Third BRAKE light stopped working

[jhilario72]

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!

 

[gooffeyguy]

Probably burnt out. The one on my 2001 has before and the entire red plastic lens/housing gets replaced and just plugs in back behind it

 

[OR VietVet]

No separate fuse. Replace the whole thing. Easy to get to and check power and ground.

 

[Fless]

Contrary to what you've heard, there's a fuse for it in the in-cab lefthand fusebox (VEH CHMSL):

 

[OR VietVet]

Contrary to what you've heard, there's a fuse for it in the in-cab lefthand fusebox (VEH CHMSL):

View attachment 416822

That's not for the 3rd brake light. That is for "Chair Heat Module Seat or Leg". Everyone knows that!!!!!

 

[OR VietVet]

If the CHMSL stands for "Chassis mount stop light". Aren't they all chassis mount? Please break down the CHMSL, thanks.

 

[Fless]

If the CHMSL stands for "Chassis mount stop light". Aren't they all chassis mount? Please break down the CHMSL, thanks.

Center High-Mounted Stoplamp

 

[OR VietVet]

Center High-Mounted Stoplamp

View attachment 416836

Ok, ok, ya got me. I effffed up. I am going to my room now.....

 

[ScottyBoy]

When my 3rd brake light went out, I unhooked it and checked the voltage at the plug while I had a helper press the brakes. I did indeed have voltage, so I just ordered a new 3rd brake light from Amazon. I don't believe they have a replaceable bulb, I think you do indeed HAVE to replace the entire housing. It was only about $20-$25 it wasn't expensive at all.

 

[mwilson62000]

Does your cruise control still work? If I remember correctly, years ago in my Tahoe, the CHMSL stopped working and I happened to notice my cruise wasn’t working either. Was the brake light switch under the dash.

 

[OR VietVet]

Pay close attention to the harness connector after you unhook from the old brake light connector. Sometimes wire memory happens and it has some pull there and the wire will suck back in there.

 

[jhilario72]

Does your cruise control still work? If I remember correctly, years ago in my Tahoe, the CHMSL stopped working and I happened to notice my cruise wasn’t working either. Was the brake light switch under the dash.

Not sure, I never checked the cruise control. Thanks for the suggestion

 

[RST Dana]

Does your cruise control still work? If I remember correctly, years ago in my Tahoe, the CHMSL stopped working and I happened to notice my cruise wasn’t working either. Was the brake light switch under the dash.

Brake light switch should be fine since he already said the other brakes lights are working properly.

 

[Fless]

I would think the first thing to check is the CHMSL fuse for voltage on both sides, as well as good pin fitment. Then see if you have voltage at the high mount stop lamp connector when the brake pedal is depressed.

If there's no voltage at the CHMSL connector, look under the truck behind the right rear wheel for the lighting connector plate. I know, it's not the best place for wiring connections (dirt courtesy of Kebler Pass, Colorado):

If I recall correctly the taillight circuits run through that, but I can't tell you which one would be for the high mount lamp. Carefully remove the connectors and check for cleanliness and pin fitment. Maybe someone has a wiring diagram that would show which wires are which.

 

[Doubeleive]

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!

99% chance it is the light burnt out, a replacement is like $20 off of amazon all day long. those brake lights fail like a fat man running a marathon on the gmt800's

 

[RST Dana]

I would think the first thing to check is the CHMSL fuse for voltage on both sides, as well as good pin fitment. Then see if you have voltage at the high mount stop lamp connector when the brake pedal is depressed.

If there's no voltage at the CHMSL connector, look under the truck behind the right rear wheel for the lighting connector plate. I know, it's not the best place for wiring connections (dirt courtesy of Kebler Pass, Colorado):

View attachment 416903

If I recall correctly the taillight circuits run through that, but I can't tell you which one would be for the high mount lamp. Carefully remove the connectors and check for cleanliness and pin fitment. Maybe someone has a wiring diagram that would show which wires are which.

Op already stated he has voltage there.

 

[Fless]

Op already stated he has voltage there.

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

 

[OR VietVet]

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

I double checked too. I don't see where the OP says there is voltage there. He says the other brake lights work though. Don't "Mess with Fless".

 

[Doubeleive]

in the last 23 years my average life for that 3rd brake light has been maybe 3 years and every one I pulled from junk yard trying to save $5 was bad, except for maybe 1
just had to replace the one on the 05 and it was newly replaced about 3 years ago.
people usually don't even notice it unless someone mentions hey you have brake light out, I occasionaly back up to a glass window at night and check my lights in the mirror.
they either burn out entirely, or get very dim, or several of the led's will fail and/or get dim at the same time.
really the only other thing that happens is the rubber seal can deform allowing water and dirt in the harness socket area causing some corrosion.
super easy to just throw a meter on the 2 socket pins and have someone hit the brake pedal to verify operation.

 

[jhilario72]

Thanks for all the great info. I thought my 3rd brake light was completely out, but upon closer inspection, it is indeed very dim. I ordered a new LED one from Amazon. Hope to get it this weekend and install it. Let you all know! Happy Holidays!

Joe