Dave AndersonOct 2

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 threadDave Anderson

Annoyingly there _are_ still a few computers involved, in a read-only capacity. And wouldn't you know it, one of those fucking computers is the one that's making me read the service manual. During some recent work the rear light modules were disconnected from the car, and while in that state the brake pedal got tapped.

This led some safety monitoring computer to notice that no current seemed to be flowing when the brake light switch was closed, and so now I have a bunch of angry diagnostic codes for "omg brake light communications failure" even though the light modules are reconnected and all the lights work, including the brake lights.

Even in a system designed to be computer-free for safety, somehow the computers found a way to be irritating and wrong about stuff.

Oct 2 at 6:09amWeb
Show threadDave AndersonOct 2

And you know, I'm grateful I _guess_ that the safety systems are Quite Worried about what looked like an intermittent brake light failure. I appreciate that. But I also wish that it'd just let me go into the vehicle settings and hit "yes I was doing a wrenching that's why the lights weren't working but it's fixed now".

Instead I have to cobble together some OBD-II nonsense with proprietary Mazda extensions to be able to clear the diagnostic codes, but only after a masonic ceremony in which I perform the Steps of Atonement to persuade the computer that everything is fine now.

By which I mean: disconnect the battery for long enough that the computer forgets to be angry. Which is apparently genuinely part of the official required procedure to clear brake light fault codes.

Show threadDavid AdamOct 2
@danderson I mean, that's the best way for me to forget I'm angry too