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post #1 of 7 (permalink) Old 07-28-2018, 09:18 PM Thread Starter
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High Mounted Centre Brake Lights

I have been considering upgrading the three high mounted centre stop light bulbs (912 bulb type) to compatible 912 LEDs, but I understand that on the Cobalt/G5 because of the low load of LEDs would cause cruise control to stop functioning as the BCM depends on the high mounted brake lamps electrical line to switch cruise mode off when the brake pedal is depressed. Is this indeed the scenario that would occur with the 3 high mounted brake lamps if upgraded to LED and would this mean that the 3 high mounted lamps must be filament bulbs (unless some form of load resistors were added)? Thanks for any help.
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post #2 of 7 (permalink) Old 07-29-2018, 04:42 AM
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You are mostly correct, where you are not quite correct is, the cruise fails to engage when all 3 bulbs have failed or high brake light circuit doesn't work. This is the vehicles way to tell you the high brake light has failed. The cruise gets release by a contact directly from the brake switch to BCM not from the high brake light.

If you leave one filament bulb in and insert 2 LED equivalent lets say on each side of the center bulb this will not affect the cruise module.

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post #3 of 7 (permalink) Old 07-29-2018, 10:07 AM
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The preferred way to convert the CHMSL to LED was to use one out of a Monte Carlo/Impala. It bolts right into the light housing, and doesn't affect cruise control.

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post #4 of 7 (permalink) Old 07-29-2018, 10:52 AM
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I did this to my 10 Cobalt. It doesn't bolt directly, I had to glue the LED module from the Impala in the Cobalt housing. The only problem with this setup is if you lose your high brake light the vehicle has no way to let you know.

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post #5 of 7 (permalink) Old 07-29-2018, 11:49 AM Thread Starter
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You are mostly correct, where you are not quite correct is, the cruise fails to engage when all 3 bulbs have failed or high brake light circuit doesn't work. This is the vehicles way to tell you the high brake light has failed. The cruise gets release by a contact directly from the brake switch to BCM not from the high brake light. If you leave one filament bulb in and insert 2 LED equivalent lets say on each side of the center bulb this will not affect the cruise module.
Thanks for clarifying this. I always had the impression that if just one of the 3 filament bulbs failed in the high mounted brake light, that cruise control would fail to engage, so I always had the impression that all 3 bulbs had to be working. However, if cruise would work with just one of 3 filament bulbs working then just having to replace 2 of the 3 filament bulbs with a pair of LED lamps could be useful because most LED lamps are sold in pairs and I was thinking that I would need to buy 2 pairs of LED lamps to do this upgrade, which I wasn't even sure it was possible or not without disabling cruise. I guess the only concern is whether the 2 LED flanking the centre filament builb would look brighter than the bulb (quite possible I would guess).

While we're on the subject of exterior bulbs, do you know if the Cobalt/G5 uses a flasher relay for the turn signal and hazard mode lights, and if so where that flasher relay is located? I looked at the owner's manual's pages for the centre console fuse board map and also the engine compartment fuse box map and couldn't find such a flasher relay. I'm not sure if the BCM is directly controlling the timing of the turn signal flash, though I do know that I would get hyperflash if one of the front or rear turn signal / brake bulbs died. So I'm thinking there has to be a flasher relay somewhere. Trying to investigate whether I could replace that with an aftermarket LED flasher relay that would just plug in directly and then I could use LEDs for front turn signals and rear brake lights without having to use load resistors, something I prefer not to have to deal with.

Last edited by gmdriver; 07-29-2018 at 12:44 PM.
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post #6 of 7 (permalink) Old 07-29-2018, 01:25 PM
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No flasher relay, these filaments are solidstate driven by the BCM. If you add an LED bulb you also have to add a parallel resistor to stop hyperflashing. Again if you do this and the LED replacement fails your vehicle will not be able to warn via hyperflashing.

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post #7 of 7 (permalink) Old 07-29-2018, 01:36 PM Thread Starter
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No flasher relay, these filaments are solidstate driven by the BCM. If you add an LED bulb you also have to add a parallel resistor to stop hyperflashing. Again if you do this and the LED replacement fails your vehicle will not be able to warn via hyperflashing.
Right, that's because the load resistor would appear like a working bulb to the BCM so even if the LED brake lamp failed there would be no hyperflash. Thanks for confirming that these lights are directly driven by the BCM. In that case, I'm going to stick with the regular filament bulbs for brake lamps. They are cheap and lasts quite a long time (years) anyway. I'll just upgrade the two side filament bulbs of the high mounted brake light to LED and I think I'm done with all exterior bulbs LED upgrades after that, if I choose to not use any load resistors.
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