Tail Lamps Being Backfed

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

  1. 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?
  2. 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?
  3. 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.
  4. 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.
  5. 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)
  6. 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).
  7. Nope, I tested the sockets before I ordered. I have the regular 3157-style sockets. Any other ideas?
  8. Have you compared your socket test to the replacement LED's received?

    See 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.
  9. 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.
  10. 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.
  11. I broke down and bought some resistors. I just finished installing them, and they didn't solve the problem either! Any more ideas?
  12. Measure and post the resistance from the rear light socket ground terminal to battery negative. Use a volt-ohm meter (VOM) with a new battery and calibrate the meter before starting. Use a long piece of scrap wire to extend the VOM test lead in order to reach the battery negative terminal.

    The value should be less than 5 Ohms. If the ground resistance is too high, I could see that adding to the problem.

    If the resistance is high, then for a confirming test, run a straight piece of scrap wire from the battery negative terminal to the rear light socket ground terminal to see if this solves your problem.
  13. The resistance between the rear light socket ground wire and the battery negative terminal is 78.8 Ohms. However, running a wire directly from the ground wire to the ground terminal did not affect the lamps' behavior in any way.
  14. Where were the resisters installed (in line) or across the LED bulbs? If across the bulbs, which filament (high or low).

    Confirm which sequential tail lights kit you have (the "splice in" or "plug and play")? Is this the instruction manual?


    Are you absolutely positive that the high/low/ground leads for any of the sockets haven't been reversed? If the ground has been swapped at even one socket, this could cause the back feed. Or if the HIGH/LOW has been swapped.

    Confirm that it does not matter which of the three sockets a regular 3157 incandescent is installed in always restores function.

    I'm also concerned about the sky high ground resistance. It may be pointing to a break in the ground path when the sequential kit was wired in. Which if true could be the source of the back feed. If this were my car, I would remove all (6) bulbs and measure the ground resistance of each socket back to a known good ground. Note, it will be important to remove all of the bulbs otherwise if there is back feeding, the measurement will include the back feed.

    The measurement should be almost the same for each socket. If one or more sockets are higher/lower than the other, investigate the cause.

    Can you post pictures of the tail wiring?
  15. wmburns, I have "plug and play" sequentials from American Muscle, but I got them from an AM employee a long time ago (back when they were Mustang Tuning) so I can't guarantee they're the same as what they sell today. I have already tried swapping out the sequentials for the factory harnesses, however, and it didn't make a difference, so I think we can rule them out as part of the problem.

    Since they are plug-in instead of splice-in, a lot of what you said doesn't apply. I will do more experimenting with bulb position and ground resistance to see if anything jumps out at me, but I agree that the ground resistance is way too high and should be addressed anyway. Do you have a resource that contains the locations of all chassis grounds, ground straps, and connections to the negative battery terminal? I have a copy of the FSM, but I've never found anything like that in it. I don't think my grounds have ever been correct (previous owner), but absent a definitely resource, how can I know for sure?

    Interestingly enough, I tested the resistance from the chassis ground on the radiator support to the negative battery terminal, and it was also in the 78 ohm range. At the very least, it looks like the negative battery cable / terminal needs some TLC.
  16. This tells me that the VOM meter wasn't "zeroed" before use. Or the VOM battery is weak.
  17. You're right; I re-tested the radiator support to ground resistance and it was 3-5 ohms.