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Dup errbody. I hve a 95 seville. During the day brake lights work perfect. But at nite the right side wrks odd. Now all bulbs r good een the breaks r pressed the far rite bulbs brighten but the bulb jst to the left( wich r on the trunk) turns of. And wen the right signal lights r on , instead of flashing together one flashes then the other.any suggestion on how to fix?
 

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98 DeVille, 97 DeVille d'Elegance
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Re: 95 seville brake lights problem

Bulbs, sockets or bad ground. Don't overlook frt lights as they can effect stop/marker lights in the back.
 

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Re: 95 seville brake lights problem

Post your replies here jesse. my response to your PM:

it has to be related to dual element bulbs or parking lights, either bad wiring, grounds or just plain crappy sockets. If you have one that's corroded I would start there. Use dielectric grease also on the bulbs. It can be as simple as a bad ground at one location.
 

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1997 Cadillac STS (BOSE w/AUX In, Massage Seats, URPAS, "Z")
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Re: 95 seville brake lights problem

Is a bad ground(s), the bulbs are dual filaments with common ground.
The big filament (the brighter one shared between brake and turning signal) has a lower resistance than the thinner one used for parking light.
The way the dual filament bulb works is one end of each filament is connected to positive voltage (one for parking lights and one for turning signal/brakes) and the other ends of the filaments are connected together to the ground.
So as long as the ground (in the center) is connected applying voltage to the other end of each filament will drain the current trough the ground thus not affecting the other filament.
Now remove the ground (corrosion, wiring issues) and here is what happens:

Situation 1. All 3 lightbulbs have ground (note the thin filaments 1,2,5 are parking, thick filament 2,4,6 are brake/turn signal).

Note I considered the parking lights are ON and the brake/turn signal are off (as it would be when driving along).

Situation 2. Let's now consider the left bulb's ground connection is broken. The parking lights are still on.

What happens in this case. Well the ground is broken so the thinner filament (1) on the left bulb will ground trough the thicker filament (2) - serial connection - then the circuit will continue as follows: a>b>c>filament4>ground and a>b>d>e>filament6>ground (filament 4 and 6 are parallel connection).
So now the current flowing trough filament 1 is also going trough filament 2 then is split half/half between filaments 4 and 6.
Since filament 1 is thin (has a higher resistance) it will glow (although a bit lesser intensity).
The current flowing trough filament 2 is not strong enough to make it glow as this filament has a lesser resistance.
Obviously since the current trough filament 4 and 6 is only half of filament 2 these filaments will not glow either.
So from your point of view the filaments 3 and 5 will work normal and 1 will look just a tad less bright while the parking lights are ON.

Situation 3. Let's now apply brakes or use the turning signal. We are still assuming the parking lights are still on.

Note how both ends of the filaments 1 and 2 are connected to 12 volts (same potential) thus no current will flow trough any of the 2. The other filaments are also connected to 12V (both ends) but the central common end is connected to ground thus the current will flow trough each filament to the central ground.
What you see, when you step on the brakes or use the turning signal the left light bulb will turn off completely. In this situation the flasher will sense the missing bulb in the circuit (the current drawn by the 2 bulbs is 2/3 of the current drawn if all 3 bulbs will be ON) so it will flash faster - as is supposed to do.

I did not draw a situation 4 but let's imagine it:
Situation 4. The parking lights are OFF and TURN/BRAKE light is ON (also ground for filament 1 and 2 is disconnected).
Well in this situation since the filament 2 has a lower resistance than filament 1 still filament 1 will be lit yet this time filament 1 will be grounding trough filaments 3 and 5. In this situation the path to the ground is actually slightly higher resistance as filaments 3 and 5 are higher resistance, but still combined the resistance (parallel connection) is just half and will pretty much match the filament no 2 thus filament no 2 could be the "bottle neck" (you can calculate). Anyway for the viewer filament 1 will be lit although the parking/brake signal is actually on. Filaments 3 and 5 will see only half (actually slightly less) than the normal current so these filaments wil remain dim.

Turn the Parking light ON and you are back to situation 3 (bulb on the left turns OFF).

In reality you have 3 more light bulbs in the circuit (other side) and actually the filaments with missing central ground will ground trough the bulbs on the other side too. Add 2 missing grounds (or a burned filament) and you get all sort of apparently "crazy" bulb behavior.
 

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Re: 95 seville brake lights problem

Need to put that in Tech Tips or Electrical Tech... along with a google translation to Non geek speak :)
 

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1997 Cadillac STS (BOSE w/AUX In, Massage Seats, URPAS, "Z")
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Re: 95 seville brake lights problem

The above translates to:
1. Check all your bulbs (replace the ones burned - easy to spot the filament is broken is you knock it with your finger you will see it vibrating)
2. Check all the bulb sockets, make sure are clean and shiny (if corroded need to be replaced).
3. The last section of the harness for the tailights is a separate piece from the car main harness. There is a connector ~1 feet away from the sockets. That's where the harness splits in 6 wires positive and 3 wires ground going 3 to each socket. That connector is also known for getting corroded, sometimes may even melt a bit because of a high contact resistance. Basically is deigned as sacrificial, if any issue with the sockets or in the connector you suppose to replace it. VERY EASY to find in any wreckyard if the dealer doesn’t carry one anymore. Is also very inexpensive.

This is the harness I am talking about:

Eldorado/Deville has 1 harness (one-piece tail light), Seville has 2 (one for trunk lid mounted taillight and one for outer taillight). This particular picture is of a outer harness of a '96-'97 Seville, the older cars are similar but the connector is different. The 3-rd smaller socket pictured is the corner light (parking light circuit), the Elodorado/Deville don't have it in the tail light.
 

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This is probably a dumb question but I really need to know, In a 1997 Cadillac Deville, can my brake lights and my turn signals still work even if my tail lights are not working?
 

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Yes, assuming the issue with the tailights is not the ground connection (common for tailights and turn/brake).
 
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