voltmeter should be in the range of 13.5 to 14.5 volts. If the voltage reading is 15 volts or more, it is likely the regulator in the alternator is malfunctioning and the battery is being overcharged. If the voltage is below 12 volts, the alternator may be failing.


Alternate Test Method

If the output terminal of the alternator is not accessible, the voltage can be checked at the battery. Touch the red lead of the voltmeter to the positive terminal of the battery and touch the black lead to the negative terminal. The reading should be in the range of 11 to 13 volts.


Retest the Battery

If the alternator is not functioning properly, the battery can easily be damaged. After performing the alternator test, turn the engine off and retest the battery voltage. If the battery voltage reads below 10 volts, it is likely the battery may be damaged to the point it will have to be replaced along with the alternator.


Know the Location of Your Vehicle’s Regulator

On some older models, the regulator is not internal to the regulator and can be replaced separately. Depending on the vehicle, the regulator was occasionally designed as an external attachment to the alternator. On some models, it is mounted on the firewall. Refer to your owner’s manual for the specific location on your vehicle.


Safe Removal of Electrical Components

The first step to the safe removal of an alternator or a battery is to disconnect the negative terminal cable from the battery. By removing the ground for the electrical system, you can safely remove the positive terminal connectors from both the battery and alternator. This will prevent an accidental electrical spark from occurring.

Keeping the electrical system of your vehicle operating at the proper voltage levels is important. A low battery can cause a no start condition and an alternator that fails can leave a vehicle stranded on the roadway. Indicators like slow starts, a flashing battery light on the dashboard, or a dash voltage gauge that reads erratically, can all be indicators of a possible electrical system problem. Testing the alternator can tell you if a problem is developing.