Why do cars have such vast wiring looms…?
Also, your assumption is somewhat wrong – most cars have a lot of digital wiring in them 9to contradict myself): here’s a bad digital copy of an older IEEE Spectrum article https://cs.senecac.on.ca/~lloyd.parker/Cars_on_Code.htm: Even low-end cars now have 30 to 50 ECUs [microprocessor-based electronic control units] embedded in the body, doors, dash, roof, trunk, seats, and just about anywhere else the car’s designers can think to put them. That means that most new cars are executing tens of million of lines of software code, controlling everything from your brakes to the volume of your radio.
Some vehicles make more use of busses – things like CAN. The main reasons a given item might not be on a bus are cost and complexity. If you’ve got a car stereo and four speakers, you could either run a two-core cable to each speaker, or run a bus and have a microprocessor and an amplifier on each loudspeaker. The latter makes the audio system more complex, and more expensive (processor for each speaker), although it might reduce the complexity/cost of the car overall. Also, bus systems that go everywhere add to the complexity of testing the system; if you’ve got a bus with 20 devices on it, you want to know how the system behaves if different systems power on in a different order, how the system behaves when the different devices are in different modes, how different things react if the battery voltage is low, how different things react if the battery voltage temporarily dips while
You may well have been an edge case, as I know that VW certainly do stock wiring looms as I have not only bought them, but stocked them (for an affiliated, semi-official arm using VW’s own parts system). I wonder if your car was over a certain age, or they just didn’t have any of that particular loom, which has led you to that conclusion. Once they have run out of the supply that they made in addition to production (tooling up is expensive, as you note, so they often make a run of components larger than the production requirement and stock them in bulk) then the cost of making an extra loom may well be prohibitive. It’s possible that your fault was common, and they burned through their supply with warranty claims and had to buy back yours as one of the later ones with no other course of action.
Most sensors are resistance or inductance sensors. These are simple, cheap, robust and reliable. The extra wires required is a small penalty in an environment where extra weight of wire (and, to an extent, packaging issues resulting) is such a small issue. In addition, the majority of the wiring is to supply voltage, rather than transmit data – all the sensors would still need power fed to them in either case. The complexity of separate control systems for the various sensors and the like to use USB would be substantial, in my opinion. It’d be a much more complex arrangement.
Interesting side note — some auto companies (Volkswagen at least) do not consider the wiring harness a part, nor do they keep them in stock. I purchased a Jetta about 6 years ago that did not have the rubber plugs installed in the underbody to protect the harness. One snowy winter later, the entire harness was corroded. I was told that to replace the harness, Volkswagen would have to shut down an assembly line, retool it for the prior years model and run my replacement. Instead, they purchased the car back form me at a very reasonable price.
