How to Check a Car Alternator

When the question of how to check a car alternator arises, it is important to understand the relationship between the battery and the alternator. They rely on each other. If one fails the other falters as well.

First, the battery is made up of 12-volt cells. If the battery is 600 cold cranking amps, it is essentially divided by the cells. The combination of all the cells amount to the sum total of amps. If one cell begins to wear out or short out, the battery could easily still supply 12 volts, however the amperage will decline. This makes the battery impossible to take a full charge and the car is hard to start. If a cell shorts out, the battery will have a continuous drain, causing it to not only take a less than full charge, but die over a short period of time. This is where the relationship comes into play.

An alternator's output, as stated on the identification plate, is maximum flash output. This means that the alternator can muster up 100 amps for a very short period of time. It is designed to operate at 30 to 40 percent of its maximum output continuously.
The reasoning for this is that the more the power requirement, the more power must be supplied to the rotor or field to create a higher magnetic field. As the rotor turns, this high magnetic field creates an alternating current in the stator, which results in increased heat generated. If the alternator is operated at its full potential for any length of time it will destroy itself.

Understanding the relationship, the battery must be fully charged to properly check the alternator. If the car is hard to start and the battery light is on but dimly lit, the battery voltage is excessively low. If the light comes on and stays on or goes out when the engine rpm is raised, the alternator is not operating correctly.

Check the battery with a voltmeter with the engine off by placing the red lead on the battery positive post and the black lead on the negative. The voltmeter should read 12.3 volts or a little higher. If the voltage is lower either the battery is bad or discharged due to an alternator failure. The way to determine which is bad is to charge the battery if possible then start the engine, or jump start the engine for a quick diagnosis. Keep the leads on the battery as before. With the engine running the voltage should be about 14.5 volts or a little higher if the alternator is working properly. If the battery is low shut the engine off as soon as the test is done so the alternator does not overheat. Replace the battery. If the voltage is below 13.5 volts the alternator is bad.

If the engine is hard to start and the alternator is working with the correct voltage, shut the engine down and keep the voltmeter on the battery as before. Have a helper start the engine while observing the voltmeter. A good battery will not drop below 10.5 volts as the starter is engaged. If the voltage drops to less than 10 volts the battery has a bad cell and should be replaced.

If the headlights, dash lights or radio begin to dim or flicker the alternator is going bad. The usual problem found in alternator failure is one or more of the diodes failing. This in effect would change the sine wave to more of an alternating current than a direct current, which causes the flickering.

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