Automotive alternators are generators providing electrical power to a vehicle’s electrical circuits while charging the car battery. The more sophisticated onboard electronics become on today’s cars and trucks, the more power an alternator must produce.
Early cars used true DC generators and voltage regulators for charging the onboard battery only. At the time, on early model cars, only the lights and starter required electrical current for operation.
Times have changed considerably. A vehicle today uses a host of electrical and electronic components such as GPS navigation, CD changers, stereo amps, electric seats, and seat heaters, power windows, advanced onboard computers, and much more. It’s easy to see how the power demands on an automobiles charging system are quite high on a typical vehicle today.
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A basic knowledge of how a car’s alternator works are helpful when attempting to diagnose and repair an electrical problem. Since the alternator and car battery work in conjunction with one another, it’s easy to suspect the battery as faulty when the problem lies with the alternator or vice versa.
The alternator mounts on the front, or side, of the engine and is driven by a belt. Most cars today use a single serpentine belt to drive the air conditioner compressor, water pump, and the alternator.
The engine rotation drives the belt, which in turn rotates a shaft on the alternator. Inside the alternator, hundreds of yards of copper wire wind around the shaft and rotate with it. This part of the alternator is often called the rotor because it rotates. Around the inside of the alternator, housing is several magnets of opposing polarity. This part of the alternator is called the stator since it is stationary.
As the alternators rotor spins, the electrical current pushes through the copper wound rotor by the magnets. This current channels into a rectifier where it converts to DC current. The DC current powers the car’s electronics, and charges the car battery. An automotive alternator is a complex component, but this is a simplified explanation of how it works to aid the do-it-yourself auto mechanic.
A dead battery in the morning could be any number of things. Obviously, the first step is to put the battery on a charger or jumpstart the car with another vehicle and a pair of jumper cables. This will get the car started and will probably run fine during daylight hours. But more investigation is necessary to root out the real issue.
A dead or weak battery is the first indication of a problem. Sure, the battery may be bad and ready for replacement. Or, the alternator has possibly weakened or failed and is no longer providing an adequate amount of current to the battery to maintain its charge while the car is running. Here are a few symptoms that may give the cause away.
Each of the above symptoms will naturally lead you to believe the battery is weak and ready to die. But before purchasing a new car battery, test the voltage output from both the car battery and the alternator.
A few items to check before getting too deep into the troubleshooting process is worthwhile. Ensure the drive belt is tight on the alternator and is not slipping. Check to ensure the hold-down bolts are tight that secure the alternator to the engine, and the alternator isn’t able to move.
And lastly, check the entire length of the electrical cables between the battery and the alternator for any signs of insulation damage, cuts, or burns in the wiring. If any damage exists in the cables, repair these before moving on with the alternator problems troubleshooting steps.
A simple tool for troubleshooting car problems with an alternator is a voltage-ohm-meter. This is not a special, scientific, meter that requires a doctorate degree to operate and understand. One usually only costs $5 to $10 dollars at any auto parts or department store. The meter should be capable of reading AC and DC voltage and ohms (resistance).
Place the meter selector switch in the DC voltage mode. Place the red lead onto the positive battery terminal, and the negative lead onto the negative battery terminal. Read the battery voltage on the meter with the car engine off. The ideal voltage is between 12 and 13.5 volts.
If the voltage is less than 11 volts, try to charge the battery with a battery charger to bring the voltage up to the ideal range and continue the test. If unable to charge the battery fully, then the battery is the suspect. The next phase of the test will help to verify if this is true.
While holding the meter leads onto their respective battery posts, have an assistant start the car and allow it to idle. The voltage should now read closer to 14 to 14.5 volts at the meter.
If it does, this indicates the alternator is supplying current to both the battery and to the car’s electrical system. If there is no change in the voltage reading with the car running, the alternator may be bad.
A more precise step to verify if the alternator is supplying power to the car battery and electrical system is with the use of an ampere probe. While this meter is not as inexpensive as the volt-ohm-meter, one can be borrowed or rented from most national parts stores.
The meter clamps around a single power lead to detect the amount of current, not voltage, flowing through the wire. Clamp an ohm-meter onto the positive lead from the alternator and repeat the steps used before.
Start the car and read the amp meter readout for the current flow. If no flow is detected, make sure that the meter is clamped onto only one wire, and that it is the positive, or red, the wire leading from the alternator to the battery. The detection of 3 to 4 amps on the meter is a sign the alternator is working, and the battery is suspect. If no amperage is visible on the meter, remove or replace the alternator.
While an automotive alternator can be faulty and not fail entirely, it usually either works or it doesn’t. An alternator that works intermittently is much harder to isolate, even for a professional with advanced tools.
However, the alternator problems troubleshooting steps discussed here are effective at isolating a complete alternator failure.
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