Mechanics

Alternator Test

Step by step guide on how to test an alternator. When this component fails it can cause the "check engine" or "service engine soon" light to come on. This article pertains to all non-hybrid car models.

Difficulty Scale: 3 of 10

Tools and Supplies Needed

  • Voltmeter
  • Protective gloves and eyewear
Step 1 - The alternator needs to output a voltage level of about 13.6 to 14.3 volts and about 30 amps under normal running conditions, if not, the alternator needs replacing. To test for alternator output connect a voltmeter to the battery terminals. Once connected start the engine and hold engine idle at about 1200 RPM's. (If engine wont start because of a dead battery visit: Jump starting.) If the alternator has failed visit: Alternator replacement. If the alternator tests okay, and the engine will not crank over visit: Battery test

Alternator Charging Voltage
Alternator Voltage Test

Car Alternator
Alternator

Step 2 - (Note: this test is for unusual charging problems, most of the time the Step 1 test is sufficient.) Testing the amperage output of the alternator is good for measuring the "volume" (not the level) of voltage the alternator can produce. This test can be tricky because if the alternator is producing insufficient voltage, it can still produce low amperage. If the amperage is low, it will slowly allow the battery to drain down. To check the amperage output of an alternator an amp meter is needed. Once the meter is connected start the engine. Next turn on all electrical accessories and raise the engine idle to about 1500 RPM. The alternator should output the max amperage it was designed to produce. Example: 90 amp alternator should output about 88 amps. Note: An alternator cannot sustain maximum output for long periods of time. If the alternator is forced to operate at maximum output it will overheat and fail. An alternator is designed to operate at max amperage but only for a brief amount of time.

Helpful Information

An alternator is responsible for charging the electrical system of a vehicle which is powered by the engine. While the engine is running the alternator charges the battery for future use. (Note: never disconnect the battery while the engine is running. If the battery cable is disconnected from the battery a spark can be generated which can cause the battery to explode or cause a major electrical malfunction to occur.) Electrical voltage and amperage are generated to recharge the battery and supply voltage to the electrical system of the car while in operation. The alternator is held in place with mounting bolts which are connected to the engine. There is a main electrical wire on the rear of the alternator that supplies voltage to a main voltage distribution box. If an alternator is not charging properly, the battery will slowly drain down until the minimum voltage threshold occurs and stops the engine from running. (The engine ignition system needs a sufficient amount of voltage to operate.)

Common Problems:

  • Alternator shorts out and stops producing electrical power. The car will stop because of the lack of electrical power.
  • The battery can fail do to age or defect.
  • Battery cables become loose causing the electrical system to fail.
 

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-15)