TAC Tech > Electrical

1979 Trans Am - Brake Lights Not Working

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steedaq:
All,

Just wanted some thoughts and trouble shooting guidance before I potentially went down the wrong path.  Last week a coworker mentioned that my brake lights weren’t working so I took a deeper dive, and this is what I found out..

Rear taillights work
Dome lamp and trunk light work
I have 12 V to the brake light switch (orange wire I believe)
Fuse #4 is good
Hazards flash in the front but don’t in the back
Turn signals don’t work

Do you folks recommend any other checks or have any idea what the root cause could be?  I appreciate any help you can provide.

TonysTA:
Hi - I just went through debugging all my lights. Start with the simple stuff 1st, like making sure the bulbs are good and cleaning up your grounds in the trunk.

steedaq:
Bulbs are good but that’s a great point regarding the grounds.  I’ll definitely check those out.  Thanks!

b_hill_86:
I don't have an exact answer for you off the top of my head but i'm leaning toward your turn signal switch to some extent. I don't know if they go bad or not, maybe someone an chime in on that, but here's what I'm thinking...

You said your stop lamp/haz fuse is good so starting from there, there's an orange wire that runs from the fuse to feed the trunk lamp, if equipped and yours is, and the dome light (which you say also works). From there, it runs to the brake light switch and feeds 12v to that. That's the orange wire you tested at the brake light switch. Now from there, when your brakes are applied, the brake lamp switch sends 12v up to the turn signal switch. The reason being, your brake lights need to be "overridden" by the turn signal since the rear signals and brake lamps share the same bulb.

I can't find a wiring diagram of the turn signal switch in the column yet but with no turn signals on, that switch would have to allow 12v through it to feed the brake lamps. Then, say you flip on your left signal, it would have to interrupt the 12v feed from the brake switch and replace it with the intermittent 12 volts from the turn signal. So my guess is maybe your switch is bad. If I find a wiring diagram for the signal switch I'll have a look but it seems like it would be possible for the switch to successfully feed power to your front signals but not the rear.

firebirdparts:
The brake lights (at the switch above the pedal) are powered all the time.  From the brake light switch, power goes up into the turn signal switch.  When you signal, the brake light on one side is disabled on American red-signal-light cars.  So there are contacts inside the column for each brake light which are totally independent. You've already stated they both don't work on 3 independent power supplies with independent switching.  (brake/signal/hazard all totally separate)

Based on all that I think you're unplugged at the column.  You can't really lose both brake lights at the same time unless you lose power on that circuit before it goes into the column, or if all the bulbs go bad.  All the bulbs bad is probably likely as strange as that sounds.  The brake light switch and that plug are only about an inch apart.  Some of that wiring inside the column might have had a mishap, I don't know.  The bulbs are certainly not reliable.

If park light filament is intact and working, that tells you nothing about the brake light filament. The filaments burn out one at a time.

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