just spent probably an hour or more diagnosing a bad headlamp on a Subaru Outback. worked on high-beam but not on low. pulled what I guessed was the low-beam bulb out and tested it with the ohmmeter: good. now instead of checking the other bulb, I decided to hook up the meter and make sure I was getting power to the bulb. I was not, and what I saw was very confusing. of the 3 wires that came into the headlamp assembly, one split off to go to each lamp. ground, right? wrong. it was +12V. wtf? this was where I went off on a wild goose chase, finally ending up pulling the power cable off the other headlamp assembly and checking its voltages, and finding them the same: +12V, Ground, +12V on low beam and Ground, Ground, +12V on high beam. so I finally had it figured out: the ground wire went only to the low-beam lamp, and the fixed +12V line went only to the high beam lamp. when high beams are on, the relay switches the common lead to ground, turning off the low beam lamp and turning on the high beam lamp. otherwise, both sides of the high beam lamp were high. that's why when I tested voltage on it, there apparently wasn't any, because there was no differential. and of course, I was on the wrong lamp; when I finally pulled the low beam lamp and ohmed it, it was burned out.

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last updated 2015-09-20 19:33:29. served from tektonic.jcomeau.com