How to Test the Alternator in Your Car




Introduction: How to Test the Alternator in Your Car

An alternator helps power all of a car's electrical systems, including the starter motor, speaker system, and lights. A bad or failing alternator can discharge your battery, cause issues with your car's electrical systems, or leave you stranded when you need to use your car.

It's a good idea to know a few simple tests and early signs of alternator failure to avoid a breakdown. Here'a a step-by-step process to test your car's alternator.

Before you get started, gather the necessary materials to test your alternator. You will need:

  • Rubber hose (about three feet)
  • Voltage meter or digital multimeter
  • Wheel chocks
  • Safety glasses

Step 1: Getting Started

It's easy to set up for the job of testing your alternator.

  1. Park the car on level ground to make it easier to work on.
  2. Turn the engine off.
  3. Secure the wheel chocks around the driver's side rear tire to prevent the car from moving.
  4. Put on your safety glasses.
  5. Open the hood.

Step 2: Locate the Alternator

In many vehicles, the alternator is located near the top front of the engine. Most alternators have a round, vented metal housing, with visible copper wire inside.

Step 3: Check the Engine's Drive Belt

A loose drive belt can cause an alternator to not charge properly, so its tightness is the first thing to check.

  1. Locate the engine's drive or serpentine belt.
  2. Check its tightness by pressing on it between any two pulleys. It shouldn't have much movement or feel loose.

If the drive or serpentine belt is loose, it should be tightened before moving on to test the alternator.

Step 4: Listen to the Engine

The sounds the engine makes can help to pinpoint problems.

  1. Start the car's engine.
  2. Listen for any unusual noises like grinding or squeaking.
  3. Use your length of rubber hose to locate the source of noises. You can use it like a doctor's stethoscope by placing one end on the metal alternator housing, and the other end up against your ear. If the grinding or squeaking noises are very loud through the hose, it may indicate a failed bearing in the alternator, in which case the alternator will need to be replaced.

Step 5: Test the Alternator With the Engine Off

Now you're ready to begin the actual testing of the alternator. First, test it with the engine turned off.

  1. Turn the engine off.
  2. Turn on your voltage meter and set it to DC volts.
  3. Connect the positive lead to the positive terminal (+) on the battery, then connect the negative lead to the negative terminal (-) on the battery.
  4. Check the battery voltage on the voltage meter. It should read about 12.5 volts.

Step 6: Test the Alternator With the Engine On

Next, test the alternator with the engine and electrical systems turned on.

  1. Turn the engine on.
  2. Check the voltage meter again. It should read between 13.5 and 14.5 volts.
  3. Stress test the alternator by turning on the car's radio, headlights, and air conditioner.
  4. Check the voltage meter to make sure there is not a reduction in voltage with the electrical systems turned on.

If the voltage does not change when the engine is started, or if it does not read between a minimum of 13 volts and a maximum of 15 volts, the alternator may be faulty.

Step 7: Finishing Up

Now that you've checked your car's alternator, it's time to finish up.

  1. Turn the car off.
  2. Disconnect the voltage meter from the battery terminals.
  3. Close the hood.

If your tests result in normal voltage readings, but you still have issues with battery warning lights, a discharging battery, or your car's electrical systems, further testing is needed. You may want to enlist the help of a professional mechanic who can test or replace the alternator for you.

YourMechanic offers car repair and maintenance services at the convenience of your home or office, 7 days a week, and saves you up to 30%. A version of this article originally appeared on How to Test Your Alternator.

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    5 years ago

    Here's a tip I would add: check to make sure your battery post terminals are clean and tight. Don't just jiggle them - undo, inspect and/or clean & re-tighten as necessary. My solution for battery/alternator monitoring - if you do not need the cigar lighter socket or plug in a splitter, is to get a little digital voltmeter about 1" x 1" x 2" and stick it with double-sided spongy tape at a convenient spot on the dash. Just a short cord with plug from an old 12v accessory, or buy one at the auto supply. If you want to get fancy you could make a cut-out in the dash and run the wire internally to the cig lighter terminals.


    Reply 5 years ago

    Good points!


    5 years ago

    The easy way is just to face you car up against a wall in the dark.

    Switch your engine on and let it idle.

    Switch on your headlights and slightly rev the engine.

    If your lights get brighter on the wall your alternator is working.


    5 years ago

    One more step.

    Turn OFF all accessories, and with the engine running - turn your DVOM to the AC setting and read the value.

    Be sure to run this test as close to the anternator output lug as posible since the nearer you test to the battery, the "softer" the signal might be and create a false negative. The battery acts as a huge "sink" or shock absorber for voltage spikes, but it won't protect devices forever that need good electricity.

    The output voltage on the AC scale must not exceed 0.002VAC or there is likely a diode problem. Modern auto computers - and newer vehicles may have more than three units - thrive on purity and quality of your alternator's output.

    Sine waves or spikes in the digital signal can cause a lot of havoc with on board ECMs, ECUs, BCMs, TCMS and such that can damage the computer or the system it controls.


    Analog meters should be used with serious caution for underhood testing as they may inadvertently get into a 5VDC signal or supply path and can load down the fragile system too much. You can damage sensitive electronics with analog meters and worse yet - test lights.

    DIY Hacks and How Tos

    Thanks for posting the tutorial. I have been wanting to start learning about how my car works.