07 Drivers headlight won't work - low beam only
 

Okay, I've seen this problem before but never found an answer...
I have a 2007 V6 Mustang. One night, for no apparent reason, my driver's side headlight shut off. I pulled the bulb and replaced it with one of the dozens in my "tool box"...still no light. So, I shut everything off, put the (possible "problem") headlight from the driver's side in the passenger side and put the fully functional bulb on the drivers side. Turned the lights on and both worked. Drove a block and the drivers side went out again. Trick is, BOTH headlights work with the high beams on! At this point I did a quick inspection of the bulbs to find that neither were bad.

Then the magic...turn the lights off, and back on and they work...until you drive 100 feet or so. Then switch them off and back on and you have another 100 feet of illuminated driving. At this point I checked all the grounds to ensure that I didn't have a lose ground. Seeing that the ground was tied in to the same spot for both lamps I sanded the connection for good measure, and reattached the ground (Ground is not the issue)
Here's where it gets interesting...as I have voltage tested everything with a multimeter..

So, it's not the bulb (I've tested new bulbs from at least 5 manufacturers)...

It's not the control module near the kick panel (replaced and reprogrammed it)....

It's not the plug itself (replaced both plugs and added ceramic, high-temp plugs)...

I've taken it to the dealership, the body shop, and an electrician and none can find the source of the problem.

I thought it might be the switch itself so I removed it. I found that nice burnt out switch light that everyone talks about but no other signs of short or damage...so I pulled out the multimeter again and everything tests fine...even when I reproduce the "100 foot and she's out" problem. Even when the low beam goes out all "sockets" are reading correct voltage. Can ANYONE help? Sick of driving around with my fogs only!

(And no...NOTHING is wired to my headlights, the harness, or anything surrounding! All aux. lighting and accessories are straight from the battery with fuses, relays, separate grounds, and upgraded wiring and connectors...so nothing "hooked" to the car can be interfering.)

When you say fogs, are they the Pony Package Fog lights or did you switch out your grill with a GT? Just curious if a mod could have caused this.

No, they are not pony package. They are Street Scene Eleanor style grill H10 fogs. However, they are not wired to the headlamps. They are wired from the battery with a remote....not connectivity of any lights or accessories to the harness, switch, or even the same grounds...and its only the driver's side headlight.

Have you tried jiggling the white wire that runs from the left headlamp?

I'm sure I've jiggled every wire in the last week but I'll go back and give that one some special attention. Its driving me crazy! I guarantee that it is something ridiculous and simple...no to fond out what that something is! :/

If your wires are all good, then it has to be the SJB causing this.
Check for +12 on the white wire at the SJB when the drivers side
goes out to confirm this.

http://iihs.net/fsm/?dir=40&viewfile... Autolamps.pdf

SJB (junction box) has been replaced...bulb has been replaced...plugs have been replaced...only thing yet to be replaced is the switch. Going to try the switch I have but highly doubt that's the cause...though the switch, when unplugged, does turn the lights on...and they both stayed on...for a day. So...I'll try the switch and then I'll test all the lines from the bulb to the ground to the sjb and to the switch.

Thanks for the great suggestion though!!! Took me two days to convince the dealership to even look at the junction box. If anything else comes to mind let me know. If I have to I'll wire the damn things hard-wired with a separate switch....as this is getting quite frustrating

I still think it's the white wire or something connected to the white wire. Measure the voltage at the white wire when the headlight switch is on, but the headlight has gone out. Maybe you need to pierce the insulation to get at it. If it reads 12 volts or so, I give up. If it is reading 0 volts, then follow the wire to the next point; I think that is the BEC, the underhood fuse box. Then it goes to the SJB. If you did as 157dB suggested, presumably the SJB is reading proper voltage, even when the headlight has gone out. So the problem is between the headlight socket and the SJB. If the white wire theory is correct, methodical testing should disclose the point where the 12 volt reading changes to 0 volts.

Best answer EVER! I will try that tomorrow night. Makes complete sense. I mean, turning the switch off and on will make the light come back on...momentarily...which, to me, would say the junction box is registering correct output...but the light is not receiving the power. Having replaced the socket/plug I have ruled out a faulty low beam connection as the high beam still works. I guess the only thing that makes me question a simple wire issue is that it comes on and goes off...though the fact that it only shuts off while moving would suggest either a faulty connection or minor short/arc...though it seems like a (not saying it is...just symptom-wise) relay or switch or circuit issue...like when it "heats up" it shuts off.

Maybe the relay is bad.