Issue with headlights

Discussion in '2011+ Mustang GT 5.0L Tech' started by Yosep, Jul 24, 2019.

  1. Yosep

    Yosep Junior Member

    Hello all, I'm new to this forum and this is my first post, if I write something inadequate or out of place please let me know

    Now I'll start with the (still unknown cause of the issue):
    About a week ago I was driving home under really heavy rain. I had all the lights on (fog + low beams + high beams), but suddenly the high beams went out (both). I continuously tried turning them on/off several times to no avail

    Next day I took the car to the dealership to be checked, but surprisingly the issue simply disappeared. It was working perfectly. This happened several times (the trip to the dealership and the trip back with no answers). All the times they told me they couldn't find the issue, they checked connectors, cleaned them and added some grease, checked for DTCs and nothing.
    I've being to the dealership 4 times already but from time to time the issue appears again.

    What I did noticed is that if I start the car and the high beams are working, they will remain working until I stop it. Also when they are failing, the flash-to-pass seems to kind of work (the beams light up for some milliseconds and then the relay shuts them down (I can hear the "tick").

    But last Sunday I was again driving under the rain (not so heavy this time) and the fog lights (both) went out (the high beams were luckily working), and it hasn't happened again (fog lights failing)

    So, in summary, this is driving me crazy...

    A few facts I've gathered:
    - When the lights fail (high beams or fog lights), both fail together
    - The frequency of the high beams failing is much lower now than the first days
    - The fog lights failing only happened once
    - All fuses are fine
    - There are no DTCs in the entire car

    Could you help me find what the problem is ?

    Thank you in advance !
  2. travelers

    travelers Senior Member

    The members will definitely let you know if you post something in the wrong place. Now what year is you car? Is the carpet on the right side near the kick panel wet or damp. It might be the SJB getting wet or damp.
  3. Yosep

    Yosep Junior Member

    The car is 2013, GT with the HID lights. The carpet doesn't seem wet, but maybe the water hasn't reached that point yet (or I didn't notice it)
    I was checking the workshop manual (for the 2011 model) and saw that there seems to be a relay in there that controls the high beams, but if what you said is the actual problem, shouldn't I see some other malfunction as well ? I don't know what else is controlled from that module other than the lights

    How could I check if the SJB went bad ? I believe it's a rather expensive part to just go and get a replacement

    In any case it's a good point to start looking :)
  4. Sky Render

    Sky Render Stig's Retarded Cousin S197 Team Member

    How do you have both the high and low beams on?
  5. Yosep

    Yosep Junior Member

    When the issue happened the first time I had all lights turned on, but I'm not sure the module controlling them had the low beams and the high ones powered together. I for sure had lights (highs) and suddenly only lows
    The fog lights issue was similar but those just turned off on their own and no amount of toggling on/off made them light up again (until I stopped the car that is)
  6. travelers

    travelers Senior Member

    Are the HID lights stock? I would start looking at the harness for repairs or spots that don't look like they came factory. With out going into my electrical book I think the windows are also controlled by the SJB.
  7. Yosep

    Yosep Junior Member

    Yes, the HIDs are stock. Curiously the windows don't seem to have any fault. I'll check the harness to the lights in the SJB, because the cables and connectors to the lights should be fine, I checked them before also when they fail, both lights fail, which is rather improbable
  8. nyuk98GT

    nyuk98GT Infrequent poster


    Check the cabin filter drains (under the hood, passenger side, lift the cowl cover (carefully). If the drains are plugged then water can drain back into the cabin into the junction block and cause some havoc.

    Look at the connector at the headlight. Any corrosion or discoloration? A tiny dab of dielectric compound on each of the metal pins should improve the connection in wet weather.

    Look at the ground(s) and confirm they are snug. Look at the wiring harness to each side and confirm they are reasonably weatherproof (no bare wire showing).

    Confirm that the high current fuse(s) for the high beams is seated in the fuse block.

    That's all I can think of for now. I hope it helps.

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