Nissan Repair: 1987 Nissan Pathfinder headlight issue, nissan pathfinder, low beams

Question
Hello,   

I have a 1987 Nissan Pathfinder, and the low beam on the driver headlight is not working. High beam is ok. Both filaments on the bulb are ok, and if I check the voltage at the connector with the bulb removed, I get 12V for both the high and low beams when selected.

However, when the bulb is actually in place, the voltage for the low beam drops to near zero when low beam is selected. If I jumper the low beam wire to the hot battery terminal, the low beam filament lights up. Same ground is used for both filaments, and I have further verified it is not the ground.   

So, I seem to have isolated the problem to the low beam hot, but not sure where to look from here. Is there a relay somewhere I should check, or how should I proceed with the troubleshooting from here?   Thanks for any advice,   John

Answer
John,

I would almost bet the headhight switch is defective.  American cars use a contact breaker switch as a safety precaution for an overheated circuit.  That is, to keep the wiring from melting and the headlights from going out if an overload is sensed a contact breaker switch pulses like a blinker.  On Japanese cars the left and right side are wired independently for both left and right for high and low beam.  Nissan uses a neat littel headlight switch with all the contacts within the switch.  But, when only one contact fails the whole switch has to be replaced since there is no replacing a single contact switch internally.  I have taken a few a part and filed the contacts and managed to get a few weeks or months out of the switch.  But, eventually it had to be replaced.  courtesynissan.com or rockauto.com are both good and cheap sources for a new headlight switch.