How to Test the Alternator and Charging System

Much like testing your car battery, a multimeter can be used to quickly tell whether your car’s charging system is adequately keeping the car battery at the proper charge, and operating the electrical system at the proper voltage as well.

Car batteries have a resting voltage somewhere between 12.45 and 12.6 volts in most cases. Your vehicle’s charging system, however, operates at a slightly higher voltage than the battery's voltage and does so for several reasons. For one, a phenomenon known as voltage drop causes voltage at the source (the alternator) to degrade as it travels through the hundreds of feet of wiring that snake throughout a vehicle.

What this means is that voltage at the Alternator may be, say 14.5 volts. Voltage at the battery may be 14.2, but yet, way out at the rear taillight bulb, this voltage may only be 13.5 volts, due to voltage drop.

Because of this, every vehicle's measured operating voltage at the battery may be different, but they should all be higher than the battery’s resting voltage. In most cases, on most vehicles, you will see a voltage of 14.00-14.5 when the vehicle is running with all accessories off. Now that you know this, it’s time to test the charging system.

What Signs Indicate a Weak Alternator?

Battery indicator light lit up on the dash panel
Car Battery Light

An alternator is built to be durable in the toughest conditions including extreme heat under the hood. Internal components are isolated well to prevent wear using high-quality bearings. Unfortunately, certain parts inside the alternator don’t last forever like the brushes made of soft carbon material and the bearings. Corrosion can form that interrupts electrical conductivity, diminishing the alternator’s output.

Symptoms of a bad alternator can be varied. They include: 

  • A battery light on the dashboard. This indicator isn’t for the battery itself but points toward a charging system defect.
  • Dim or excessively bright lights. An alternator that’s underperforming will often exhibit dim lights while an overcharging condition could make lights brighter than they should be.
  • A burning smell. Should the alternator have an electrical issue inside, a burning electrical smell can develop, while a burning rubber smell can pop up if the belt is slipping on the pulley.

Check out our resource What are the Signs of A Bad Alternator to learn all of the symptoms of a failing alternator.

Keep in mind that low voltage conditions can cause abnormal electrical operation in today’s highly electronic vehicles. Some otherwise unexplained symptoms could be related to a bad alternator.

How to Test an Alternator

1. Prepare Your Multimeter

Set your multimeter to voltage and ensure it’s adjusted to 20 DC volts, or if your voltmeter does not have incremental settings (2,20,200,2000) then simply set it to DC volts.

2. Touch the Probes to Your Battery Terminals

Do not start the car yet!

Press each probe to the correct terminal, touching negative to negative and positive to positive. Your multimeter and battery are probably both color coded, so the negative terminal and probe will be black while the positive terminal and probe are red.

If your vehicle has a battery that is tucked away, like under the rear seat or fender well, you will often find jumping or access terminals under the hood, or at least a positive terminal often covered with a large, red plastic protector. If you cannot find a negative lug to touch off to, touching off to any clean bolt that isn’t painted on the chassis or engine compartment will work.

Take a reading on the battery. You should read between 12.4-12.6 volts. Write down this reading. If it’s below 12.4 volts, check out our article on how your battery could be bad. Now, start the vehicle.

3. Check the Reading

Again, touch the same location you did when checking the battery. Your charging system should be supplying voltage between 14.0-14.5 in most cases, sometimes higher, but never over 16.5 volts. What you are looking for is a number higher than the initial battery voltage. If the voltage is still only 12.4-12.6 once the engine is running, then the charging system is not functioning.

In most cases and in most vehicles, you will have a battery light illuminated that indicates that the alternator is not charging. Contrary to what may seem like a bad battery, an illuminated battery light does NOT mean the battery is bad, it means the battery is not being charged properly.

Very low differences in voltage between the battery and alternator should also be suspect – say – 12.8 volts at the battery. The first course of action is to check the wiring carefully that runs between the alternator’s main power wire and the battery. A worn, loose, or damaged connector can cause a massive voltage drop.

If everything appears fine, the alternator should be removed and bench tested, which can be done for free at AutoZone.

4. What if the Alternator Tests Fine?

Electricity is frustrating, or at least it can be. While an alternator can be an expensive piece to purchase, nothing could be more frustrating than buying one only to find out that the one you replaced was fine and another part was the problem. Keep in mind, just because the Alternator tests fine, does not mean the system is fine.

The charging system is composed of wiring that not only supplies the battery and system with DC power, it also “senses” the voltage that’s out in the system in order to properly set the voltage leaving the alternator. Couple this sensing wire (known as a field wire) with several other wires, connections, and even fuses and fusible links, and you have a charging circuit that can fail in various spots even if the alternator is perfectly fine. Keep in mind, when you test your charging system while the car is running, or AutoZone does the same, the system voltage is being tested at the battery, and if it’s not getting a charge, the test cannot determine why.

Either the alternator itself is at fault, or the wiring between it and the battery are. Either case, the alternator will need to be removed, tested, and the wiring and fuses related to the circuit checked as well.

How to check your car’s starting and charging system to identify any issues.

If any testing or replacement of your alternator or charging system becomes too difficult of a job, consider visiting one of our Preferred Repair Shops in your area can help you diagnose and repair any faults in the charging system.

Do you need a more in-depth look into your vehicle to help you with your project? ALLDATA, the leading provider of automotive repair information, is now providing DIYers with the same information that the pros use with ALLDATAdiy’s single-vehicle subscriptions.

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Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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