No brake lights

Busterbrown

Original poster
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Dec 4, 2011
253
Can anyone point me in the right direction regarding my brake lamps? As simple as I thought it would be but none of the checked fuses are blown. The high center tailgate works oddly enough. Rear seat fuses I inspected were 34(vehicle stop) and 51(brakes). The only engine compartment circuit checked was labeled stoplamp (12). Should I check the bulbs in the am?
 

The_Roadie

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Nov 19, 2011
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Portland, OR
It can't be front fuse #12 or the pedal switch or rear fuse #16 if the CHMSL works.

If you have a voltmeter, check to see if rear fuse #34 has 12V on it when you step on the pedal. This works even if the ignition switch is off.

Otherwise, as my buddy Ray is fond of saying - when you lose a lamp in your house - is your first thought that the lamp is bad or the circuit breaker? In this case, suspect two bad lamps.
 

DDonnie

Member
Mar 26, 2012
2,631
the roadie said:
It can't be front fuse #12 or the pedal switch or rear fuse #16 if the CHMSL works.

If you have a voltmeter, check to see if rear fuse #34 has 12V on it when you step on the pedal. This works even if the ignition switch is off.

Otherwise, as my buddy Ray is fond of saying - when you lose a lamp in your house - is your first thought that the lamp is bad or the circuit breaker? In this case, suspect two bad lamps.

I know it was common with the s10 platform, any chance it could be the multi switch?
 

The_Roadie

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Nov 19, 2011
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The multi switch as in the multifunction stalk switch that runs the turns signals? No possible interaction. Same with the flasher unit. Our brake lights are separate from the turn signals, except on the trailer connector, which does go through the flasher unit.
 

DDonnie

Member
Mar 26, 2012
2,631
the roadie said:
The multi switch as in the multifunction stalk switch that runs the turns signals? No possible interaction. Same with the flasher unit. Our brake lights are separate from the turn signals, except on the trailer connector, which does go through the flasher unit.

Cool beans. I always thought that was a bad design with the s10 series. Good to hear they dont go through that any more.
 

RayVoy

Member
Nov 20, 2011
939
the roadie said:
Otherwise, as my buddy Ray is fond of saying - when you lose a lamp in your house - is your first thought that the lamp is bad or the circuit breaker? In this case, suspect two bad lamps.
Bill, your buddy is agreeing, it is so easy to suspect something complex and to ignore the simple, just because both sides are dark.
 

Busterbrown

Original poster
Member
Dec 4, 2011
253
I perused an older thread on the OS in which most of those having the same brake light failure found salvation via bulb replacements. Just need to find a few minutes to make the purchase and install. Results pending...
 

Menthol

Member
Dec 8, 2011
177
I bought 2 bulbs last week only to find out it was fuse 34. 2 exta bulbs for those long summer trips :smile:
 

meerschm

Member
Aug 26, 2012
1,079
did you look at the blown fuse #34 closer, try to change, or measure the fuse?

Just curious.
 

Busterbrown

Original poster
Member
Dec 4, 2011
253
meerschm said:
did you look at the blown fuse #34 closer, try to change, or measure the fuse?

Just curious.

It's not the bulbs. Had a few minutes this afternoon to pull the brake light bulbs on my Rainier and replaced both. It seems the in-store catalog from Sylvania listed the incorrect replacement bulb number (3157). I should have looked at the existing bulb as I now know they are 3057. But I digress, both side rear brake lights still don't light. All three bulbs in the rear taillight assembly are 3057 and I swapped out the suspected middle bulb (brake light) with the reverse. Still nada. I've replaced the #34 fuse under the seat too. I'm going to get my volt meter out now to see if the fuse socket is getting power to it.

Any other suggestions? A 5 minute fix is turning into a afternoon project.:hissyfit:

Edit:
Not showing any voltage to fuse #34. Just to experiment, tested the #16 fuse and didn't see 12 volts either.

Edit #2:
The only fuse showing voltage in the rear fuse panel is #37 (Front parking lamps). CHMSL is working when pedal is depressed. So far, all other systems are operating as expected. I'm stumped.
 

Busterbrown

Original poster
Member
Dec 4, 2011
253
fletch09 said:
bad board assembly ?
not sure if they are the same as the TB.
but that happened on my 03 EXT. :twocents:

I hope not. All other systems are operational. Rear wiper, seats, HVAC, radio, sunroof, perimeter lights, cabin lights, power seats, door modules. I've tested everything. When ignition is on, there has to be other fuses on board that light up...

Did you go to a salvage yard for a used board?
 

fletch09

Member
Nov 20, 2011
1,982
Busterbrown said:
I hope not. All other systems are operational. Rear wiper, seats, HVAC, radio, sunroof, perimeter lights, cabin lights, power seats, door modules. I've tested everything. When ignition is on, there has to be other fuses on board that light up...

