Both Headlights Blow Out at the Same Time

Both Headlight Blow Out - 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)

This article pertains to all cars

Difficulty scale 1 of 10

Tool and Supplies Needed

  • Voltmeter

Alternator Test

Step 1 - Using a voltage meter connect the meter leads (red to positive and black to the negative terminals of the battery). Then start the engine and allow to idle (do not drive) the voltage should be between 13.6 to 14.8 volts. Now have a helper increase the engine speed to about 2000 RPM's. The voltage should not go higher than 14.8 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 and electrical system damage. Most alternators are designed with internal voltage regulator that cannot be replaced unless the entire alternator is disassembled. In this case its best to replace the alternator. After the alternator or voltage regulator replacement is complete replace the headlight bulbs and any other bulbs effected by the malfunction and recheck system.

Best Practices

  • When replacing the alternator use quality parts


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-01-02)