How To Correctly Test An Alternator

A faulty alternator is a common cause of engine problems and often require expensive work to be done by a certified mechanic. If you’re looking to save on your alternator repairs, troubleshooting the problem yourself may be a good idea. Such a task can be performed by almost everyone once you have mastered some basic electrical troubleshooting notions. Read on to learn everything you need to know to test your car’s alternator, how to replace it and save a lot of money along the way.

Automotive charging system diagram

How Alternators Work

A vehicle’s alternator is the main component of the charging system. While the battery is exclusively used to start the engine, the alternator’s job is to take the relay once the vehicle is running and recharge the battery.

When you turn the ignition key and start your engine, the starter drains a significant amount of the power stored in the battery. By itself, the battery could not make the car run for longer than 10-15 minutes. The alternator creates electricity to power every electrical system on your car and send the excess to be stored inside the battery for future use. That way, the battery stays fully charged at all times and is always ready to start your car once again the next morning.

The alternator can be compared to a dynamo in the sense that it transforms kinetic energy into electrical power. This is made possible by a simple magnet rotating inside a copper coil. In motion, the magnet displaces the electrons on the copper atoms thanks to its magnetic field. This operation is ensured by the alternator belt which transmits the power of the engine, which is why the alternator only fulfills this function when the engine is on. The electrons rotating in circles create the electricity that will then be transferred to the battery.

Main Components


The rotor is the rotating part generating a magnetic field via an excitation coil fed by brushes. It is composed of two shells nested one inside the other, forming an assembly of three electromagnets. The rotor is connected to the crankshaft via the drive belt and constantly turns at the same speed as the engine.


Charcoal brushes are used to feed the electromagnet with power while it turns on itself. A standard connector is not enough as the wire would basically wrap on itself as the rotor turns. Exactly as with starters and steering wheels, a connection allowing contact between two moving parts in rotation is needed and this is where the brushes come into action.


The stator, as the name suggests, is the static part of the alternator. In the case of a three-phase alternator, the stator is composed of three coils. Thanks to the magnetic force induced by the magnet of the rotor, electrons move and generate alternating current in each of the coils.

Voltage Regulator

Modern alternators have an electromagnet in their center. The PCM can modulate the intensity of the magnet using a voltage regulator making it possible to control the amount of power produced by the alternator.

Diode Assembly

Since a three-phase alternator generates an AC current and cars only run on DC current, the power needs to be rectified before behind sent back into the system. To do this, manufacturers use a diode assembly. Diodes only allow current to go in one direction. Connecting them in series will then redirect the current produced by each coil to ultimately convert the AC input into a DC output.

Cutout view of an alternator

What Happens in the Case of an Alternator Failure

Whenever your alternator starts misbehaving or stops working completely, the first thing you’ll notice is the charging system warning light lighting up in the dashboard. This warning light is there to let you know that the power in the system is slowly dropping and is now under a normal threshold.

The battery warning light is usually shown whenever the voltage reading of your electrical system goes under 12.6v. When the alternator stops producing current, the battery will take the relay and power everything in its place. As said earlier, the battery itself won’t be able to power the car for long, though. If the condition remains the same, the engine will shortly die.

In such a situation, turn off all the electrical accessories to save as much power as you can and quickly find a safe place to stop the car. Turn off the radio, the headlights, the climate control, the navigation, and the entertainment system to be able to go as far as you can before it dies and save on tow truck fees.

Most Common Causes of Alternator Failures

Broken Drive Belt

The drive belt is what’s making the rotor spin. If the drive belt breaks, the alternator will stop turning and producing current altogether. In the case of an alternator failure, always begin by inspecting the belt and make sure it’s not too loose or damaged.

If the belt is completely torn up, the drive pulley might be stuck and will need to be replaced.

Worn Out Internal Components

Any worn-out component can cause the alternator to stop working. Charcoal brushes, for example, are often prone to premature wear due to the constant friction of the rotor. Water and rust can also cause an open circuit or short to ground condition to the rotor and stator.

In any case, buying an alternator rebuilding kit instead of replacing the whole thing might be a good solution.

How to Test an Alternator Using A Multimeter

Auto mechanic testing an alternator using a multimeter

Before starting to troubleshoot an electrical problem, make sure you get yourself a good multimeter. Try to buy one equipped with an amp clamp. This feature really helps to speed up the process of measuring amps which is, otherwise, kinda complicated.

Check the Battery Voltage

First, check the battery voltage while the engine is turned off. This will give you a reference value to compare with the alternator output voltage. Connect the device to the battery terminals: the red wire of the multimeter must be connected to the red terminal of the battery and the black wire directly to the black terminal. The voltage reading should be around 12.6v. If it’s not, you’ll have to charge your battery with a smart battery charger first or you might end up with false readings.

