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We recently even had that for our car.
ABS stopped working, along with other things like remote car opening and also the turn signals for some reason (rest of lights still worked)
Repair shop didn't find any errors in the electronic log, but on a wimp disconnected the battery and let it sit there for a while.
All just worked normally again after that.
While this is typical software glitch behaviour, it is slightly disturbing in a car, especially involving safety relevant functions like ABS and ESP...
That's not software, that's almost certainly purely electrical. More specifically, disconnected/floating ground/return lines. Car computers need 0 Volts to look like 0 Volts and 12V to look like 12V, but floating grounds can cause 0 to look like -4 and 12 to look like 20.
Strong chance that the battery terminal connections were corroded or not tightened down enough to get good contact and that by disconnecting/reconnecting the new connections were better.
-- spacecraft software/electrical engineer who cosplays as a backyard mechanic
I don't think so.
The negative terminal of the battery just has a single 30cm long line directly connecting it to the car chassis.
And if that is "floating", the main current source is disconnected and the whole car is completely without power.
So, sorry to have to disappoint you, but that's not it.
Not necessarily
In electrical engineering floating just means there is nothing forcing a particular conductor one way or another. All a battery does is try and force its two terminals to be ~12V apart, what happens coming off the battery terminals is a different story. If you had a bad connection to the terminals a resistance could have been there, itself driving a voltage difference depending on the current passing through it.
Interesting that your car routes all return through the chassis, I've never seen that before. Admittedly I've only ever worked on US or JDM cars, but the ones I've worked with typically have separate return lines for the electronics, a dedicated thick one for the engine and then the engine is what connects the chassis to battery return.
No disappointment, just a fun interesting problem and learned something about European cars.
Interesting... All my cars up to now (mostly German brands) had the direct connection between negative battery terminal and chassis.
But as already said, newest car is already 15 years old, so might have changed by now...
The topic of contact resistance is a separate one, would show as voltage drops during heavy load situations. Similar to high internal battery impedance during winter especially for an older battery, when all the lights go out when starting the engine. This is mainly due to the high start current drawn by the Diesel engine (Diesel engines also being uncommon in the U.S. afaik?), so the electronics are designed to cope with this severe voltage drops.
Yeah, US and JDM cars are predominantly gasoline engines. Engine start current draw is similar between the two engine architectures but gasoline engines with their spark plugs would certainly cause noise on the ground line during normal operation which is probably the biggest reason for the dedicated ground line. The digital electronics would also be sitting behind a down regulator that I'd be willing to bet isolates the ground as well.
I still lean towards the original topics failure mode being electrical based not software. Software faults tend to be highly repeatable and almost always persist across full power drains since, assuming no underlying electrical issues exist, computers execute software instructions perfectly every time. Given the exact same set of inputs and the exact same timing, they'll get back to the same state. And that would have been happening since the factory.
Degraded electronics could be feeding new or unexpected inputs into the computers that trigger different software state transitions that then lead to unintended or unexpected behavior, but things would have to be going pretty off the rails for the system to pass all of its built in tests and not realize something has gone wrong.
Another possibility is the mechanic found loose harnessing, connectors, did a few different unplug/plug cycles and then only told you about the battery.