Center High Brake Light Won't Work- No Power - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 9 (permalink) Old 07-18-2018, 07:02 PM Thread Starter
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Center High Brake Light Won't Work- No Power

Hey all, 2010 Cobalt I just bought (not SS). Cruise doesn't work nor high brake light (side brake lights work fine).

I looked in the owners manual and checked the fuse and relay under the hood for chmbl (or such) and both seem fine. (I switched with the AC relay and no luck. AC works fine.)

The CHMBL relay sockets (terminals) measured no voltage in the two toward the drivers side. Passenger side front was 12.5 volts and steady. The passenger side rear wandered from zero up to about 0.3volt, then dropped to zero and repeated over and over. Then, if you opened the door, it would go to about 10.3volt and stay for maybe 3 or 4 minutes after closing door before dropping to the 0-0.3volt cycling again. It doesn't seem that 10.3Volts is enough to throw the relay as I don't get the 12.5v on to the fuse. Pressing the brake pedal made no changes to any values??

Any ideas on where to go from here? Since side brake lights work I'm assuming switch should be good? I have continuity from CHMBL connector (in trunk), one side to ground, other to the 10Amp fuse. I just can't figure why I'm getting some, but not enough voltage to throw the relay and why it's variable on opening the door and not pressing the brake pedal!!!
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post #2 of 9 (permalink) Old 07-18-2018, 09:09 PM
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As far as whatís going on with the brake lights I canít really say for sure, but the cruise will not work if the brake light is out
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post #3 of 9 (permalink) Old 07-19-2018, 05:54 AM
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The 2 tail brake lights are controlled by the brake pedal, the high brake light is controlled by the BCM. If the BCM get's the brake switch signal it will then light the brake light. 2 separate signals come from the brake switch. Try re-seating the BCM connectors maybe.

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post #4 of 9 (permalink) Old 07-19-2018, 01:00 PM Thread Starter
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I'm going back to basics as I really can't understand the logic or operation of the schematics. I've no idea how the system detects a burnt out bulb to disable the cruise control (or why?), nor why I am getting a voltage surge when I open the door. The third brake light isn't supposed to light when you open the door, does it???
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post #5 of 9 (permalink) Old 07-19-2018, 01:11 PM
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Quote:
Originally Posted by Droz View Post
I'The third brake light isn't supposed to light when you open the door, does it???
No

Quote:
Originally Posted by Droz View Post
I've no idea how the system detects a burnt out bulb to disable the cruise control (or why?)
It can only detect if all bulbs are burnt. There's a sense resistor to ground, if there's no current flowing through this resistor because all 3 bulbs are burnt it disables cruise. Since you can't see behind your car this is a way the car lets you know your high brake light is out. Same as you lose background lighting if you lose both tail lights.

PS: you have checked the bulbs right?

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post #6 of 9 (permalink) Old 07-20-2018, 08:43 AM Thread Starter
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Ah, thanks...makes a bit more sense.

I ran 12V direct to the light bar and all 3 lit fine, so the bulbs are good.

Following backwards from the light bar connector....I have continuity to ground and to the fuse, so that wiring should be good. Fuse to relay, again good. So I'd say the issue is that the BCM isn't sending enough voltage to throw that relay. With all connected, I should hear that relay click with someone else pressing the brake, and I don't.

Voltage at the relay is varying with the interior lights, not the brake pedal, so something must be crossed up. I glanced at the BCM, 3 wiring harnesses connect on. Does anybody know which wires (and which harness) are brake switch signal in and CHMBL voltage out? I'm going to disconnect and reseat them now, see if there is any change or if I can see anything burnt out or such.

(If the brake switch has 2 separate signals, it could possiblly be that as well, though it wouldn't explain the voltage when you open the door.)

Thanks for the info thus far! I will get this sorted.....
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post #7 of 9 (permalink) Old 07-20-2018, 02:08 PM
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Pin #54


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Last edited by Coby7; 07-20-2018 at 02:12 PM.
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post #8 of 9 (permalink) Old 07-22-2018, 12:19 PM Thread Starter
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Quote:
Originally Posted by Coby7 View Post
Pin #54

Thanks Coby, (I searched the forum enough to see how helpful you are to us Cobalt drivers with electrical troubles!)

I did the ridiculous yesterday and disconnected the battery for a few minutes then hooked it back up. Not a minute ago my daughter was leaving for work and I ran out to see if it had any effect. What do you know, the 3rd light was lit!! I had her go on/off the brakes a few times and it worked perfectly. I will test the cruise control another day, but I would guess that will work now too.

Who knows if it will last, but for now will stay away from it and observe.

Thanks for the help.

Richard
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post #9 of 9 (permalink) Old 07-22-2018, 01:20 PM
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YW

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