NEED HELP Right brake light not working

[Steevo]

My right brake light is not working. The left one works fine.
Naturally, I changed the bulb, no difference.
So I took the left light fixture which was working and plugged it into the right wiring harness.
The problem stayed the same. The right brake light is inop.
So I asked around and was told that the brake signal goes through the multi function switch turn signal contacts to the brakes.
So does that make sense? Should I order a turn signal switch to solve a brake light problem?
I changed out my turn signal switch with an ebay one three years ago, I guess it could have failed.
Why did I buy on ebay? Because at Auto Zone that switch is $178. I paid about $30. Big enough difference for me. And ya know, with *everything* coming from China now, brand names just don't matter anymore.

 

[TollKeeper]

The tail light boards are known to corrode and fail. Start with checking that you are getting a signal to the pigtail going into the tail light.

 

[Steevo]

I did swap the left with the right, the problem stayed on the right. So it's not the fixture.
I agree about testing at the pigtail. But that was a little complicated. The contacts are recessed.
The question is whether the brake lights go through the turn signal switch or not.
Because that is the common failure point.

 

[TollKeeper]

I cant honestly answer that. I know on the G-vans, where the brake light and turn signal are the same bulb, it goes thru the multi-function switch. But on these trucks, they are separate, so I dont know.

 

[Steevo]

The brake and turn signal are the same bulb, I think. I am gonna go look.
I was hoping someone could answer definitively. This is where the smart guys are.

 

[mrrsm]

Unfortunately... The GM Engineers created a bizarre maize of wiring, fuses, Class 2 Network and such that can leave us all confused by these things. Here is the GM OEM "Brake Warning" Schematic. Grab a Cup of Coffee and Zoom Around on this Damned Thing to get a better view on where the problem(s) might be with Switching Modules, Bulb Fixtures and Fusing:

 

[TollKeeper]

The brake and turn signal are the same bulb, I think. I am gonna go look.
I was hoping someone could answer definitively. This is where the smart guys are.

If you fast forward in this vid to 9:25, they are seperate

 

[TJBaker57]

Could not find where you stated the year of your truck. Looked at a 2002 wiring diagram. Shows wires straight from rear fuse block to the lamps.

 

[Mooseman]

Stupid question but I gotta ask... Did you check the fuses?

 

[Steevo]

Stupid question but I gotta ask... Did you check the fuses?

I did not check the fuses, because for brake lights I can't imagine it would be a separate fuse for left/right. But maybe I'm wrong.

 

[TJBaker57]

I did not check the fuses, because for brake lights I can't imagine it would be a separate fuse for left/right. But maybe I'm wrong.

Diagram posted above shows same fuse for both lower brake lights. The center high mounted light has its own fuse.

 

[Steevo]

Diagram posted above shows same fuse for both lower brake lights. The center high mounted light has its own fuse.

If that's so then I am back to the turn signal switch. But I just dunno. I have never seen a problem like this.

 

[TJBaker57]

If that's so then I am back to the turn signal switch. But I just dunno. I have never seen a problem like this.

You can see on the wiring diagram that the turn signal switch is NOT involved!

Since both brake lights come from the same fuse it HAS TO BE somewhere between that fuse in the rear fuse block and the actual light harness. That or a bad ground at the right side.

 

[Mooseman]

Parking lights and turn signal also not working would likely point to a bad ground.

 

[Steevo]

Parking lights and turn signals all work. Tail light is same bottom bulb as the non functioning brake light. And it works. In fact the two bottom bulbs are turn signals, even the non functioning brake light. The top is amber, the bottom two bulbs are red. And the red taillight, same bulb as the non functioning brake light.

 

[TollKeeper]

They arent the same bulb, see video above. Turn signal and brake are seperate. Brake light is top, turn is middle, reverse in lower. So you would need to probe the harness plug at the tail light to see if current for the brake light is making it there. If it is, you know its in the tail light (board, socket, corrosion, etc). If it isnt, you will have to find the next harness plug, and check there. Narrow it down to where the signal is and isnt making it to.

Dont forget to look into the harness plug end, and see if corrosion is inside of it. Then back probe to see if the current is at least at the wire, if the terminal end is corroded.

 

[christo829]

How does your connector to that right tail assembly look? When I got my TB, one of the tail light circuit boards had been replaced, but the other had not. Then the brake light failed on that side. Turned out that the connector had been damaged, probably by that original circuit board. One of the connectors inside the housing had slightly melted and been pushed back in to the plastic, just enough to eventually stop completing the circuit. It wasn't visible from the outside of the plug.

