The Normal Alternator Output Voltageby Jay Motes
An automotive alternator generates electrical power to run the engine and charge the battery. An alternator that is putting out a reduced charge will eventually cause the engine to stall and not properly charge the battery, which can cause starting problems.
An alternator should put out between 13.5 and 15 volts of power. The alternator should be tested with the engine idling and electrical accessories, such as the radio and headlights, turned off. A reduced output may be a sign that the alternator is failing, but the drive belt and wiring should be inspected before replacing the alternator.
A stretched or slipping alternator drive belt may cause the alternator to not produce a full charge. The belt should be inspected to assure it is properly tensioned and not damaged. A drive belt should be soft and pliable. Belts that show cracks, uneven wear or have a glazed appearance should be replaced.
The wiring and connections attached to the alternator should be examined for looseness, corrosion or other damage. Corroded battery terminals and cables are a common problem on many vehicles, and this corrosion may cause a discharge of voltage and should be repaired.
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