Dave AndersonOct 2, 2025

Reading through car's maintenance manual, it's interesting to see how even in very fancy, technologically advanced vehicles, there's a certain... distrust of computers.

This car has a "combination" rear light module that has sections for normal position lights, braking, turn signal and reverse. The whole module is fed by a single wiring harness, which I assumed these days would be all CAN/LIN bus stuff.

And I'm mostly right! Many of the lighting functions happen by some central processing unit sending CAN messages to another more different processing unit, which sends more commands over LIN bus to turn lamps on and off. The wiring diagram is an honest to god networking diagram with 3-4 entities involved.

The one exception is the brake lights. Want to know what the wiring diagram for the brake lights looks like?

Battery+ -> brake switch mechanically connected to the brake pedal -> wire down the length of the car -> lamps -> chassis ground return. No computers at all, just push pedal receive lights.

Show threadEwen McNeill

@danderson I suspect getting your safety certification is *much* simpler if your trust domain is “there’s a battery, switch, wire, and a light” 😃

But lots of other systems want to know if the brake is applied, so there’s an observer pattern.

I gather it gets even more complicated in EV that support single pedal driving (part of pedal travel is regen breaking, part is acceleration). Which seems to be at the edge of safety certification as when the brake lights is “work in progress”.

Oct 2, 2025 at 7:20amWeb
Show threadHenryOct 2, 2025
@ewenmcneill @danderson If you hadn't already seen it @TechConnectify did a video on this problem a couple of years ago: https://www.youtube.com/watch?v=U0YW7x9U5TQ that some forms of slowing down that aren't 'braking' slow the car fast enough that it should active the brake light, but doesn't. I think that has improved in some models since then.
Electric cars prove we need to rethink brake lights

YouTube