 

[Steevo]

They arent the same bulb, see video above. Turn signal and brake are seperate. Brake light is top, turn is middle, reverse in lower. So you would need to probe the harness plug at the tail light to see if current for the brake light is making it there. If it is, you know its in the tail light (board, socket, corrosion, etc). If it isnt, you will have to find the next harness plug, and check there. Narrow it down to where the signal is and isnt making it to.

Dont forget to look into the harness plug end, and see if corrosion is inside of it. Then back probe to see if the current is at least at the wire, if the terminal end is corroded.

The brake light is the bottom one. It's red. That is a much later model. My reverse lights are separate, in the bumper cover.

 

[TollKeeper]

So you have a Envoy, 9-7x? You never indicated what vehicle, or year, you have.

Edit: The Trailblazer tail light design never changed during its entire line 2002-2009. Even the earlier model 1995-2001 S-Series Trailblazer had the same tail light layout, with a different mold.

Edit2: The other option which you hinted at earlier, and I just now caught.. Try taking the Taillight that isnt working, and plug it into the other side that is. Does it still work? This would rule out the onboard circuit board.

 

[Steevo]

How does your connector to that right tail assembly look? When I got my TB, one of the tail light circuit boards had been replaced, but the other had not. Then the brake light failed on that side. Turned out that the connector had been damaged, probably by that original circuit board. One of the connectors inside the housing had slightly melted and been pushed back in to the plastic, just enough to eventually stop completing the circuit. It wasn't visible from the outside of the plug.

I'll have to look into that. They appear immaculate to me. No corrosion, but this vehicle is only in CA and AZ. So that is as expected.

 

[Steevo]

So you have a Envoy, 9-7x? You never indicated what vehicle, or year, you have.

Edit: The Trailblazer tail light design never changed during its entire line 2002-2009. Even the earlier model 1995-2001 S-Series Trailblazer had the same tail light layout, with a different mold.

Edit2: The other option which you hinted at earlier, and I just now caught.. Try taking the Taillight that isnt working, and plug it into the other side that is. Does it still work? This would rule out the onboard circuit board.

It's a 2003 Envoy XL.

No, I didn't try the right light fixture on the left. As I said when I connected the left fixture to the right plug the problem stayed the same. The left fixture works properly on the left, on the right the brake light is inop.

 

[TollKeeper]

So the 2002-2006 Envoy XL all had the same tail light, and in fact was the same as the Shorter Envoy 2002-2009 tail light as well.

Per the wiring diagram posted above by @TJBaker57 , the wiring schematic is the same for Trailblazer and Envoy and Bravada, it was just the lights themselves, and the internal board orientation that changed. So you would be back to what I suggested, you will have to back probe the wiring, starting at the tail light harness, and see where the power loss is. The turn signal switch has nothing to do with the brake lights on these trucks.

 

[Mooseman]

Yep, basically the same wiring except the backups are in the bumper. Turn signal is on top and amber, brake is lower.

 

[Steevo]

Diagram posted above shows same fuse for both lower brake lights. The center high mounted light has its own fuse.

On further investigation, the Center High Mounted stoplight is also inop. I found the fuse under the rear seat and it's good. I tested for voltage at the yellow wire that connects to it, and there is no 12v there when the brake pedal is depressed.

So I have my right stop light and the center high mounted stop light inop.

 

[Steevo]

Well, I am thinking of throwing a stop light switch at it. But I can't see from the wiring that is a likely cause of this problem.

 

[TollKeeper]

So the CHMSL is a known failure point by themselves. They are quite exposed up there, heat, moisture, freezing, etc. So this may, or may not, be related.

I reiterate, that you need to have your circuits tested at your tail light harness with either a multi-meter, or even a light. If you want to change your brake light switch, this may or may not fix your problem. As far as I know, it just runs a signal to a switch/relay, to activate both brake lights + CHMSL. I could be wrong, as I will admit I HATE wiring diagnostics with a passion, as its NOT my most knowledgeable area.

But until the lack of power is indicated at the tail light harness itself, your just shot gunning parts at it.

 

[TJBaker57]

On further investigation, the Center High Mounted stoplight is also inop. I found the fuse under the rear seat and it's good. I tested for voltage at the yellow wire that connects to it, and there is no 12v there when the brake pedal is depressed.

So I have my right stop light and the center high mounted stop light inop.

Looking at the wiring diagram I see NO yellow wire connected to the vehicle stop fuse #34. There is a white wire from the stop light switch and light blue wires from the fuse to the 2 rear brake lamps. The yellow wires I see are one from the center high mounted stop light fuse #16 to the high center brake light, and a second
Yellow wire from the turn signal hazard flasher to the Left turn signal.

