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Thread: Low Beam Headlight fuse questions / help

  1. #1
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    Low Beam Headlight fuse questions / help

    I have somewhat of a mystery on my hands. My right low beam headlight no longer is operational. I've successfully been running HIDs for 5 or so years, and I've checked both L-R ballasts/bulbs, and they both work. But the right side does not. Fuse #10 keeps blowing. I have changed nothing since the last time everything worked fine. I replaced both lighting harness looms a few years ago with new OEM units.

    Can someone give me any ideas as to why Fuse #10 keeps blowing? And also, the fusebox nomenclature states "7.5/15" for Fuse #10 and Fuse #11 (which is for the Left headlight). Why does it say 7.5a / 15a? I've been running stock OEM 7.5a fuses in the #10 and #11 slots for years with no errors. All other headlight functions are working.

    I can see in the ETM that Fuse #3 (7.5a), Fuse #5 (10a), Fuse #2 (7.5a) are related and all fuses are intact. Is P90 Front Power Distribution Box part of potential failure? I have a Euro LKM that Ethan sold me many years ago before he was made to walk The Plank around here. And yes, my batteries are primo-charged and running well. I am not an electronics guy. I have primitive understanding but I am seeking opinions




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  2. #2
    jcs1966's Avatar
    jcs1966 is offline SoCal Eights - CA BMW CCA Member
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    I would unplug the harness from the HIDs and see if you are still blowing fuses. You have a short to ground somewhere. If it just started happening, there is most likely a failed component.
    Jeremy Smith
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  3. #3
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    With the key off, headlight switch off, Using a multimeter set to Ohms: pull fuse 10 and 11 out, put one meter probe on a good ground, (engine metal, ground lug, etc) and the other probe on the terminal that the fuse plugs into, I am not sure which of the 2 terminals of the fuse will be the "power" side so check both sides. What you should find is that one fuse terminal from each fuse reads open, (usually "OL" on the meter display) This is the side that comes from the switch. The other side of each fuse will have a different reading on fuse 10 vs 11. The reading on 10 should be very low, low resistance means high current flow, which blows the fuse.

    Now to find where the short circuit is you can unplug your ballast from the harness and the meter should read open. If it still reads low resistance then the problem is between the plug and the fuse holder, if it reads open/ OL the the problem is somewhere in your ballast/bulb etc.

    If both sides have the same resistance during the first test then it is possible that there is a problem when the ballast tries to ignite. I would switch the ballast from left to right and see if the problem follows it or stays.
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  4. #4
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    Dave, if it's easy enough to swap the ballasts and see if the problem moves to the left side (fuse blows on right side) then it's most likely a failing ballast.
    Taylor in Carlsbad

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  5. #5
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    Thnx Gents, I did swap the ballasts around. They both work perfectly. I also switched them both with the fog ballasts, L&R respectively.

    This is a big let down since I can't drive this lovely beast at night now!
    Last edited by Koizumi; 05-22-2014 at 08:40 PM.
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  6. #6
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    Is your LKM throwing any codes?
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    Following, having the same exact problem.

  8. #8
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    7.5A is probably a little too low for a ballast startup. I always changed those fuses to 10A when I did my HID conversion. You may want to try it and see if that can resolve your issue for now.

  9. #9
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    I'll try 10A right now but should I worry about the small gauge of the stock harness?
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  10. #10
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    Update: Well the 10A blew instantly too
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  11. #11
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    Hmm!!! Then Something is wrong with either the Ballasts or you have wiring issues. How old are your harnesses? I replaced my harness sets and polarity reversal extension from Bill (MaxLumens). It may be something for you to explore. Alternatively you could buy another set of HID's?

  12. #12
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    I replaced my harnesses with new OEM 2-3 yrs ago, right before Bill started offering his custom units (darn!). I have 2 different and new sets of HIDs that all test well (ballasts and bulbs). I have no idea what the hell is going on, arrrgh!
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  13. #13
    jcs1966's Avatar
    jcs1966 is offline SoCal Eights - CA BMW CCA Member
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    I could be between the harness and the fuse box. You may want to trace the wire back.
    Jeremy Smith
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  14. #14
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    It's that hood. You should probably trade it with mine for testing purposes.
    Taylor in Carlsbad

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  15. #15
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    I switched everything around (ballasts, bulbs) and more fun. The fogs on both sides L/R stopped working. So now the R lows and fog are no longer working. And the L fog is no longer working. Now all fuses are intact, #2, #10, #11. No more blowing fuses. Just more woes. Am I right in thinking LKM? Or harness? I found my crappy multimeter and got #10 running big juice but I fear that my crap meter isn't working right. I lost my good one when I moved into a new house.

    I'm really wondering about the harness. It's literally new and my car has seen only 500 miles in the past 2.5 yrs. And very seldomly has it been driven at nite.

    This is very daunting and I'm not an electrical guy, more mechanically inclined. Forgive my ignorance but dang I don't know how to proceed.
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  16. #16
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    PS. Taylor, the hood is helping me 'vent' with my lighting issue ;-)
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  17. #17
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    After some more due diligence, an old thread that Forrest started got me to inspect Fuse #9 (horn). It was blown. The fogs work thru the #9 Horn fuse. So my fogs are working again.

    But alas, I'm back to square one. Damn Right Low Beam is still non-functional!
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  18. #18
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    Anybody have a spare LKM I could borrow? I'm happy to provide a deposit if need be.
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  19. #19
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    Solved! Thnx to finding my Fluke finally (buried deep in box after box) and finding a short in the headlight harness. Crappy insulation despite being only a couple years old replacement. It's a wonder that the original harness lasted since '94 with this particular 8. The replacement is already wonky and chaffing, grrrr
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  20. #20
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    Quote Originally Posted by Koizumi View Post
    Solved! Thnx to finding my Fluke finally (buried deep in box after box) and finding a short in the headlight harness. Crappy insulation despite being only a couple years old replacement. It's a wonder that the original harness lasted since '94 with this particular 8. The replacement is already wonky and chaffing, grrrr
    Self fusing tape comes in handy here. When I did my Euro lamp conversion, I used the cable sheething for IT equipment. It was rates up to 200 Celius. Seems to have held up nicely for the last year. I also used Bill's excellent wiring harnesses. Pricey, but worth the cost. Probably still cheaper than OEM.
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  21. #21
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    Very nice (fusing tape). Great stuff! I used some to fix/seal some A/C hoses on my other German prima donna. I got two new OEM harnesses just prior to Bill's units being offered. I'm amazed at the small gauge and terrible insulatory properties of the OEM offering. It's top dollar crap.
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