Third brake light not working?

AZDANNO04

MEMBER
2008 CHEVROLET COBALT
2.2L
4 CYL
2WD
MANUAL
134,455 MILES

I replaced the bulbs and the fuse on the inside passenger center console. Friend of mine tested the bracket that holds the three bulbs, and they came on but won’t when I put everything back on. Didn’t know if there’s more than one fuse? I believe he told me he can see a very dim light for the third light didn’t have an issue before. I know when I checked if all light bulbs were working so I don’t get a ticket at night. I found inside the bracket that holds the bulbs was rust that had turned into clay like material since water had gotten in the little crevasses. I scrapped as much as I could out with dental scraper and then quick drying electrical cleaner to flush the rest out. I will take another look but if the lights came on when my buddy was able to make a connector with a 9volt battery for the lights to come on...

7 Replies

JACOBANDNICKOLAS

MECHANIC
108,217 POSTS

Hi,

Is it only the brake lights that aren't working? Do the turn signals, parking lamps, backup lamps, and license plate lamps work? I ask because there could be a ground-related issue.

I ask because the center high-mount stop lamp (CHMSL) is powered via a relay. It also has its own separate fuse.

Next, if the turn signals work, then the wiring to the light sockets is good because the signals use the same wires.

That leaves the body control module. Do me a favor. See if there is power to the yellow wires at the light sockets when the brake pedal is pressed. Don't use a 9v battery.

I attached the wiring schematic below showing the exterior lighting circuits. I highlighted areas of interest. I need to know if there is power. If there isn't, we will need to work backward to see where power is lost.

Let me know.

Joe

See pics below.

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Saturday, November 11th, 2023 AT 9:31 PM

AZDANNO04

MEMBER
265 POSTS

It's just the third brake light that’s not working. The taillights work.

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Monday, November 13th, 2023 AT 8:09 AM

JACOBANDNICKOLAS

MECHANIC
108,217 POSTS

Hi,

If it is only the CHMSL, there is a fuse and relay in the under-hood fuse box. See pics below. Note, if you are seeing a dim light, then check to make sure the ground is good. Also, using a multimeter or voltmeter, check to see if there is battery voltage at the light blue wire on the CHMSL. If there is, then either we have a weak ground or a poor connection at the light itself.

Let me know what you find.

Joe

See pics below.

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Monday, November 13th, 2023 AT 9:35 PM

AZDANNO04

MEMBER
265 POSTS

Used the voltage meter got 18.60 so more likely a poor ground. I wonder if that ties in with the trunk release button since that doesn't work and my horn, my speedometer, rpm, fuel sending gauge, airbag safety check message on instrument panel, as well as Power Steering message on panel. Even though my Power Steering still works. And coolant temperature warning light comes on and off. As I scroll across the options on panel to check coolant temperature. Temperature fluctuates also.

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Tuesday, November 14th, 2023 AT 10:19 PM

JACOBANDNICKOLAS

MECHANIC
108,217 POSTS

Hi,

It sounds like you have more than one problem. Are you saying there was 18.6V coming to the light? Was the engine running? If it was, the voltage regulator is bad. It shouldn't reach that high of voltage.

Try this test and let me know the results:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Take care,

Joe