How to Check a Car's Alternator

The alternator provides 100% of the engine's electrical power after it starts.

The days of removing the negative battery cable with the engine running in order to determine if the alternator is functioning properly are long gone. The electronics in today's computer-controlled vehicles can be seriously damaged by a voltage spike through the electrical system when the battery is disconnected with the engine running. Proper diagnostic procedures and equipment must be used to safely test the charging system in order to prevent damage to sensitive electronics. However, even given the electrical system concerns, the average home mechanic should be capable of accurately diagnosing alternator problems.

Things You'll Need

• Digital volt/ohm meter
• 12-volt test light

Instructions

1. Place the transmission in "Park" and apply the parking brake securely. Turn the ignition key to the "Off" position and wait 10 minutes to allow any timed circuits, such as automatic headlights, to completely turn off.

2. Set the digital volt/ohm meter to read D/C volts. Touch the test leads of the digital volt/ohm meter to the battery terminals. Charge the battery before continuing testing if the voltage is below 12.5 volts. A partially discharged battery can cause a good alternator to test faulty.

3. Clip the test light cable to the negative battery post. Touch the probe end of the test light to the charging post -- the large gauge battery charging wire is bolted to the charging post -- on the back of the alternator. Repair the broken wire or replace the charging system fuse if there is no voltage at the charging post and the test light doesn't turn on.

4. Turn off all accessories, such as the stereo, headlights and the blower motor. Start the engine and check the battery voltage again, in the same manner as before. With the engine running, the battery voltage should be between 13.8 and 14.8 volts. Replace the alternator if the observed voltage, when the engine is running, is not in this range.

5. Turn the blower motor on high and turn the high beam headlights on. Test the battery voltage a final time. The voltage should stay above 13.8 volts. Replace the alternator if the battery voltage drops below 13.8 volts when the accessories are turned on and does not recover rapidly to the proper range. It is normal for the voltage to momentarily drop below 13.8 volts when the accessories are initially turned on, and then rise to the 13.8 to 14.8 range.

References

• YouTube: How to Test Alternator Voltage