Computers and Multiplexing

Like everything else in our lives, computers are taking over the controls of everything on the modern automobile. How many computers does the average car contain? You might guess one or two but you would be wrong. Maybe you never even realized that cars had computers controlling everything. The most basic stripped down featureless car nowadays contains at least 4 or 5. One that controls the engine and/or transmission, one that controls, the instrumentation, one or two that control body related functions such as windows, doors, or even the dome light, and one that controls the airbag system, and so on. Very high end, feature filled cars could have 20 or more different control units all tied together in a network of control.

A typical engine control computer

Dome light you say, how and why is that controlled by a computer? Remember back in the old days when you would leave the car door open overnight only to find the battery drained the next morning? With a computer controlling the dome light it can time how long that door has been open, and after a reasonable amount of time it will shut the dome light off automatically. This saves the battery from excessive discharge so you can start the engine.

Don’t complicated computer controls just make things more likely to break, or more complicated to fix? The answer here is yes and no, not necessarily in that order. A system that is computer controlled might be complicated to fix but only if you don’t know what you are doing. Any good automotive technician needs to know how to fix every system on a vehicle regardless of how the systems work. If they know what they are doing then it doesn’t really matter how the system functions. On the other point, cars today are so much more reliable, even with computers on board, compared to what they used to be, that it’s just not reasonable to assume that complexity makes things break down more often.

All of these control units onboard each modern car or truck, are wired together in a network of control nodes, sharing information with each other. This data sharing scheme is referred to as multiplexing. This helps to simplify each individual control system because one sensor or input can be used to send information to multiple controllers. For example, back in the late 80’s and early 90’s when computer controls were becoming very popular but multiplexing had not yet hit the scene, some vehicles had 4 separate coolant temperature sensors. One sensor would tell the engine what the temperature was, one sensor would tell the temp gauge what kind of reading to display, one sensor would tell the cooling fan when to turn on, and one sensor would tell the transmission control unit what the engine temperature was. By sharing this data on a serial bus between control units, any system that’s interested in this information can take it or leave it. This means less wiring and fewer sensors. One sensor sends a signal to one controller and that controller puts that data out on the network.

Coolant temp sensor

 This sounds rather complex but it really isn’t once you know how it works. With computer controls there are so many new features that are possible, it’s simply a matter of thinking up new things to program into the system. So at what point are people going to figure out a way to upload a virus into this network? Theoretically it could be done but it’s not too much of a worry until all of our cars have access to the internet (which could come sooner than you think). The other issue preventing computer viruses from threatening our vehicle is that cars don’t run common operating systems like our home computers do. Operation programming can vary substantially from one make and model to another, and the auto manufacturers don’t share this coding information. Who knows what could happen in the future. Perhaps we will have more thing to be afraid of when we turn our vehicle over to a valet or a technician at a quick lube joint.