Brake Light malfunction 66' Coupe - Ford Mustang Forum

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post #1 of 7 (permalink) Old 10-11-2013 Thread Starter
batfink0's Avatar
Joined: Oct 2013
Location: Edmonton
Posts: 2
Brake Light malfunction 66' Coupe

Odd thing here...
My left brake light doesn't come on unless the right signal light is on.

Side note....
Both signal light works, as well as the park lights...

Any ideas?

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post #2 of 7 (permalink) Old 10-12-2013
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Try replacing the bulb and check the ground wires for the lights. Make sure the bulb is installed correctly as well. It should be a 1157 bulb.

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post #3 of 7 (permalink) Old 10-12-2013
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Either the turn signal switch is bad or something is wired incorrectly. Use the wiring diagram link on the previous page and a voltmeter to find out what is going on.

'65 Coupe, 302, Autolite 4100, C4, 3.00
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post #4 of 7 (permalink) Old 10-12-2013
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When was the last time everything worked correctly? What has been done since then?

These classic Mustangs used a 2 filament bulb to do 3 things. So the wiring is not easy to understand unless you really work with it. I had to draw my own simple version to get it, which I just scanned and attached.

Sounds like you have a chaffed wire problem or a bad turn signal switch. The stop and turn wires combine at the turn signal switch

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1968, hardtop, 289, C4, seafoam green.
1966, hardtop, 289, original 3 speed, on the road and fun to drive.
1968, hardtop, 200, 3 speed, new project car, Nov 2015
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post #5 of 7 (permalink) Old 10-12-2013
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Check bulb- it has two filaments: One turn one stop.

Check ground on left brake light.

This can be done by using a jumper wire from ground of bumper or other ground and other end to Brass Receptacle of light- which is ground, then apply brakes and have some one check to see it lights are on. Bad grounds can cause strange results.

Joe - K4JOE
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post #6 of 7 (permalink) Old 10-12-2013
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Hello. It is true that the tail lights have a two-filament bulb, but, the turn signal and the brake lights use the same filament. If either the turn signal or the brake light works, the problem is not the bulb, or any wiring issue in the trunk area. What you have is almost certainly a bad turn signal switch. If the problem was upstream from the turn signal switch, neither brake light would work correctly, because there is only one wire going from the brake light switch to the turn signal switch. If the problem was downstream from the turn signal switch, both the brake light and the turn signal would have the same problem on the same side. Since both turn signals work correctly, that means that the problem isn't down stream from the turn signal switch. That leaves the turn signal switch.
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post #7 of 7 (permalink) Old 10-14-2013 Thread Starter
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Thanks all.. Special thanks to 'Driveway'
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