Did you go to a salvage yard for a used board?

maybe boards is not the right terminology, i was talking about the the assembly where the bulbs actually get pushed into,
on a TB they get attached to the the tail light.
like i said, not sure if they are the same as the TB
 

MAY03LT

Member
Nov 18, 2011
3,420
Delmarva
Busterbrown said:
Edit:
Not showing any voltage to fuse #34. Just to experiment, tested the #16 fuse and didn't see 12 volts either.

Was the brake pedal pushed when you did the test? 34 and 16 will only have 12v when the pedal is pushed.
 

Busterbrown

Original poster
Member
Dec 4, 2011
253
fletch09 said:
maybe boards is not the right terminology, i was talking about the the assembly where the bulbs actually get pushed into,
on a TB they get attached to the the tail light.
like i said, not sure if they are the same as the TB

Ok...but it's difficult to believe that both bulb assemblies are failing at the exact same time. Just several weeks ago, each side worked. A failure on 1 side would result in both being inoperable???
 

Busterbrown

Original poster
Member
Dec 4, 2011
253
MAY03LT said:
Was the brake pedal pushed when you did the test? 34 and 16 will only have 12v when the pedal is pushed.

Yes, I had a 10 foot 2x4 pressed up against the pedal which was snaked in from the rear. The high center light was illuminated throughout the process. So, if 12 volts isn't reaching the rear fuse panel, where do I look now?

And it seems that Rockauto list both 3057 and 3157 as replacement bulb numbers, in which AC/delco identifies its replacement as the 3057. Wonder if there is a difference.
 

MAY03LT

Member
Nov 18, 2011
3,420
Delmarva
Busterbrown said:
Yes, I had a 10 foot 2x4 pressed up against the pedal which was snaked in from the rear. The high center light was illuminated throughout the process. So, if 12 volts isn't reaching the rear fuse panel, where do I look now?

Cool. If the chmsl is lit then 16 should have power. I only say should have because I'm referencing the 06 service manual (the ones posted here) and sometimes their diagrams don't break down slight differences in models. So double check that your rear fuse block calls 16 something like chmsl. Then check that your meter is grounded good and you can get it to show 12v on 16. Then move to 34 and hit both sides of the fuse. That test will determine what to do next.

There is one wire for brake lights that goes into the rear block. Inside the block it splits into two circuits - one for the chmsl (16) and the other for the sides (34). So with the chmsl working, you can rule out everything up to the rear fuse block.

Busterbrown said:
And it seems that Rockauto list both 3057 and 3157 as replacement bulb numbers, in which AC/delco identifies its replacement as the 3057. Wonder if there is a difference.

Allegedly the park filament of the 3157 lasts 5 minutes longer. Allegedly.
 

RayVoy

Member
Nov 20, 2011
939
Just to continue with the wiring discussion that MAY03LT started, the power to the rear side lights is common (both lights are fused from fuse #34), the power to the centre light is separately fused (fuse #16). As MAY suggested, there has to be 12v at fuse #16 if the centre light is on.

When you get 12v at fuse #16, use the same procedure to check for 12v at fuse #34.

If you get power at #34, then go to the socket boards in the taillights.

The wire for the brake light on both sides is light blue.
 

Magyver

Member
Apr 9, 2012
64
This happened to me a short time ago. Actually it was right around the time of the start of this thread. Problem solved by replacing a bad bulb. I don't see the logic in designing a system where one side depends on the other but I'm not arguing with the results either, by rebuilding the wiring when it seems to be fixed. I don't know how long my brake lights didn't work, and wouldn't have known until a co-worker spotted it and called me. I guess the cop would have told me......
 

RayVoy

Member
Nov 20, 2011
939
Magyver said:
I don't see the logic in designing a system where one side depends on the other .
Not true, each side will light independent from the other side. They both, however, require the same brake signal (12V) through the same fuses. But, unlike, the old "series" Christmas tree lights, one burnt out lamp will not kill the other lamp.
 

Busterbrown

Original poster
Member
Dec 4, 2011
253
UPDATE:

Embarrassed am I! After much diagnosing with my voltmeter, I couldn't get the #34 fuse to light up with the brake pedal depressed. I finally realized my ground lead was broken, replaced the lead, and tada...14 volts on both tests ports of the fuse. So, I headed to the tails (AGAIN) and did some soul searching. Checked the blue wire on each side. Light up as intended from both sides. What the fudge??? New bulbs and nothing. Ok... Checked the bulb on the top of the taillight assembly this time. Replaced it and depressed the brake pedal. Voila!!! I had focused on the middle lamp on the board, which are turn signals, not brake lights. Replaced the other side with the same results.

