Third Brake Light Problem

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stinkinlinkun

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The third brake light on my 2000 LS quit working. The bulb is good but the socket shows 12 volts there all the time. The two corner brake lights work fine. I don't understand why there would be 12 volts at the third brake light socket when the brake pedal is not being pressed. What should I suspect as the problem?

Thanks for some help.
 
Are you reading the voltage with a bulb installed (and on), or with the bulb out? If you are reading it with the bulb out, then what you see is probably normal leakage. The light is turned on and off by electronics, and there is a circuit that tests to see if the bulb is good or bad. It does this by applying a small current to the bulb and sees if the resistance of the bulb pulls the voltage down to near zero. Put a good bulb in, and I'll bet your voltage reading goes down to nothing.
 
I am not sure and someone please correct me if I am wrong, but the brake lights are a negative trigger. A meter will read 12 volts, however when the brake pedal is pushed it will apply a negative trigger and should read a lower number IE: 0.
 
My test method was to take the bulb out of the socket and apply 12 volts to the wire contacts on the bulb base. The bulb lit. I then read the voltage at the contacts in the bulb socket. 12.3v without the brake pedal pressed, same 12.3v with the pedal pressed. I don't understand why that 12.3v at the socket won't light the bulb. In fact, I would think that 12.3v at the socket without the pedal pressed would keep the light on all the time. What kind of smoke and mirrors hocus pocus is this?
 
With the bulb out - Check the other side with an ohm meter. I'll bet a nickel it will read open (or mega-ohms) until the brake pedal is pressed, and near zero when it is. This is just like many door switches which ground the circuit to light the dome light.

Now, if the above doesn't happen, I'll also bet you've a bad switch or open wire. Easily tested by hitting the same side (with the bulb in place) with a 12VDC test light, and seeing if it lights. It will be dim because the bulbs are in series, but if it does light, you've found your trouble.


Andy
 
Thanks TrickVert. I will give your method a try when I get home. Brake light circuits used to be pretty simple. I guess the design and operation had to change to accommodate the third brake light so that it would be independent of the turn signal operation?
Thanks, all. I will post the outcome.
 
Thanks everyone for your help. In spite of my efforts to understand why the bulb would work when connected to 12v independently, but not when installed, I decided to simply try a new bulb and voila, let there be brake light. The best part is that I got the new bulb free (saved a whole $1.88) from our city maintenance shop.
 
It's possible the filament is/was broken such that it made contact inside when in one position, but not another. Stranger things have happened. Glad it was an easy (the easiest) fix.

Andy
 
I had the same problem...mine went out without notice...but to tell you the truth..I like the look of the LS without the third light......
 
PetesSweets86 said:
I had the same problem...mine went out without notice...but to tell you the truth..I like the look of the LS without the third light......
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Me too, but around here, I'll get pulled over and not really looking for that to happen because I've had about 5 warnings for tint. I haven't even looked at it but does anyone know how to change the bulb?
 
Is the third brake light even needed? I always thought that only if a car came with it it needs to be operational.

How ever if you really like the look without it and I am correct, you can swap out the rear deck board or what ever for a gen 2 if you find one at the junk yard with matching interior color.
 
Oh really? I must be talking about the fourth then:p :D
 
JWerner2 said:
Is the third brake light even needed? I always thought that only if a car came with it it needs to be operational.

How ever if you really like the look without it and I am correct, you can swap out the rear deck board or what ever for a gen 2 if you find one at the junk yard with matching interior color.
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i think all cars built after a certian year had to have a third brake light.
and if your car came from the factory with one it HAS TO BE OPERATIONAL. at least in michigan, if you remove it, dont be suprized if the cops start pulling you over like you were driving a powdered doughnut.
 
Is the third brake light even needed? I always thought that only if a car came with it it needs to be operational.
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Gumment safety mandate in the late 1980s IIRC. My 87 Caprice didn't have one, but Ive seen plenty of the same body style that do have them. Pickup trucks were exempt until like 1994. But now every new vehicle sold in the US has to have it.

I haven't even looked at it but does anyone know how to change the bulb?
Click to expand...
You pull the cover off by pulling it straight towards the front of the car and twist the socket out.
 
I remember the Porsche 911 that had the stupid stick up light on the US version because of that rule. It made a beautiful car look rediculous.

Here's an example:
Porsche.jpg
 
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