Why do cars have such vast wiring looms...?

April 29, 2009 10:37 AM

Why do cars have such vast wiring looms...?

I tried to trace an electrical fault in my car the other day and suddenly realised just how many hundreds of meters of wire there must be inside a standard road car.

Why do cars still use so much wiring just carrying a single binary signal when (for example) a USB cable could carry so much more data down just a few wires.

What's the great appeal over so many cables as opposed to a few microprocessors and some digital connections...?

posted by twine42 to technology (14 comments total)

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.
posted by Brockles at 10:48 AM on April 29 [1 favorite has favorites]

What type of wire are you talking about? I don't think many of the wires you'd be coming across are carrying data at all, just electrical current.
posted by winston at 10:48 AM on April 29 [1 favorite has favorites]

Also, different components have different power requirements. Much easier to just use a different wire for each.
posted by smackfu at 10:51 AM on April 29

Noise. Auto electrical systems are extremely noisy. And power - most auto system require power to drive motors/servos and not just a signal. Some cars are moving to a digital bus architecture though but the added complexity and cost is still significant.
posted by GuyZero at 10:51 AM on April 29

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.

posted by GuyZero at 10:54 AM on April 29

Here's the proper link: This Car Runs on Code.
posted by GuyZero at 10:55 AM on April 29

Another money quote from that article (sorry for the multiple comments): "a few years ago, some Mercedes drivers found that their seats moved if they pushed a certain button; the problem was that the button was supposed to operate the navigation system." yay Software!
posted by GuyZero at 10:57 AM on April 29

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 cranking the starter motor, and so on.

In other words, when multiple subsystems are interacting on a shared bus, there's a greater number of possible behaviour combinations, which makes testing more complicated.

Ultimately, though, I think it's inevitable that high-end cars will use busses heavily for communication if they don't already; if you just look at the door of a high-end car you have the electric window, the driver's control of the electric window, two loudspeakers, a headphone socket and headphone control panel, electronic locks and door-open detection, and maybe motorised wingmirrors that can tilt in two directions and fold in another. Run many more cables from the car body into the door and it will become difficult to open and close the doors!
posted by Mike1024 at 1:20 PM on April 29

For what it's worth, CAN BUS motorcycles have been available since at least 2005 (the R1200GS is the first mass-production CAN BUS bike I'm aware of.) According to BMW, that change alone saved more than 15 pounds of wiring.
posted by workerant at 2:10 PM on April 29

Part of the reason is engineering conservatism. The senders (sensors), receivers, and systems in a modern car have been refined over the better part of a century, over a very large product volume, and in a legal and regulatory environment that creates strong financial incentives not to screw up.
posted by Good Brain at 12:58 AM on April 30

Part of the reason is engineering conservatism.

Run a QA team for a month and you'll realize why this exists. it's not just an old-guard mentality. Proving software correctness is nigh-on impossible. Proving a DC servo works is nearly trivial.
posted by GuyZero at 9:42 AM on April 30

Oh, I wasn't saying engineering conservatism was a bad thing in this situation.
posted by Good Brain at 11:06 AM on April 30

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.
posted by rtimmel at 11:23 AM on April 30

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.
posted by Brockles at 5:11 AM on May 1