Auto Repair Help by Professional Mechanics
Troubleshooting
/ Both
Headlights Go Out
Both Headlights Go out at the Same Time
Headlights are a basic electrical bulb that depends on a constant voltage supply
to operate. If both headlight bulbs go out at the same time the system voltage supplied
to the vehicle is too high. The car's electrical supply level is controlled by the
alternator and
voltage regulator.
To test for this condition follow the procedure below: (always wear protective safety
goggles and gloves before work begins)
Alternator Test
Troubleshooting
- Using a voltage meter connect the meter leads (red to positive and black
to the negative terminal of the battery). Then start the engine and allow to
idle (do not drive) the voltage should be between 13.6 to 14.3 volts. Now
have a helper increase the engine RPM's. The voltage should not go higher
than 14.3 volts, if the voltage continues to rise beyond the max limit the
voltage regulator has failed and needs to be replaced to prevent further
bulb damage. Most alternator systems are designed with internal voltage
regulator that cannot be replaced unless the alternator is disassembled. In
this case it is best to replace the alternator.
- After the alternator or voltage regulator replacement is complete replace
the headlight bulb and any other bulb effected by the malfunction and recheck system.
For exact specific repair procedures for any of the steps above visit our
online repair
manual page.
Troubleshooting
/ Both
Headlights Go Out
Check Manufacturer Specific
Car
Headlight Questions and Answers
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