No Brake lights!!! But everything else works

Babas11911

Original poster
Member
Aug 15, 2012
22
Ok my brake lights are not working. I first checked the fuses they are good. Then I checked the bulbs and they were good. I then changed the circuit board and they were good and that didnt fix my problem. So my next step was to change the switch on the brake pedal and that didnt work either. Its really aggravating because the light for the running lights is the same as the brake light and that works. So if anyone has any idea please help me. This is one of the last two things I have to fix on the truck the other is just rear shocks. So please help!
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
It might help if you fill out the info about your truck. I know there are differences between the Trailblazer and Envoy taillights.
When you checked the fuses, did you use a test light or meter? Or just look at them?
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
Babas11911 said:
... the light for the running lights is the same as the brake light ...
No it's not. Separate filaments, even if they're in the same bulb.And I've got an Envoy but I never remember until I take it apart if they're in the same bulb.

In any event, the critical thing to look for is - does the CHMSL (third brake light) come on or not. If it does, then check rear fuse #34. If it does not, suspect the brake pedal switch or FRONT fuse #12.

Do you have a meter and some experience running it? Any sort of electrical DIY work goes 100 times faster with a meter than by guessing and wasting money and time on the frustration.

How are you checking fuses and lamps without a meter? Visual checking can miss microcracks.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
I have a test light and a meter the fuses are good and the electrical in the box i checked the third light which doesnt work but the fuse is good and so is the voltage. I disconnected it. Anyways i have checked 12 under the hood the one for the third and every thibg else. I checked the plug to see if there was power coming thru and its not so from the rear
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
OK, does fuse #34 have 12V on it, while plugged in, when you press the brake pedal? If not, check the wires at the brake pedal switch. One should be hot all the time, the other gets hot wne you press on it.

Fastest way to troubleshoot an electrical circuit issue is to divide the circuit in half and eliminate either the source or the destination half. Lather, rinse, repeat.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
the roadie said:
OK, does fuse #34 have 12V on it, while plugged in, when you press the brake pedal? If not, check the wires at the brake pedal switch. One should be hot all the time, the other gets hot wne you press on it.

Fastest way to troubleshoot an electrical circuit issue is to divide the circuit in half and eliminate either the source or the destination half. Lather, rinse, repeat.

Well i have power at all the fuses including 34 so i have taken my seatsz out and i am going to trace the wires because I got nothing in my tail lamp circuits. Do I need to check where the wires r running out of the fuse box?
 

Babas11911

Original poster
Member
Aug 15, 2012
22
Babas11911 said:
Well i have power at all the fuses including 34 so i have taken my seatsz out and i am going to trace the wires because I got nothing in my tail lamp circuits. Do I need to check where the wires r running out of the fuse box?

Oh and I dont have power in any of the brake lights. I dont know about the third but I disconnected it to see if they would work cause I think that bulb is blown. I cant get that third light assembly apart. Its a pain I got it out of the joint but the whole piece comes how can I get to the bulb without busting it?
 

McGMT

Member
Jun 17, 2012
621
Babas11911 said:
Oh and I dont have power in any of the brake lights. I dont know about the third but I disconnected it to see if they would work cause I think that bulb is blown. I cant get that third light assembly apart. Its a pain I got it out of the joint but the whole piece comes how can I get to the bulb without busting it?

Its not a bulb...
 

McGMT

Member
Jun 17, 2012
621
Babas11911 said:
Ok lol what is it then cause dude at advance lied to me

It is an LED light.... Well, its not actually an LED but it is when you replace it... But it is an assembly... I realllllly doubt it is bad, you just have a ground issue out back or a bad wire. If all those brake lights are out there is something amiss.
NEVER listen to the guy at advance, they are just cash register jockeys...
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
Babas11911 said:
...dude at advance lied to me...
I doubt they (strictly speaking) lied. That would imply they KNEW the truth and told you otherwise. :no:

Parts clerks may know some, but not all the parts in all the platforms. The collective wisdom of trailvoy, now GMTNation, can know virtually everything about our platform because we're specialists.

My opinion of many parts clerks is they're failed mechanics. The few experienced professionals are worth waiting for if you have some experience with them.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
the roadie said:
I doubt they (strictly speaking) lied. That would imply they KNEW the truth and told you otherwise. :no:

Parts clerks may know some, but not all the parts in all the platforms. The collective wisdom of trailvoy, now GMTNation, can know virtually everything about our platform because we're specialists.

My opinion of many parts clerks is they're failed mechanics. The few experienced professionals are worth waiting for if you have some experience with them.

Ok so i should start at the rear fuse panel and trace it to the rear?
 
Dec 16, 2011
110
McGMT said:
Its not a bulb...

It is, in fact, a bulb, but it's a xenon bulb (same thing currently used in movie projectors, only smaller, dimmer, and much much cheaper). In any case, you do have to replace it as an assembly, like so many things on GM vehicles.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
UMEXT said:
It is, in fact, a bulb, but it's a xenon bulb (same thing currently used in movie projectors, only smaller, dimmer, and much much cheaper). In any case, you do have to replace it as an assembly, like so many things on GM vehicles.

So at the two fuses I am getting right around 12v but when I splice the line about a foot above the fuse box going to the lights I was only getting 0.32. So any ideas?
 

McGMT

Member
Jun 17, 2012
621
Babas11911 said:
So at the two fuses I am getting right around 12v but when I splice the line about a foot above the fuse box going to the lights I was only getting 0.32. So any ideas?

Well that ain't right... Let me go brew a cup, break out the diagrams and I will get back at ya...
 

The_Roadie

Lifetime VIP Donor
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Nov 19, 2011
9,957
Portland, OR
I'm not clear when you say you "splice" the line. Was it broken? Splice is a repair process.

To measure the voltage INSIDE an insulated wire I often push a tiny pin through the insulation into the center conductor strands. That won't damage the conductors.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
Well the guy that was helping me accidentally cut the line so when i went to repair it i tested it first just to see and thats what i got.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
the roadie said:
Ahhhhh, you probably have a flaky fuse block socket or connector.

The fuse box is getting good power to it so how can i remedy the connectors? Should i take the fuse box apart i have it loose and when i was shifting it to get to my wires the interior lights were flashing on and off too!
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Babas11911 said:
The fuse box is getting good power to it so how can i remedy the connectors?
Take it apart and inspect them for oxidation.
Should i take the fuse box apart i have it loose and when i was shifting it to get to my wires the interior lights were flashing on and off too!
You're doing this with the battery connected?!?!?! :eek: :no: Please get somebody with some experience before you burn the truck up.
 

Babas11911

Original poster
Member
Aug 15, 2012
22
the roadie said:
Take it apart and inspect them for oxidation. You're doing this with the battery connected?!?!?! :eek: :no: Please get somebody with some experience before you burn the truck up.

I didnt take the fuse box apart with the battery connected I took the seats out and the bolts loose. I havent done anything I shouldnt with the battery connected. I dont think its possible to test the wires for voltage with the battery disconnected.
 

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