Tail Lamps Being Backfed

Discussion in 'SN95 4.6L Mustang Tech' started by Flghtmstr1, May 22, 2014.


  1. Flghtmstr1

    Flghtmstr1 Member

    Joined:
    Apr 1, 2005
    Messages:
    556
    Likes Received:
    3
    Trophy Points:
    19
    Location:
    Newtown Square, PA
    I bought LED tail lamps from VLEDs.com, and when I turn the key on and the lights on, the tail lamps are way too bright. I think they are being backfed by something, but I can't figure out what. Any ideas?
     
    #1
  2. wmburns

    wmburns SN Certified Technician

    Joined:
    Aug 14, 2009
    Messages:
    3,172
    Likes Received:
    58
    Trophy Points:
    69
    Location:
    Houston Texas
    Help me understand. Is the problem the parking lights work but are too bright? Or the brake lights come on when only the parking lights should be on? If so, how have your confirmed because the details matter. What happens when the brake pedal is depressed? Does the high mount 3rd brake light work?

    If the original bulbs are put back in, does everything work?
     
    #2
  3. Flghtmstr1

    Flghtmstr1 Member

    Joined:
    Apr 1, 2005
    Messages:
    556
    Likes Received:
    3
    Trophy Points:
    19
    Location:
    Newtown Square, PA
    With the key on and head lamp switch on, the parking lamps work but they're too bright. When I hit the brakes, they get a bit brighter and the third brake lamp comes on.

    With the key off, all rear lamps run correctly (parking and brake).

    I didn't notice this issue with the old incandescent lamps, but that doesn't mean it wasn't happening, as LEDs are much more voltage-sensitive.
     
    #3
  4. wmburns

    wmburns SN Certified Technician

    Joined:
    Aug 14, 2009
    Messages:
    3,172
    Likes Received:
    58
    Trophy Points:
    69
    Location:
    Houston Texas
    OK, it seems to me that back feed is not your problem. If it were, then the symptom would be "brake lights always on when parking lights are on".

    Besides, the car's electrical system is still 12 volts DC. Having an additional route for the current to flow isn't going to make the voltage higher.

    It seems to me that your problem is the parking lights and the brake lights are reversed. Either wired backwards, or installed backwards, or the bulb is built backwards.

    Are these LED's direct socket replacements for the Stock bulbs? If so, do they fit inside the socket and are they fully seated?

    Have you done any wiring modifications?

    I encourage you to compare the old to the new. Use a 12 volt source to test the old. Put negative on the bulb case and touch positive to each of the terminals on the end. Note which terminal produces a bright light and which produces a dimmer light.

    Perform the same test on the LED's using the same terminals. The lights should be dim or brighter just like the factory.

    Note on LED lights. Don't reverse the positive and negative. If no built in protection is provided, the LED will be ruined.
     
    #4
  5. Flghtmstr1

    Flghtmstr1 Member

    Joined:
    Apr 1, 2005
    Messages:
    556
    Likes Received:
    3
    Trophy Points:
    19
    Location:
    Newtown Square, PA
    I have the Raxiom / American Muscle sequential tail lamp kit, so all three segments of each tail lamp are always lit (when the lights are on). I should have mentioned that earlier. These are the lamps I'm using: http://www.vleds.com/bulb-type/brake-lights/3157/3157-red.html. They are 3157-style direct replacements.

    As to your question of whether I've done any wiring modifications, the answer is yes. My car is extensively modified mechanically and electrically, which makes things much trickier to diagnose. Here are the electrical modifications I've made (that I can remember):

    Sequential tail lamps (as previously mentioned)
    Bosch intercooler pump tied into random "key-on hot" wire in the passenger's side corner of the engine bay (I know, I know, I should be using a relay, but it's been working great for 5 years)
    Speedhut gauges wired into head lamp switch
    8 gauge wire feeding twin Walbro 255s, grounded to grounding lug in trunk (this is why I'm suspicious of back-feeding)

    However, I haven't made any wiring modifications since installing the LEDs, and everything was working great with the regular incandescent bulbs.

    Does this shed more light on the situation? (haha)
     
    #5
  6. wmburns

    wmburns SN Certified Technician

    Joined:
    Aug 14, 2009
    Messages:
    3,172
    Likes Received:
    58
    Trophy Points:
    69
    Location:
    Houston Texas
    Looking at the link provided gives detailed information how to tell a 3157 style from a 3157-CK style. I could see how use of the incorrect "type" could cause one of the circuit to power BOTH high and low. Which sure sounds like YOUR problem to me.

    In the end, I think your problem is far more basic (wrong part).
     
    #6
  7. Flghtmstr1

    Flghtmstr1 Member

    Joined:
    Apr 1, 2005
    Messages:
    556
    Likes Received:
    3
    Trophy Points:
    19
    Location:
    Newtown Square, PA
    Nope, I tested the sockets before I ordered. I have the regular 3157-style sockets. Any other ideas?
     
    #7
  8. wmburns

    wmburns SN Certified Technician

    Joined:
    Aug 14, 2009
    Messages:
    3,172
    Likes Received:
    58
    Trophy Points:
    69
    Location:
    Houston Texas
    Have you compared your socket test to the replacement LED's received?

    See Occam's Razor

    http://en.wikipedia.org/wiki/Occam's_razor

    When the replacement LED is tested outside of the car, the results should match what happens "inside" the car. If they don't, the odds favor the reason as the solution to the problem.

    IF the LED's behave the same way outside of the car, then it follows that the problem is with the LED's themselves.
     
    #8
  9. Flghtmstr1

    Flghtmstr1 Member

    Joined:
    Apr 1, 2005
    Messages:
    556
    Likes Received:
    3
    Trophy Points:
    19
    Location:
    Newtown Square, PA
    It's not the LEDs. If I include at least one regular bulb in one of the six tail lamp segments, the LEDs will work correctly. It doesn't matter which position the lone incandescent occupies. Does that help narrow it down? I think it is an electrical issue with my car, but I haven't been able to figure it out.
     
    #9
  10. trombonedemon

    trombonedemon Well-Known Member

    Joined:
    Jun 25, 2009
    Messages:
    2,588
    Showcase:
    3
    Albums:
    1
    Likes Received:
    35
    Trophy Points:
    59
    Location:
    U.S.A, U.S.A., U.S.A.!
    You might need a resistor, I tried to put LED's on my tail lights a the electrical system went haywire. Now some have built in resistors.
     
    #10

Share This Page