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sprinkles08, ASE Certified Technician
Category: Car
Satisfied Customers: 22816
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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GMC Sonoma CHMSL / third brake light issue

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Do you have a moment for a CHMSL / third brake light issue? My 3rd brake light doesn't work, but all other lights do. I've reviewed your instructions on: The brake light relay was clicking, the fuses were all OK. When I removed the brake light relay and jumped pins 30 to 87, the brake fuse blew. That doesn't make sense to me, I double checked the numbers, and I was jumped the correct ones. Not sure what next steps are. Any suggestions?


Hello and welcome to JustAnswer!


The third brake light is powered from a dedicated circuit from the brake switch rather than through a relay. Replace the fuse that blew and reinstall the relay and verify that all other lights do still work.


Start by checking the orange wire at the brake switch and make sure it has power, this comes from the stop lamp fuse in the power distribution center under the hood.


If that power feed is ok then press the brake pedal with the switch plugged in and make sure there is power on the white wire leaving the switch. If there is no power then you have a bad brake switch.


If power there is ok then go to the fuse labeled VECHMSL in the box under the hood and make sure it isn't blown, if ok make sure it has power on it with the brake pressed. If there is no power but there was power on the white wire at the brake switch then the wire is open in between.


If you do find power at the fuse then the next step would be to check for power and ground at the third brake lamp. Power leaves this fuse and goes to the lamp, if you're missing power and it was ok at the fuse then the circuit is open in between.


If you have power at the lamp but no ground then you have an open ground circuit or the ground is ground itself is corroded, off or loose.


Customer: replied 4 years ago.


The brake switch wires are difficult to get to, so I started at the fuse. I measured 12v with no press on the brake, then 0 volts when brake pressed. (The fuse was jumpered with a paperclip.)


Since that seems wrong, I moved down to under the dash.


Is the white wire in A1 ( the same as the white wire you mention?


On A1 I measured 12v when brake pressed and 0 with no pressure.


This doesn't seem definitive to me, am I missing something?

The wire does branch off and go to the multifunction switch on the steering column so that's probably the same circuit and this tells us the brake switch is working.

There are two fuses labeled CHMSL, one for trailer and one for vehicle. YOu did test at the one that said vehicle?

If so, let's remove the trailer CHMSL fuse and again check for power at the fuse with the brake pressed. Unless there is voltage coming into the circuit from another source then it won't have power unless the brake is pressed, if there was truly power there without the brake pressed then the brake lamp would be lit all the time.
Customer: replied 4 years ago.

I tested at the correct CHMSL fuse this time. Good call :-)


But I get the same results, voltage with no pressure on the peddle.Drops to near zero when brake is pressed.


It may be because of a "bias voltage" ( see 5mins through 12 mins)


But I'm not 100% certain. I only have a multimeter, not a test light. I'm not sure if the bias voltage applies to this.


This is just a hardwired circuit without any modules controlling it. Power goes from the fuse through the switch, through this fuse and then to the lamp.

Do you show continuity between this fuse and the white wire you found in the steering column harness?

Customer: replied 4 years ago.

Continuity... 100ohms... Yes, but seems high?

Yes, that's quite a bit of resistance if it's actually the same wire.

How much jumper wire do you have? Are you able to check resistance between the CHMSL fuse and the power wire at the light itself?

Customer: replied 4 years ago.

I checked both sides of the fuse to the electrical connections in the third brake light. One side of the fuse+light gave .8-1 ohm. Pretty good. The other side of the fuse+light was about 80 ohms.


I wonder if it's a bad ground?


I tried to pry out the light fixture to examine the back but it didn't want to come. It seems like it will break before it comes out, unless I'm missing something.

It sounds like the bulb hasn't been checked yet then? The first thing I'd recommend is checking the bulb and then testing the power and ground to see what is actually missing.

I'm seeing that the lens should just have two screws that need to be removed and then it should come off. Does yours have the two screws?

It's possible that someone could have used some silicone sealer or something similar when reinstalling it at some point and that may be what is holding it in.
Customer: replied 4 years ago.

What I meant is that the fixture (the part the bulbs are mounted in) won't come out easily.


The cover came off just fine, and that's how i was able to get the test results above, after removing the bulbs and testing from the bulb connections to the fuse.

I misunderstood, sorry.

With the brake pressed do you have battery voltage at the brake lamp?

Customer: replied 4 years ago.

so .. there is actually 12v at the outlet. :/


I don't have a test light, but i did jumper up a rear turn light by hand. I couldn't get it to light up any more than the existing fuse-style lights are. :(

Did you check ground? Let's check for ground on the other wire at the third brake light now that you did find power.

Customer: replied 4 years ago.

2 ohms to ground on one side. 90 ohms to ground on the other side... thoughts?

At the third brake lamp you have a yellow wire and a black wire, correct? There is power on the yellow with the pedal pressed and 2 ohms to ground on the black wire?

Customer: replied 4 years ago.

Yes. The 90 ohms to ground is on the yellow (left) side. The 2 ohms to ground on the black side.

If you have power and ground at the lamp connector then the problem is the lamp, bulb or connection. Make sure the terminals are ok.


Is it an LED unit or an incandescent bulb? Either way the problem is connection at the lamp connector or it's the bulb(s).

sprinkles08 and 8 other Car Specialists are ready to help you
Customer: replied 4 years ago.

Ok, so bad assumption on my part to start with: I didn't think BOTH bulbs could have gone bad, and they still "looked" OK with a filament running through them. But they were both bad.


Thanks for your help.