Low voltage to brake lights

One of my friends was driving behind me at night and let me know that my brake lights looked very dim when illuminated. I checked the fuse for the stop lights which seemed to be good, but replaced it just to be sure, but it did not solve the problem.

After digging a little bit deeper I pulled the tail light out and checked everything visually which also seemed to be good. I then unplugged the tail light to check the voltage from the harness. The brake light only seemed to be getting around .5v when the pedal was depressed while the tail light and turn signals remained at 12v. I can't seem to find the issue or think of what else could be the cause, and was wondering if any of you guys have an idea of what it may be!

 

Check your alternator. just start your car and take off the negative and see if the car turns off if it stays on then your alternator is still good that's all I no about that if that helps.

 

I would check to be sure the ground is secure and has no resistance then check the brake light switch. The alternator should not have anything to do with it . The signals get 12 volts so you know the voltage from the battery is good.......let us know how you make out. good luck.

 

What year is your truck? I'm wondering if it has to do with the LED brake lights newer 2nd gens have. Maybe the previous owner converted from LED units to the older incandescent style and didnt use a new wire harness? Just cut the wires and spliced them to work with an incandescent brake bulb? Just throwing out ideas.

 

I strongly advise you NOT to do this. This was a questionable practice even back in the day of points ignition. With all the electronics on modern vehicles you can do serious damage to important, and expensive to fix, systems on the vehicle. Computers do not like huge voltage spikes.

 

Looked into the idea of it being a bad ground so I found a wiring diagram of the tail light harness. I disconnected to ground mounted onto the frame rail by the drivers side rear wheel and gave it a good cleaning then reconnected. I pulled the brake light switch and checked resistance throughout the connectors and all was good. Had my brother push on the brakes and all is good now! Must have just been a bad ground. Thank you!

 

Im glad you found the problem.....Its always a good place to start by checking grounds when you have an electrical problem.

 

I just recently swapped out the trendy "altezza" style LED taillights on my '09 for a set of '08 lights. The "pre-" and "post-" LED vehicle harnesses are the same, as the "electronics" for the LEDs are built into the taillight itself. The LED tail/stop assembly then has a pigtail with a "lamp base" style plug for connecting to the common vehicle harness...

 

+10x for @fathomblue's comment. I'm just reiterating for any future readers of this thread. Do this on any ECU controlled vehicle. You know, like since the late 70's, early 80's.

You 'might' get away with it, or you might spend a bucket-'o-bucks correcting the fried hardware. Do you feel lucky?