Alternator Test

Easy step by step guide on how to test an automotive alternator, this guide pertains to most vehicles using a conventional charging system.

Difficulty Scale: 3 of 10

Tools and Supplies Needed

  • Voltmeter
  • Protective gloves and eyewear
Begin with your vehicle on level ground, engine "OFF" and in park, with the emergency brake set.

Step 1 - Identify the alternator location and inspect the serpentine belt tension which must be present for the alternator to operate correctly. Visit - Serpentine Belt Tensioner Replacement

Automotive Alternator

Step 2 - A voltmeter set on DC voltage is needed to check the alternator output.


Step 3 - You can change out the test probes to include alligator clips to help testing without holding the test probes to the battery terminals.

Alligator Clip

Step 4 - Connect the voltmeter lead to their respective terminals, red/positive black/negative.

Connect Voltmeter to Battery

Step 5 - Once connected, the voltmeter will read battery and electrical system standing voltage.

Battery Voltage

Step 6 - Start the engine and hold the idle to about 1200 RPM (just above idle), observe the meter, it should read between 13.6 and 15.8 volts depending on battery condition and state of charge.

Alternator Charging Voltage

Step 7 - To load test the alternator, turn all interior, exterior lighting systems and air conditioner system "ON", while holding the idle up slightly, the voltage should stay about the same, if the alternator fails either of these tests replacement is needed.

Alternator Load Test

Helpful Information

Never disconnect the battery while the engine is running, this can cause a major electrical system malfunction. (Note: The test of removing the battery cable to test the alternator was used for vehicles made before 1976).

If a voltmeter is not available, turn the vehicle headlights "ON" with the engine "OFF". Observe the headlight brightness while starting the engine, the brightness level should momentarily fade, then get brighter once the engine has started if the alternator is charging. When an alternator fails it can cause the "check engine", "service engine soon" or "battery light" to come "ON".

An alternator cannot sustain maximum output for long periods of time, it will overheat and fail. When an alternator diode fails it can leak AC voltage which can cause disruptions in the electrical system including battery failure. To test for this condition switch the voltmeter to AC voltage with the engine at about 1200 RPM, the meter should read "0", if voltage is present one or more diodes have failed and the alternator should be replaced.

An alternator is used to charge the battery for future use, while supplying additional voltage to the vehicle's electrical system. If an alternator is not charging properly, the battery will slowly drain down until the minimum voltage threshold occurs stopping the engine from running.

Best Practices

  • Avoid unnecessary sparks near the battery if possible to thwart accidental ignition of hydrogen gases present inside the battery causing it to explode.
Common Problems

  • Bad battery causing the alternator to fail prematurely.


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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