Alternator Test
Test Alternator Before Replacing
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.
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
Alternator Voltage Test
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. Next switch the voltmeter to AC voltage and run the engine at about 1500 RPM, the meter should read 0, if voltage is present a diode inside the alternator has failed and is leaking AC voltage into the system and the alternator should be repalced.
- Voltmeter
- Protective gloves and eyewear
Alternator Voltage Test
Alternator
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