68 Mustang Headlights Out - What Next? - Vintage Mustang Forums
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post #1 of 11 (permalink) Old 09-17-2019, 09:54 AM Thread Starter
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68 Mustang Headlights Out - What Next?

About a month ago the low beams stopped working on my 68 coupe. The high beams still worked, though. A couple weeks later, the high beams stopped as well. Dash lights, taillights, turn signals, parking lights, etc all still work.

So, I started reading and on the advice of more than one thread I replaced the headlight switch. That didn't fix the problem, so I replaced the dimmer switch. The problem persists.

Where do I go from here? In pretty much every thread I found about headlights out, the fix was one of those switches. Any suggestions would be hugely appreciated.
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post #2 of 11 (permalink) Old 09-17-2019, 10:37 AM
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Have you checked the bulbs themselves?
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post #3 of 11 (permalink) Old 09-17-2019, 11:00 AM
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Diagnosis instead of throwing parts and money at it. Do you have power coming out of the headlight switch? Out of the dimmer switch? Are the bulbs good? Buy yourself a simple test light and start tracing back to where you lose power.

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post #4 of 11 (permalink) Old 09-17-2019, 11:04 AM
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The diagram.
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post #5 of 11 (permalink) Old 09-17-2019, 11:51 AM
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Grab your test light and find a suitable ground under the dash... a courtesy lamp housing is usually a good one...

I usually unscrew the dimmer switch from the floor but leave connected for ease of testing.... turn the headlamps on and probe the plug from the back side...

You SHOULD have 12V in TWO places. When you depress the dimmer switch you should STILL have 12V in TWO places, but one of them changed positions.

If you DON'T have 12V ANYWHERE, disconnect the plug from the dimmer switch, wait 30 seconds, then probe the plug again. If you now have 12V in ONE position that didn't have voltage a minute ago, you most likely have a short or extremely high resistance between the dimmer switch and the bulb(s) that is tripping the headlamp switch internal circuit breaker. If you didn't OBTAIN 12V after disconnecting the plug you probably have an open circuit between the plug and the headlamp switch. Check the headlamp switch plug that all the terminals are firmly secured and that the new headlamp switch has the same terminal arrangement as the old one.

If you have 12V at TWO positions in the plug with it attached to the dimmer switch, but no headlamps working then check the bulbs and/or the plugs at the bulbs. If they're good, look for a split in the headlamp wiring somewhere between the bulbs and the dimmer switch.
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post #6 of 11 (permalink) Old 09-17-2019, 01:59 PM
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Nice write up Woodchuck...



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post #7 of 11 (permalink) Old 09-17-2019, 04:02 PM
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its also a good idea while your in there to put some dielectric grease on all your external sockets.
(clean them up first)
do it once and youll be good for many years to come if not a lifetime
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post #8 of 11 (permalink) Old 09-17-2019, 09:07 PM
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Quote:
Originally Posted by Blues Power View Post
its also a good idea while your in there to put some dielectric grease on all your external sockets.
(clean them up first)
do it once and youll be good for many years to come if not a lifetime
Actually, you want "bulb grease". Silicone dielectric grease is an insulator. It works great on spark plug boots and although it may protect against corrosion when applied to a metal surface it isn't the best for "conductivity".
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post #9 of 11 (permalink) Old 09-18-2019, 07:58 AM
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dimmer switch
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post #10 of 11 (permalink) Old 09-18-2019, 07:07 PM Thread Starter
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Thanks for the advice, everyone. I pulled the headlights out today. I've got 12V on both plugs and zero continuity between the poles on the bulbs.

I'll get new bulbs tomorrow. Hopefully that fixes things, but I'll be worried that they'll burn out also. Seems unlikely that the headlights died together of natural causes.
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post #11 of 11 (permalink) Old 09-19-2019, 08:31 AM
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That's pretty strange. Just for good measure check out the headlight grounds while you're up there in the front of the car.
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