I put LED bulbs in my brake light lenses today and I noticed that they seem like the brake pedal is already pushed in because the 3rd brake light is on when I turn the headlights on. The passenger side signal works and the driver side does not. The parking lights work fine as well. Anyone have a clue how to fix this? I have a 99 blazer lt

 

And I also have upgraded the signal flasher. And I changed all 4 bulbs to LED. Forgot to mention that.

 

From your description, you now have a bad ground on the driver's side, rear.

 

Ok, how do I check that so I know for sure that's the problem?

 

Ok, so I took the led bulbs out of the driver side, when I took the top one out the 3rd brake light went off Luke it's supposed to be. For now I pit the old bulbs back in until I can figure this out. The LEDs make nice reverse lights on the flipside

 

Reason that happns is that dual filament bulbs are wired with GRD as common point:

Stop + WWWWW GRD WWWWW tail +

So the return path is GRD. If:

Stop + WWWWW {?} WWWWW tail +
then you see that tail + goes through bulb to the stop circuit.

In tungsten bulbs both are dimmer because they are in series. I dont know the LED config so I cant tell you for sure about how bright they would be but it would def feed the stop circuit as you can see.

 

Thanks for the info man. I appreciate it.

 

I put my beer down to read that & re-read that & once more to make sure I read it right the 1st 2 times. ... Yup I feel like I'm taking grade 10 E=MC sq.
SOME LED's need to be flipped around, not sure why it's just the way they are. 2 prongs & how can that get messed up?! But the circuity of the way they're made may dictate so. Or you may need a LED box/inline voltage booster so your vehicle doesn't register a burnt out bulb & blink fast. That's because LED's require/draw less power & that messes up the stock configuration. That makes the vehicle "register" a burnt out bulb & your signals & other things run amok.

Got change for my 2 cents worth?! lol

<edit> my 1st thought was a ground issue as well... I just got carried away. Seems like something is making a false ground & causing issues. How are the terminals where the bulbs plug in? How are the wires to & from? Check all that out as far as you can.

 

So I checked the terminals on the bulb sockets, they seem fine and rust free. I also checked the wires as best as I could and they look fine. I ordered some load resistors for the bulbs so hopefully this solves my problem.

 

Well because LED stands for Light Emitting Diode and Diodes pass current one way and different characteristics when polarity is reversed. Depending on how they are made. Some LED's are made to glow one color for one polarity and different the other. That is assembled back to back.

I could illustrate that, too... but dont wanna make you spit yer beer.


There are no 12v LED's... they light at about 1.5 volts and because of that there's all sorts of ways to ballast them so they will operate at 15 volts without burning out.
And sticking other resistors in parallel with them so blinker and bulb monitor systems work just seems stupid to me. Reason for LED in the first place is they're more efficient.

Whatever... back to the OP... I have a feeling that the problem may be with the one LED bulb. Especially if it's back to normal with the tungsten bulbs.

If you didnt mark it, you're gonna have to start over.