 

[Steevo]

So the CHMSL is a known failure point by themselves. They are quite exposed up there, heat, moisture, freezing, etc. So this may, or may not, be related.

I reiterate, that you need to have your circuits tested at your tail light harness with either a multi-meter, or even a light. If you want to change your brake light switch, this may or may not fix your problem. As far as I know, it just runs a signal to a switch/relay, to activate both brake lights + CHMSL. I could be wrong, as I will admit I HATE wiring diagnostics with a passion, as its NOT my most knowledgeable area.

But until the lack of power is indicated at the tail light harness itself, your just shot gunning parts at it.

There is no power at the center high mounted stop light, I checked that thoroughly today with a dvm.
Essentially, I did check the harness by attaching the functioning left hand light fixture to the right hand wires, the problem stayed on the right. Fuses are all good.

I am down to actual wiring on the right and for the CHMSL. It would be helpful if anyone knows a common point of failure for those two lights. Which is why I am down to the stop light switch.

 

[Steevo]

Looking at the wiring diagram I see NO yellow wire connected to the vehicle stop fuse #34. There is a white wire from the stop light switch and light blue wires from the fuse to the 2 rear brake lamps. The yellow wires I see are one from the center high mounted stop light fuse #16 to the high center brake light, and a second
Yellow wire from the turn signal hazard flasher to the Left turn signal.

The yellow wire to the CHMSL. From it's fuse, I presume.

 

[Steevo]

Here are the wiring diagrams for this specific truck, courtesy of Oreilly.

 

[TJBaker57]

The yellow wire to the CHMSL. From it's fuse, I presume.

Did you test for 12 VDC power at both the chmsl fuse #16 and the vehicle stop fuse #34 when the brake pedal is depressed?

That is where I would begin. No power to these two fuses when brake pedal depressed equals bad brake switch or underhood fuse #12.

Have power to rear fuses #16 and #34 with brake pedal depressed then test all 3 wires (2 light blue and one yellow) leaving the rear fuseblock to all 3 rear brake lights. Lacking power on any of those three means a bad fuseblock.

Have power on all three of those wires leaving the rear fuseblock but don't have the same power back at the light fixtures? That means bad wiring between the rear fuseblock and the light fixtures.

Step by step, follow the power.

 

[Steevo]

Probably a good approach, I am flummoxed by the properly working left brake light, which according to the wiring diagrams is fed by a single contact in the stop light switch. If those were separate, OK, but they don't seem to be.

 

[TJBaker57]

Probably a good approach, I am flummoxed by the properly working left brake light, which according to the wiring diagrams is fed by a single contact in the stop light switch. If those were separate, OK, but they don't seem to be.

That single contact in the stop switch goes through the rear fuseblock. Underneath the rear fuseblock they split up into separate wires.

 

[coolasice]

Check for voltage at the fuse block on the Appropriate fuses then move down the line checking for voltage at the lamp board connector. It is a Process of elimination. Always work one way or the other either from the source or from the load. Check for continuity to ground at the connector. If you can verify there is voltage coming from the fuses you can work down the line and find where the problem is. More then likely either a broken wire or bad connector since the other side is working and they are tied together at the fuse block.

 

[TollKeeper]

Work one way or another after splitting the circuit. You know the fuse block is about the half way point, so thats the point to check from. Power going into the fuse, work towards the back. Power not getting to fuse block, work forwards towards switch.

Halving the circuit as much as possible as harness connections allows you to check for corrosion at the harness end, without damaging the wire.

 

[coolasice]

Work one way or another after splitting the circuit. You know the fuse block is about the half way point, so thats the point to check from. Power going into the fuse, work towards the back. Power not getting to fuse block, work forwards towards switch.

Halving the circuit as much as possible as harness connections allows you to check for corrosion at the harness end, without damaging the wire.

He's getting power to one side, so the switch is working.

 

[coolasice]

for reference here is a high quality image for both the stop lamp schematic and the tail lamp connector pinout (right side only). I highlighted the connector pins that apply to your vehicle (gmc).

@Steevo does your right turn signal work?

 

[Steevo]

Yes, both turn signals work. Separate bulb. Amber, at the top of each fixture. The brake light is the bottom bulb.

 

[coolasice]

Yes, both turn signals work. Separate bulb. Amber, at the top of each fixture. The brake light is the bottom bulb.

They are separate bulbs, but they share the same ground, that rules out the ground. I would check for voltage on pin A at the connector when brakes are applied. From the sounds of the problem, you have either a bad connector or a break in the wire from the connector to the fuse block.