So, what I learned after hours wasted: make sure you're replacing the correct bulb. Top = parking lights and brakes, middle = turns, and bottom = reverse. Just one jacka$$ helping a future prospect. :rotfl:.

Btw, Roadie was spot on with his original reply. Suspecting two bulbs at the same time is just hard to swallow.
 

Menthol

Member
Dec 8, 2011
177
#34 Rear block 15A for the brake light keeps blowing when brake depressed. I'm thinking one of th rear boards. whick one?
Why the price difference below from Rockauto?



DORMAN Part # 923009 {#16525938, 16532713, 70006}
Category: Tail Lamp Connector Plate CAD$12.77

DORMAN Part # 70006 {#16525938, 16532713, 923009}
Category: Tail Lamp Connector Plate (Only 1 Remaining) CAD$24.03
 

meerschm

Member
Aug 26, 2012
1,079
you may have a short someplace pull the board and see if the fuse still pops ( or get fancy and measure...)

both part numbers sub for each other.

search for the part numbers on Amazon. looks like the second one is a pair of boards, the first is one board.

http://www.amazon.com/dp/B001C0A0MU/?tag=gmtnation-20
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
The fuse is doing its job, then - to prevent melted wiring. You have a short to ground somewhere in the wiring from the fuse block to the lamps. I'd remove both side taillights, unclip the connectors to separate the taillight circuit boards from the truck's harness, and check it again. If the fuse blows again, inspect the wiring on the tuck side very carefully, including under the carpet near the fuse block. If the fuse doesn't blow, you have a problem in the taillight circuit boards or lamp area.
 

Menthol

Member
Dec 8, 2011
177
I unplug both lights, pressed the brake and the fuse blew. Now to start check wiring any common areas.
Roadie-: Can I do this to narrow it down and make it quicker (only 8 deg Cel in Toronto)
If I stay at the back and set my meter to continuity, test from each pin on the plug to frame, I should only get sound on the green wire correct? or is my assumption wrong?
 

The_Roadie

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Nov 19, 2011
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Ground in our platform is usually black, but you're right. Ground will show continuity to the frame. But since you're blowing a fuse, the stop light feed wire is where the short is. That one is light blue. On both sides. But since both sides are spliced together in the harness, you can't even narrow it down to the left versus right.
 

Menthol

Member
Dec 8, 2011
177
What's the dark blue wire for? Looks like a ground too because I was getting continuity to the body.
Got lucky! decided to start at back driver side interior quarter panel and found the exposed wire there. all is well for now. Thanks All.
 

The_Roadie

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Nov 19, 2011
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Portland, OR
Dark blue in my manual says right turn signal. But oddly enough, it says light blue for left turn signal, and that would conflict with the brake light schematic, so I don't know without looking physically what's true.
 

RayVoy

Member
Nov 20, 2011
939
Bill, the brake wire is LT Blue, for light blue, the left turn uses a LT Blue, which stands for left turn blue :rotfl:. I've noticed that wiring combo, as well, and thought it strange; I guess there are only so many colours, without a trace colour, available, but it is an odd place to over-laping colours (Canadian spelling).
 

Glenn Duke

Member
Oct 28, 2013
1
Hi Roadie,
New to this forum and any for that matter. I was reading your post about break lights on 2006 Rainier, I am having the same problem but the middle upper hatch light is not working also. Checked all fuses mentioned in the thread and are ok. However I do not have 12volt on fuse #34 as you mentioned at rear seat. Do you know what that means next for me and what does CHMSL stand for. Thanks in advance, Glenn
 

Sparky

Member
Dec 4, 2011
12,927
CHMSL stands for "Center High Mount Stop Light"

Thanks GM for a random acronym in the fuse box to confuse us :crazy:
 

RayVoy

Member
Nov 20, 2011
939
Fuse 34 protects the front of the truck from the back of the truck. It is a fuse that protects the flasher module from a problem occurring at the rear lights. It is a mid-circuit fuse.

The power starts at fuse #12 under the hood, goes to the brake light switch, to the flasher module and then to fuse #34.

Fuse #34 is not part of the high centre brake light.

If fuse #12 is good, you have a bad brake light switch.
 

Wishbone

Member
Dec 5, 2011
343
Busterbrown said:
UPDATE:

...[snip]...

Btw, Roadie was spot on with his original reply. Suspecting two bulbs at the same time is just hard to swallow.

I'm so glad I consulted this site before any troubleshooting. I had both brake lights out, but the CHMSL was working. Last night I replaced both bulbs and we're back in business! :wootwoot:
 

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