Brake light filament not working - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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Old Nov 17th, 19, 7:21 PM Thread Starter
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Adam
 
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Brake light filament not working

Hey fellas, brand new to the forum here. I am having a few electrical issues with my new purchase (1970 Chevelle)



The left tail/brake light situation is weird, simply because power is getting to the rear light, but in a weird way. I've changed the bulb to no success, so it's something in the wiring. Ultimately, the bright filament isn't getting power. Of note, everything else works fine (front flasher, running lights, etc) Here's the situation on the rear left side:


Headlight switch off: brake pedal activates tail light filament (dim)

Headlight switch on: tail light is normal (dim), but when I hit the brake pedal, there is no light (neither filament)



Both turn signals work, but again, weird situation on the left
Headlight switch on OR off: tail light flashes (dim) instead of brake light filament


I checked wires, and the yellow wire on the left was scuffed, but still intact. I'm confused as to why the bright filament isn't getting power, but the turn signal is still working, but on the dim filament. Is a wire crossed somewhere? The right has no issues.



Pretty basic wiring, but I'm stumped.
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Old Nov 17th, 19, 7:37 PM
blm
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Re: Brake light filament not working

Bad ground? On my 69 the brake light socket gets its ground by contact with surrounding body structure. Assume 70 is the same.
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Old Nov 17th, 19, 7:45 PM Thread Starter
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Re: Brake light filament not working

If it were a bad ground in the socket, would any of the lights work? seems odd that the tail light filament will light up, but when I press the brake with the headlights on, no light at all from the filament. When I press the brake with no headlights on, only the dim tail light filament comes on. I'd think ground would affect tail light, brake, and turn signal equally??
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Old Nov 17th, 19, 8:37 PM
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Marco
 
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Re: Brake light filament not working

It seems like you are looking in the most logical places.

Assuming that the socket is getting full 12v.
Assuming all the wires are firmly connected to the socket.
Assuming socket is not corroded or gunked up with grease.
Then the next step will be the ground. A weak or corroded ground could lead to some weird stuff so it's worth checking out.

After that start tracing wires back to the switch. Though I would look at the switch before working through the wires in the interior.

Those are the generic steps I typically use when trying to solve electrical problems. I find they require a lot of time and patients.
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Old Nov 17th, 19, 8:44 PM
blm
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Re: Brake light filament not working

To exclude the possibility of a bad ground do the following. On any of the conditions that you experience a dim light, remove the socket and bulb intact, take a length of stranded wire and touch one end to the metal part of the socket and the other end to a metal part of the trunk. If the light improves it is a bad ground. If it does not then grounding is not the issue.
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Old Nov 17th, 19, 8:50 PM Thread Starter
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Re: Brake light filament not working

So, with my conditions mentioned above, power is clearly getting to the tail light AND brake light, but just not enough to activate the bright filament...am I understanding that correctly?

With that, would replacing the socket itself just be easier? Unless it can be disassembled, then I can take my dremel wire brush and try to clean it.
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Old Nov 17th, 19, 8:52 PM Thread Starter
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Re: Brake light filament not working

Additionally, when I disassembled the tail light, I noticed that neither reverse bulbs were in; do you think the previous owner removed them for a reason (blowing fuses, switch not installed, etc), or should I put a bulb in a see if they work? What's the worst...a blown fuse?? What's the bulb? I have a handful of random extras laying around that I could try.
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Old Nov 17th, 19, 8:53 PM
blm
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Re: Brake light filament not working

See post #5. It's called trouble shooting.
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Old Nov 17th, 19, 9:07 PM
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Re: Brake light filament not working

Listen to Brad. Do it.
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Old Nov 17th, 19, 9:26 PM Thread Starter
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Re: Brake light filament not working

Quote:
Originally Posted by blm View Post
See post #5. It's called trouble shooting.

will do! just doing my research tonight before I get into tomorrow, definitely will try this; just trying to see if I'm missing anything simple, thanks.
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Old Nov 17th, 19, 9:53 PM
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Re: Brake light filament not working

What you are probably experiencing is what they call back feeding. Due to the desigs of the older style combination marker light/turn signal bulbs (1157 style) is if a light ground is not present, when the brake is depressed power is then sent to the bulb but then without a proper ground, now power gets fed (backfed) into the other part of the bulb witch is the parking light circuit and screwy thing happen. It can also have the opposite type of action in that if the parking lights are turned on and a front end 1157 bulb is not getting the proper ground, one can see an indash turn signal indicator light up.

Jim
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Old Nov 17th, 19, 11:49 PM
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Re: Brake light filament not working

With bulb removed, do you have power to the yellow wire with the brake pedal depressed? Do you have you have power to the brown wire with lights on? The problem your describing is usually the socket terminal is not making good contact with the bulb or is corroded.

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Old Nov 18th, 19, 11:42 AM Thread Starter
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Re: Brake light filament not working

Quote:
Originally Posted by oldcutlass View Post
With bulb removed, do you have power to the yellow wire with the brake pedal depressed? Do you have you have power to the brown wire with lights on? The problem your describing is usually the socket terminal is not making good contact with the bulb or is corroded.

Ya, the more research I do, the more it points to a faulty socket; I went ahead and ordered one and I'll install this weekend. All the operations work properly (tail light, turn signal, and brake), but they just aren't lighting the bright filament at the right time. I used a small wire brush dremel tool on the contacts, but it may be deeper in the socket where the corrosion is...???
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Old Nov 18th, 19, 4:16 PM
blm
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Re: Brake light filament not working

Adam,
Did you check your ground as suggested? Did you use a meter to see if you had 12 volts at the contact? You ordered a new socket so just wondering how you were able to determine your socket was bad? Shotgunning parts is frustrating and can get expensive not to mention time consuming and unnecessary. A multimeter is a very valuable tool which will save you time and money.
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Old Nov 18th, 19, 4:22 PM
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Re: Brake light filament not working

Th OP's description of the problem is almost always a bad ground. try the jumper to ground to see if that solves it.

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