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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 06-05-2013   #1 (permalink)
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Default Tail and Brake lights on 66

Maybe someone can point me in the right direction....
I have a 66 coupe - when I turn on the tail or head lights, both filaments in the back light up. When I push on the brake pedal, both filaments light up (as they should).
I have replaced the turn signal switch already, is there something that I am missing? Not having brake lights work correctly makes it dangerous to take out on the road, I'm worried someone will plow into me from behind.


thanks in advance
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Unread 06-05-2013   #2 (permalink)
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If I understand you correctly, when turning on the headlight switch both filaments in the tail lights light up. Then you turn off the headlight switch, hit the brakes and both filaments light up.

Looking at the schematic, when headight switch is turned on, wire 14 should have 12v on it and that filament should light up for the tail lights.

Wires 9 and 5 coming out of the turn signal switch should not have 12v on them unless the turn signal is turned on, but apparently you have a short in the new switch or the switch is just bad. The aftermarket switch's are known to have problems...let me guess Scott Drake maybe? Are you using the stock steering wheel or an aftermarket as lots of people have problems with the Grant's being a source of problems.

Double check your wiring color code versus the schematic and if you didn't make a wiring error, or cause the short, you can blame it on a faulty switch.

Its a shame thats such a hard place to get to and work. If I ever have to replace mine I'm going to bring all of the leads out and run them through an accessable terminal block so troubles can be tested easily.
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Unread 06-05-2013   #3 (permalink)
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I don't have a schematic, which is wire 14? Also not looking at the car right now but I don't remember that many wires in the turn signal switch...
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Unread 06-05-2013   #4 (permalink)
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JSH... here's a link to the wiring diagram for the exterior lighting on a 66. There's a sticky on the main forum page which links to other schematics.

http://hammar.dyndns.org/~djhamma/wi...6/66exter1.jpg

I'm sure Veronica can diagnose this in a heartbeat. In the meantime, my guess is that it's still a problem in your turn signal switch, as that's where the brake filaments get their power. Or, perhaps something is bridging the circuit in the light sockets themselves.
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Unread 08-26-2013   #5 (permalink)
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Well, just getting back to this....

Got a new repro switch from CJ Pony parts and put it in yesterday morning. Really helps to take out the two phillips head screws so that the cup or whatever it is can slide down the column - that way you can see the wires going into the slot. BFH helped there too... I did find that one of the wires on the switch was torn through the insulation, green with white stripe.

But, still no love on the brake lights. Guess I'll have to trace back more to ensure I'm getting power in all the right places.


Oh, and the wires on the new switch were a funky bright pastel color. You can tell what color they are supposed to be, they just don't match the originals at all.
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Unread 08-26-2013   #6 (permalink)
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You are missing a ground connection at the light sockets, either the sockets or the bulb base are corroded and not making connection, or your ground wire for the wiring harness is not hooked up for the bulbs. The only other way this can happen is if a bulb is broken and the filament of one falls on the other filament (I have never seen this but it is possible). I always remove the lens on all the turn signal,parking, taillights, so I can view all the filaments. You need to only have one type of light on at a time to troubleshoot this problem, verify the brakes first, with the brake pedal pressed and tail lights off, the heavy filament in the brake lights. Good Luck.

Last edited by rex1965; 08-26-2013 at 09:59 PM. Reason: more info
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Unread 09-13-2013   #7 (permalink)
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Success of sorts....

While rummaging around in the trunk I found that the driver side was unplugged from the socket, so I fit that back together. Feels rather loose even with the 47 years of use. Also, while in there I unscrewed the ground wires on either side of the taillights, sanded the ring terminals and the floor underneath and then screwed them back in.
Later, I turned on the lights and then hit the brake pedal and things appear to work correctly now!
So thanks for the answers and guidance, now on to more problems/opportunities with the car....
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