dollar7499

Owned car for 2 months, 1999 911 cab 3.4l - car had no low beam headlights when I bought it, high beams and daytimes worked fine. Thought both bulbs wouldn't be burnt out at the same time, so I bought a switch. Installed the switch and found that both bulbs were indeed burned out. Replaced bulbs, ran good for 1 month. Swapped battery and bulbs blew. Headlights were not on. Replaced bulbs, worked for 2 days, disconnecting battery several times to work on steering wheel (added paddle switches and kept mis-aligning steering wheel splines) and now my bulbs blew again. Again, lights were not on. What makes bulbs burn out like that? If the same H7 bulb is used for high beams, surely the low beams can handle much more power? What should I check for? I am buying bulbs again (3rd set) and replacing the low beam relay just to do something. (oops now I see there is no low beam relay) I really should be able to disconnect my battery without this happening.

dporto

Sounds like basic electrical troubleshooting to me. The insulation on the wiring to the lights is know to deteriorate, opening the potential for shorts. In the case of a dead short close to the bulb, the bulb is acting like a fuse (if it is indeed “blown”). The first thing I would do is check the bulbs for continuity. No continuity = bad/blown bulb. If the bulb is blown, start checking the insulation on the wiring. If the insulation is degrading it should become obvious where the short is - burn marks partially melted insulation etc. Solution: Fix insulation (by either replacing the wires or re-insulating (liquid electrical tape, shrink tubing, electrical tape (very temporary) ). Try these out and let us know what happens.

dollar7499

This is going to make for a bad thread. The lights the first time were burnt out - no continuity - but when I went and checked this set that weren't working last night- they had continuity - so I put the one with upgraded wiring back in and it worked! So I took the one with trashed wiring and coated all the wires and plugged it back in and it worked.. so some shorting was causing the lights not to come on last night, but not blow any fuses or the bulbs.. I'm going to have to think about this.. maybe some safeguard internal to the switch??

dporto

Ok, so that's progress right? What's to think about? You've got some funky wiring that needs some type of remediation - how that goes is up to you. It seems like there may be some intermittent/questionable connections as well. Pull the connectors and spray them out with contact cleaner, check to make sure there aren't any bent pins etc. Like I said in my first post; pretty basic trouble shooting - don't over think it (no "internal safeguard" on the switch - it's just a simple switch - but it is wired into a fairly complex circuit so it can cause many seemingly unrelated problems). Start with the most plausible explanation (dead bulbs) and work out/back from there. You now know it's not the bulbs, should be a piece of cake! :-)

dollar7499

With the lights working again, all I could do was coat the wires and see if this ever happens again....