How to Test Your Alternator with the Engine Off

Your motor vehicle's alternator has the responsibility of charging your car's battery, as well as energizing its electrical system when the car is running. When the alternator fails to do this particular job well, your car will gradually lose electrical power. The engine will eventually stop running because of the insufficient voltage it is getting to operate the ignition system. The result? You may find yourself stranded on the roadside!

When to Check Your Alternator

Whenever your battery warning light is on, or you recently replaced your battery or alternator, you need to test your voltage output. Testing is also essential if you hear a buzzing radio sound through your speakers. This may indicate alternator diodes gone bad, and leaking AC voltage.

battery warning light

If your car battery is overcharged, it will trigger a chemical reaction that will emit a noxious sulfur-like smell. This will indicate a voltage regulator failure that paved the way for the overcharging of the system when this happens. Your battery indicator light will not only turn on. The "service engine soon" as well as the “check engine” will also turn on.

If you need to check the charging system, it is never a good idea to disconnect the car battery while your engine is still running. This may trigger a major electrical system malfunction like a main computer short circuit. Before 1976, it was common practice to remove the battery cable for testing the charging system.

The battery is utilized as an electric shock absorber that helps stabilize the system. If you disassemble the battery cable, it will trigger a voltage spike that can potentially cause damage to major electrical components such as the computer. Thus, this is no longer done for motor vehicles today.

As mentioned, the alternator helps in powering all your car's electrical systems. These include the starter motor, lights, and speaker system. A malfunctioning or lousy alternator can cause a battery discharge that may result in electrical system problems in your car. Worse, it may leave you stuck in the middle of nowhere just when you need to be somewhere else.

It is, therefore, a good idea to be aware of some simple tests, and knowing the early indications of alternator malfunction to avoid a car breakdown.

Having said that, we’re providing a detailed step by step guide on how to check the alternator of your car.

Things You Need, When Checking Your Car’s Alternator

Before proceeding, you need first to gather the materials you'll need for testing your alternator:

  • Voltage meter or digital multi-meter
  • Rubber hose (around 3’ long)
  • Safety glasses
  • Wheel chocks

Testing Your Alternator with the Car Engine Off

Step 1: Prepare for the test.

  • Park your vehicle on level or even ground. This will allow you to work more efficiently.
  • Turn the car engine off.
  • Place the wheel chocks on the rear tire along the driver’s side. This will keep your car stable.
  • Open the car hood.
  • Open the car hood

    Step 2: Find the alternator.

  • In most cases, you can find the alternator somewhere near the engine’s top front. Alternators often have a vented, round metal housing, and containing some visible copper wire.
  • Step 3: Check the drive belt of the engine.

  • Your alternator will not correctly charge if your drive belt is loose. Thus, it is essential first to check the belt’s tightness.
  • Find the drive or serpentine belt of your engine.
  • Check for tightness; press on the belt between 2 pulleys (you can use any 2). The belt shouldn’t feel loose; neither should there be too much movement.
  • If the serpentine belt is loose, you should tighten it first before you proceed to the next step.
  • belt loose

    Step 4: Watch out for unusual engine noises.

  • Listen to your engine. Various engine sounds may indicate problems.
  • Start the engine.
  • Listen and beware of any unusual noises like squeaking or grinding.
  • To pinpoint where the noise is coming from, use the entire length of your rubber hose. Use it just like how a doctor uses a stethoscope. Put one end of the hose on the metal housing of the alternator, and then the other against your ear. If you hear loud squeaking or grinding noises through the hose, it could be an indication of a damaged alternator bearing. If this is the case, you may need to replace the alternator.
  • drive belt of the engine

    Step 5: Test the alternator with the engine turned off.

  • This is where the actual alternator testing starts. But, you need to make sure that you have already turned off the engine.
  • Turn off the car engine.
  • Turn your voltage meter on. Switch the setting to DC volts.
  • Get the positive lead and connect it to the battery’s positive terminal (+), then get the negative lead and connect it to the battery’s negative terminal (-).
  • Check the voltage meter’s battery voltage reading. It should be around 12.5 volts.
  • Step 6: Test the alternator with the engine turned on.

  • It’s now time to test your alternator with the engine, as well as the electrical systems switched on.
  • Turn on the car engine.
  • Again, check the voltage meter reading. It should be somewhere from 13.5 to 14.5 volts.
  • Perform an alternator stress test by turning the car’s headlights, radio, and air conditioning unit on.
  • Go back to the voltage meter and check to see if there is any voltage reduction when you turned the electrical systems on.
  • If you don’t notice any changes in voltage readings even with the engine running, or if the reading is not from 13 to 15 volts, then you may have a faulty alternator in your car.
  • Check the voltage meter’s battery voltage reading

    Step 7: Finish up.

  • After checking the alternator, make sure to finish up.
  • Switch off the car engine.
  • Disengage the voltage meter from the terminals.
  • Close the car hood.

  • To make sure that everything for your car is fine, use an obd2 scanner to diagnose the problems.

    Final Words

    If the results of your tests show regular voltage readings, but problems such as a discharging battery, battery warning lights turned on, or car electrical systems issues persist, you may need to perform further testing. It is a good idea to seek the assistance of an expert mechanic who can perform a better job of testing or replacing your alternator.

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