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Discussion Starter · #1 ·
This one has had me wondering more than a few times while driving my Ascent. I had always assumed the lights were displayed at the same time brakes were applied and the taillight brake lights illuminated. At first I just though the ACC was really good at downshifting and slowing down, but now I'm convinced that it applies the brakes well before the brake lights on the combination meter display are shown.
I'm taking her in tomorrow to have the dealer check for rotor warpage because the brakes have been feeling a little rough when they're hot. Because of this, I've been able feel the difference between when its just downshifting and when its applying the brakes while on ACC, especially when going down a mountain.
Does anyone have any insight on how its supposed to work? Do the actual brake lights come on prior to the combination meter display lights? I'm getting to the point that I'm curious enough to figure out a way to test this...
 

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Up to a 1-2 second delay, but ACC does use engine braking as well. At some point in deceleration, it activates the brake lights.
 

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Discussion Starter · #3 ·
Up to a 1-2 second delay, but ACC does use engine braking as well. At some point in deceleration, it activates the brake lights.
Is that in any of the manuals? It’s a little odd to me that they show up when physically pressing the brake pedal right away, but not right away on ACC.
 

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Is that in any of the manuals? It’s a little odd to me that they show up when physically pressing the brake pedal right away, but not right away on ACC.
Physically pressing it is driven by the brake pedal switch detecting operation. Since the system does not use the brake pedal switch to activate it, it has to send a signal through/from the BCM through the CANBUS connection on the head unit, which then needs to send signals to both the upper Eyesight display and the combo display in the driver's pod, so, the reason the response is different is because the triggering action is different.

I would believe that the BCM directly fires the brake light relay that the brake switch is wired to, which is why the brake lights themselves have no delay in either manual operation or ACC operation.

The manuals don't go into the technical information on the underlying operation, but do indicate that when ACC applies the brakes, it illuminates the brake lights on the combo display. Though not stated, it also illuminates it on the Eyesight ACC display on the center info display for cars equipped with one.
 

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Discussion Starter · #5 ·
Sounds pretty technical indeed. I don’t think I would’ve designed it that way but I’m not a software/vehicle engineer either.
 

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Sounds pretty technical indeed. I don’t think I would’ve designed it that way but I’m not a software/vehicle engineer either.
I think it's pretty much the only way it can work, sans mechanically moving the brake pedal.
 
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