The problems of conventional automotive wiring harnesses and how they can be minimised by multiplexing are discussed. The multiplexed wiring harness replaces the conventional harness with a cable bus carrying power and time-division multiplexed control signals to electronic modules that decode the control signals and activate power switches accordingly. An experimental multimicrocomputer multiplexing system is described. It has a centralised processing unit (CPU) and up to eight local processing units (LPU) interconnected to the cable bus. The electronic harness is controlled by the CPU which processes all inputs to the system and then transmits commands to the LPUs for control of each function of the electrical system. The CPU is located in close proximity to the dashboard and the majority of control inputs are directly wired into the CPU board. The LCUs are located in areas of high load density in the vehicle. Connections between the LPUs and the peripheral electrical equipment are made by small subharnesses, in most cases by a single fly lead. An LPU will only respond to a command signal from the CPU when the command address and its own address match. To eliminate the problem of spurious commands occurring owing to electrical interference, the CPU employs the repeated transmission method for communication. The switching channels have two commands associated with them: one to turn on and one to turn off. Following reception of these commands, there is a time slot available during which the LPU sends back to the CPU diagnostic information relating to the performance of the load controlled by the channel. The system incorporates a number of failsafe features to avoid potentially dangerous situations arising in the event of a CPU failure or damage occurring to the system.