Alternator Test Made Easy
Your Alternator And It's Function
The simplest way to find your alternator is to follow the drive belt. When the engine is running, your alternator generates voltage and amperage for two basic functions. One is to recharge the battery, while the other is to supply voltage to the electrical system of your vehicle. On the rear of the alternator, you will find a main electrical wire that supplies voltage to the main voltage box of your cars electrical system. If your alternator is not performing correctly, your battery will be slowly drained from the operations of your vehicle's electrical systems. As your system drains the battery, you will soon find your car stalled on the side of the road. One of the most familiar signs of alternator trouble is when your dashboard lights dim as you turn on your lights, or flickering headlights.
Alternator Cable Test
Before going through the trouble of testing your alternator, the problem may be the connection. Be sure to clean all contacts both on the rear of the alternator and on the battery posts.
1.Testing the positive cable:
Connect the voltage meter's negative lead to the positive terminal of the alternator and the positive lead to the positive terminal on the battery. Start the engine and set at 1500 rpm. Turn on all electrical accessories and read the voltage meter. If it reads greater than .25 volts for a 12 volt circuit, and .50 volts for a 24 volt circuit, be sure to check all connections thoroughly, if all connections seem satisfactory, you need to replace your cable.. If it reads less than the a fore mentioned values, then your cables are in good shape.
2.Testing the negative cable:
Connect the voltage meter's negative lead to the negative terminal of the battery, and the positive lead to the negative terminal of the alternator. Run the same test as described above for the positive cable.
Testing Your Alternator Output
To truly diagnose the function ability of your alternator, is to purchase a voltage meter from your nearest auto-parts store. (These run in the price range from $10 and up.) Start your vehicle (if your battery is dead from power drain, either jump-start your engine or use a battery booster. Note: if the problem is the alternator, your vehicle will still run for a few minutes. If it won't stay running, your looking in the wrong place, the problem is a faulty or bad battery.) Connect your voltage meter to the posts of the battery. Red on positive post, black on the negative post. With your engine idling, you should be reading between 13.6 to 14.3 volts. If not within these specs, you need to replace the alternator.
Checking The Alternator's Amperage Output
Why test for amperage output? The amperage output is basically to measure the amount of voltage your alternator can produce, verses the level of voltage produced by your alternator as formerly mentioned above. (Note: The alternator may still produce low amperage even when weakened; however, it will still be to weak to recharge the battery.) In this test you will need an amp meter. Connect the positive connection to the positive-out on the rear of the alternator, and the negative contact to the negative-out. (Negative on driver side, positive on passenger generally. Refer to automotive manual.) You now start the engine.
When your engine is running, turn on all your electric accessories and raise the idle to about 1200 rpm. The amperage output should never reach it's maximum available output. (For example: a 90 amp alternator, should run at about 88 amps.) If your alternator is producing it's maximum capacity amperage, it will overheat and fail. Maximum amperage produced by your alternator was designed for short periods of time.
