Today's vehicles are chocked full of electronics and electric powered devices - electronic fuel injection, antilock brakes, infotainment and navigation systems, and power everything. Now, electric steering, transmission, suspensions and even electric air conditioning are appearing in the latest models. All these system require wiring, and lots of it. Besides adding weigh and taking up space, the huge amount of wiring means assembly and maintenance headaches.

Cadillac Allante was the first car to use multiplexing Cadillac Allante

In the days of wind-up windows and cassette players, wires from each electrical switch ran to the device they powered. As the number of electrical powered devices increased automotive engineers had to develop an alternate to evermore complex wiring harnesses. The solution was multiplexing. As a point of history, the 1990 Cadillac Allante was the first domestic vehicle to use a multiplexed electrical system. Here, multiplexing was used just for the lighting system.

Basically, multiplexing uses the same wires, called a 'data bus', sharing command signals, feedback signals and data between devices. This drastically reduces the number of individual wires needed in a vehicle. This results in lighter weight, smaller and less expensive wiring harnesses.

With multiplexing, input and output commands from devices in one part of the vehicle are combined in a module that contains one or more microprocessors. Multiplexing is used only for "switching" circuits, that is used only on-off commands and other data that are combined on the 'data bus' wire. There are still power wires and grounds connected to each device. Typically, only a power wire is needed with the vehicle itself serving as the ground connection.

1990 Cadillac Allante used multiplexing to control power windows1990 Cadillac Allante Interior

As an example of multiplexing is the module in the driver's door that controls many powered functions - windows, mirrors, door locks and maybe even the driver's seat. Rather than many wires to power and control each window, mirror, door, etc., a data bus is used. Now, when you use the mirror adjust controls, the module sends a packet of data onto the data bus to another module that turns on the motor in the power mirror to move it in the desired amount and direction. The same data bus can control other functions like winding up a window or locking a door.

Communications on the data bus consist of individual packets of binary coded (0's or 1's) information, not a continuous stream of information. Each transmission takes only a few microseconds, so much information can be sent and received along the data bus in a very short amount of time.

Communications traveling on the multiplex data bus pass by every module attached to the bus. However, information is only received by the specific module to which it is addressed; the other modules ignore the message.

Traffic on the data bus is two-way, that is the receiving device can also communicate back to the module and to inform that the message was received. This feedback capability allows self-diagnosing of faults. If there is no response to a command, it can trigger a diagnostic trouble code that can be read later by a technician. This capability can also be used to display an alert, such as that a taillight is not working, to driver.