Measure the Voltage of the Alternator

Start the engine and read the battery voltage again. The voltage should now be between 13.3v and 14.6V

  • If the voltage is less than 13.3 V, your alternator is not producing enough current and could cause the engine to stall. Driving while the voltage is under 13v will damage the battery in the long run and could even cause the engine to stall unexpectedly.
  • If the reading is between 13.3 and 14.6 V, everything is alright with your alternator and there’s nothing more for you to do. In case your engine stalled or the warning light appears in the dashboard, the problem is most probably related to another component of the charging system.
  • If the voltage is greater than 14.6V, the voltage regulator might be faulty and is no longer able to limit the supply of electricity. The over-voltage generated may cause accessories to bust. On recent vehicles equipped with “stop and start” features, an alternator can charge well over 15V or under 12V. In that case, buying an automotive manual for your vehicle might be necessary.

Amp Output Test

Now that you have checked that the alternator is producing enough current to recharge the battery, you still need to test it to see if it’s outputting enough amps to sustain the whole electrical system when all accessories are turned on.

Testing amps in an electrical system can be quite tricky since the circuit needs to be opened and the multimeter connected in series. Using an amp clamp can definitely help you out here. Amp clamps can measure the magnetic field produced by the current flowing in the circuit without having to open it at all.

To test the amp output of an alternator, start by locating the power output wire. A typical alternator will have two sets of wire: the control side, which is usually composed of multiple small wires wrapped up in electrical tape coming from the engine harness and the bigger wire going directly to the positive post of the battery. This is the output wire.

Install the amp clamp around it. Always make sure that the little arrow on the clamp points in the same direction as the flow of current. In this case, the current flow from the alternator to the battery.

Every car is different and manufacturers choose to install different output rating alternators for various reasons. Standard units commonly range from 60A to 200A but make sure you find the correct specification in your car’s repair manual.

In all cases, the reading on your amp clamp should be within the threshold or the alternator will have to be replaced or rebuilt.

Testing the Ground of the Alternator

To work, all electrical components installed on your vehicle must be connected to the ground and this is also true for the alternator. A bad ground connection will significantly reduce the amount of current produced by the alternator to a point where the engine could stall.

Instead of using a ground wire connected to the body of the car, an alternator gets its ground directly from its casing which is bolted onto the engine. If the engine ground wire is damaged or the contact point of the alternator and the bracket is rusted, the ground circuit might be disrupted causing various charging problems.

To test the ground of the alternator, the first thing to do is to test the circuit for continuity. Begin by selecting the Resistance/Ohm function on your multimeter. Touch the casing of the alternator with the red probe and the negative post of the battery with the black probe. If there’s continuity, your multimeter will either beep or a resistance value will appear on the screen. If the circuit is in an open loop, you’ll need to find the damaged wire and repair it.

Alternator Load Test

Auto mechanic technicians often conduct a load test to have a better idea of the general health of a charging system. A conventional load test requires a battery tester but can also be done using a multimeter.

Start the engine and measure the current at the battery. As mentioned earlier, the reading should be between 13.3v and 14.6v. Now, ask an assistant to sit in the car and turn on all the accessories at the same time. Wipers, lights, rear defrost, heater blower, everything.

The reading on the multimeter can fluctuate a little but ideally, it should quickly stabilize back at 14.6v. If the reading is a little lower, your alternator is aging and it’s getting weak. Keep an eye on the charging system warning light in the dashboard and re-test your alternator regularly.

If the reading goes back down to about 12.6v, on the other hand, you might want to think about replacing your alternator soon or it could have a hard time recharging your battery when all the accessories are on. This is especially important for car owners driving in harsh winter and hot weather conditions.

Recent Vehicles Particularities

It’s worth mentioning that this whole article is based on the most common vehicles in the market right now. Newer or higher-end vehicles often come with more complicated alternator setups. BMW, Audi, and Volkswagen vehicles, just to name a few, are well-known for their alternator clutch pulleys. This feature allows the PCM to engage and disengage the alternator depending on the state of charge of the battery and the current consumed by the accessories.

If you own or work on one of these cars, always remember that if the alternator’s voltage output is at zero, the clutch may simply be disengaged by the control module at the moment. Turn on some accessories to make it clutch.

In the same line of thought, if the alternator is not charging when it should be, try inspecting the pulley for a seized clutch. The alternator could appear like it’s not doing its job when, in fact, the clutch could be the culprit and it is not engaging anymore. Always double check your tests and make sure you understand how the charging system works on your vehicle before replacing any costly component.

Last Words

Electrical problems aren’t always easy to diagnose. Due to their relative simplicity, troubleshooting faulty alternators is a great way to try your hands at it. Start by mastering some of the most basic notions and, with a bit of practice, you should be able to learn how to correctly test an alternator in no time!

About Jean-Claude Landry

Jean-Claude is the editor of and has been a certified auto mechanic for the last 15 years, working for various car dealers and specialized repair shops. He turned towards blogging about cars and EVs in the hope of helping and inspiring the next generation of automotive technicians. He also loves cats, Johnny Cash and Subarus.


  1. Avatar

    Hello Nishal, I’m glad I stumbled into this article. My old 95 Camaro that’s been giving me problems lately. Every time I accelerate, the dashboard lights and headlights dim a bit. It’s been happening for two weeks now. It’s just recently that I found out it’s an alternator problem and I want to try fix it myself instead of taking the car to a mechanic.
    Anyway, wonderful guide!

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