Third BRAKE light stopped working

[RST Dana]

Where do you see that? I see two posts by the OP and neither one indicates the results of any testing.

Ok. It wasn’t op. Just another member.
As you were…..

 

[RP_NY]

On my 2003 Denali XL, I noticed that the third break light stopped illuminating when breaks are applied. The normal break lights go on. Is there a fuse or something that controls the third break light? Or is it a single bulb that burned out?

Thanks!

You can dis-assemble the Unit by carefully prying the Red len from the black body of the light. You will most likely find that the (1) 12V/1W Zener Diode on the circuit board needs to be replaced (do to corrosion) ($1 part)

Image below is what it looks like on 1999 K1500 Suburban

 

[jhilario72]

Thanks all. I just ordered one from Amazon for $25 and it worked. Nice bright LEDs

 

[gfleck]

my 03 denali xl failed state vehicle inspection due to third brake light not working. shop offered to replace the fixture for $150. I chose to investigate. Light fixture (original) still illuminates fine. Fuse VEH CHMSL is also intact. My next conclusion is a mouse chewed thru wire(s) over the last 2 years. Does anyone have a wiring diagram that includes the CHMSL and/or know how the wires route once they enter the ceiling and head toward the front?

 

[Fless]

my 03 denali xl failed state vehicle inspection due to third brake light not working. shop offered to replace the fixture for $150. I chose to investigate. Light fixture (original) still illuminates fine. Fuse VEH CHMSL is also intact. My next conclusion is a mouse chewed thru wire(s) over the last 2 years. Does anyone have a wiring diagram that includes the CHMSL and/or know how the wires route once they enter the ceiling and head toward the front?

I believe that the CHMSL wiring goes through the junction block shown in the video below. Wiring diagrams at charm.li on the web. Just select your vehicle using the links at the top of your browser there.

So it sounds like the 12v or the ground is not making it to the lamp socket in the liftgate. Check the wires in the flexible piece that connects the liftgate wiring to the body. All that flexing tends to compromise wires and contacts.

 

[gfleck]

I believe that the CHMSL wiring goes through the junction block shown in the video below. Wiring diagrams at charm.li on the web. Just select your vehicle using the links at the top of your browser there.

So it sounds like the 12v or the ground is not making it to the lamp socket in the liftgate. Check the wires in the flexible piece that connects the liftgate wiring to the body. All that flexing tends to compromise wires and contacts.

THANKS! I'll report what I find.

 

[gfleck]

Mucho thanks to Fless for the guidance and introducing me to the manual and drawings library! I found all the wiring protective coverings intact and the junction block in decent condition. Although, some of the wiring colors do not match between what I observed on board and on the schematics. I re-installed the original light since I had to take the vehicle in for bearing replacement, and I didn't want the space available for water intrusion. The third brake light now works. So either the inspection tech did not observe it properly, or my simply cleaning the fixture and ensuring the connection was tight corrected the issue.

 

[OR VietVet]

Mucho thanks to Fless for the guidance and introducing me to the manual and drawings library! I found all the wiring protective coverings intact and the junction block in decent condition. Although, some of the wiring colors do not match between what I observed on board and on the schematics. I re-installed the original light since I had to take the vehicle in for bearing replacement, and I didn't want the space available for water intrusion. The third brake light now works. So either the inspection tech did not observe it properly, or my simply cleaning the fixture and ensuring the connection was tight corrected the issue.

Sometimes, just unplugging and plugging a connector back in, is enough of a cleaning to make the current flow again.