3rd Brake Light Always On?

[packman]

I noticed that when I put power to the CV system, the 3rd brake light (by the rear window) is in constant illumination. The brake pedal is not depressed and the e-lights are not engaged. I didn't notice this before; but I wasn't looking behind me when I made those 1st start attempts. I'm not an electrician, so I don't know offhand why this is so. Or how to fix it?

[friskyfrankie]

What else is on the fused circuit? Are they energized as well? That might provide a clue. Could be the brake switch under the brake pedal. Are the brake lights on as well as the 3rd light? Sorry for so many questions but just trying to narrow things down.

[kishy]

Have you replaced the brake light bulbs with LEDs?

If so, that is the reason. The cruise module (ECM in years with ECM-integrated cruise) outputs a low voltage onto the brake light circuit which is enough to illuminate LEDs but is not enough to illuminate incandescent bulbs.

If not, it'll be time to study a diagram, but as circuits go this one is fairly straightforward so the list of things to check should be short.

[gadget73]

If its not LEDs being fired by the cruise module, the brake switch is almost certainly bad, or the pedal is part way down for some reason. There isn't a whole lot going on here. The fact that all of the brake lights aren't on make me think you have multifunction switch issues too. The left and right brake light wiring passes through that, the third light runs straight off the brake switch.

[Mainemantom]

I agree with LEDs affecting the third brake light if you have LEDs. I would replace the brake light switch too. Get a good quality one like a Motorcraft. The switch may look ok but be defective.
I had a mechanic cuss me out one day. At that time I was recovering and could not work on cars. I told him to replace the brake light switch to correct a problem. I told him to just do what I wanted done even if the switch looked good. New switch fixed the problem. Don't have to deal with that mechanic any more...he died.

[86VickyLX]

I would imagine the other lights would be lit as well if it was a failed brake lamp switch. Best way to tell is to disconnect the switch and see if the light is still on. Unless you have multiple problems. I have LEDs all around on my car. But I'm running CANBUS LEDs for the 3rd brake light so I can still retain cruise control. If you have some low quality cheapie LEDs I could see maybe having some issues. As I've had that problem in the past with cheap LEDs.