Alternators generate electric currents to charge your car battery while it is running. If your car’s alternator fails, you will struggle to start the car within a few hours of the failure, as the battery will have run down. A faulty alternator will likely cause your car’s "Check Engine" light to come on, and the headlights will appear dim when the engine is idling. Certain testing procedures will point out whether your alternator needs to be replaced, but these are best done by an auto technician.
Check for Physical Indications
There are a few physical indications that signify a faulty alternator. Your car’s battery warning light might illuminate randomly when the alternator brushes are worn out. When alternator bearings become dry, you will be able to hear a rough growling sound from the alternator case. Technicians sometimes use a common stethoscope to determine the source of the sound.
Testing Alternating Current
In some cases a short or open-circuit failure can occur due to a diode fail. Alternating current voltage will leak into your car’s electronic system, causing unpredictable problems with your car’s on-board electronic systems. A multimeter is often connected to the battery and set to the AC voltage position to detect alternating current. With a normal, fully-charged battery the voltage will dip below 12.6 volts and quickly recuperate.
Testing the Battery
Your car’s battery should be tested as part of the alternator testing procedure. A battery with a bad cell will not achieve normal charging voltage, causing the alternator to overcharge the remaining cells and boil the electrolyte. Battery testing should be done by a trained technician, as a charging battery produces explosive hydrogen gas.
A conductance tester is used to measure the electrical resistance in the battery, by applying a mild alternating current to battery terminals. Some technicians use load testers to discharge the battery at half its rated capacity. During this load test, the battery has to maintain at least 9.6 volts at its terminals after being discharged for less than 15 seconds.
Charging System Test
Corrosion often increases electrical resistance of the ground circuit, causing the voltage regulator to sense less than the actual B+ voltage, resulting in overcharging of the battery. In older car models, technicians install an auxiliary ground wire from the regulator base to the alternator. The voltage regulator is then disconnected to accurately test the alternator only.
With newer models, a professional scan tool is used to measure the charging voltage and retrieve any possible charging system errors. This scan tool can often be used to activate or control the field current to evaluate the alternator’s output.
Conventional charging systems are often tested with an adjustable carbon pile tester. Battery discharge rate and the alternator charging rate are measured. This load-test will quickly indicate drive belt slippage on the alternator. Technicians should, however, be cautious not to overheat the alternator when testing it for maximum output.