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Q & A: Determining the lead connections of an auto headlamp

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Most recent answer: 05/31/2015
Auto lamps often have two connections at the base. One for bright, other for low. The collar has a "return" lead but also pegs to control proper insertion of lamp into socket. My question: How to visually determine which of the to base connections is for bright and which for low?
- Larry (age 85)
Ronninge, Sweden

Hi Larry,

If you can't see inside the lamp then you have to use an alternate method.   One way is to get an Ohmmeter and measure the resistance between each terminal and the return.  The one with the lower resistance is probably the 'high' lead.  The reason is that presumably the high beam lamp requires more power than the the low beam.   Since the power flowing into the lamp is proportional to the voltage squared divided by the resistance, the one with less resistance is the bright beam.


(published on 05/31/2015)

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