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Left headlight dim - stumped on this one!

Discussion in 'Fisher Engineering Discussion' started by mtnbktrek, Feb 13, 2016.

  1. mtnbktrek

    mtnbktrek Senior Member
    from NEPA
    Messages: 144

    Left headlight is dim on low beam
    Switch to high beam no change still looks like dim low
    Changed headlights side to side still issue only on left
    Unplug left light have 12v when testing the pigtail
    Plug light in and test comes to 5.8 volts
    When I unplug plow lights from vehicle the vehicle lights work fine
    Grounds for lights I believe go to vehicle harness.
    Right headlight is fine this entire time

    I believe I have the 12pin plug the headlight pigtail colors are white black blue -blue being ground

    It's a lil colder than -10 w wind chill temps are dropping and both garages are full. Any help?

    Also this just happened while driving nothing was changed
     
    Last edited: Feb 13, 2016
  2. fhafer

    fhafer Member
    Messages: 66

    You have a high resistance connection...could be the hot or ground. Figure out which one by running a wire from the negative post on your battery to the first connection back from the light on the ground wire. If it stays dim the bad connection is on the positive leg...trace it back to find the bad connection.

    If the light goes bright with the ground jumper your bad connection is on the negative leg...trace the ground wire back until you find the bad connection

    Hope you find it quickly! Good luck
     
  3. Too Stroked

    Too Stroked Senior Member
    Messages: 555

    Also could be simply a bad ground. Always check them first.
     
  4. fhafer

    fhafer Member
    Messages: 66

    A bad ground would affect both sides I believe.
     
  5. fhafer

    fhafer Member
    Messages: 66

    The bad connection will be located in the wires not common to both lights, I would guess it is in a connection exposed to the weather. Dielectric grease is your friend.
     
  6. basher

    basher PlowSite Fanatic
    from 19707
    Messages: 8,992

    jumper a ground to the light. if it fixes it throw a frame ground to it. After the storm you can track it down and fix it right.
     
  7. mtnbktrek

    mtnbktrek Senior Member
    from NEPA
    Messages: 144

    First thing I did was jump the ground off the hlight to the ground on the running lights - no Change.

    Have not tried to ground direct to battery yet

    Assume it is a ground I saw a scotch bloc on a wire I hate them going to look at it better in am
     
  8. fhafer

    fhafer Member
    Messages: 66

    Check the bulb socket closely too. You can jump a hot wire to the light also to test the socket. Most headlights are switched on the + leg; you can verify this by doing a continuity test on the - wire to ground with the lights off.

    With the lights off run a hot wire from your + battery post to the first hot wire connection back from the headlight. If it's dim check the socket. I put a cheap inline fuse in my test jumper wire for doing these tests in case I short the test lead.
     
  9. dieselss

    dieselss PlowSite Fanatic
    Messages: 10,963

    Since you didn't specify.
    What lights are these? And 3 port Iso with 3 plugs at the grill?
     
  10. mtnbktrek

    mtnbktrek Senior Member
    from NEPA
    Messages: 144

    They are the sealed beam

    It has 3 plugs hanging off front and 4 plugs on iso
     
  11. dieselss

    dieselss PlowSite Fanatic
    Messages: 10,963

    So the square ones or the oval new style ones?
     
  12. mtnbktrek

    mtnbktrek Senior Member
    from NEPA
    Messages: 144

    Square.

    Found problem the left headlight groundwire on the truck side of the plug was swollen a few bends the insulation cracked and was all green dust. Of course it happened flush w the plug and needs a new one
     
  13. Too Stroked

    Too Stroked Senior Member
    Messages: 555

    Glad you found the problem. The reason I originally posted up my suggestion is that a really good Service Manage I work with once told me: "All stupid electrical problems are a bad ground." And you know what? He's almost always been right!
     
  14. fhafer

    fhafer Member
    Messages: 66

    The insulation may have cracked due to overheating. I'd inspect all your plugs, clean them well, and use dielectric grease when you reassemble. The grease inhibits moisture intrusion... the leading cause of corrosion and high resistance connections.

    Glad you found the bad ground wire.