Networking and Multiplexing

Even the most basic modern vehicles include many electronically controlled systems. If each electronic system had its own ECU, harness, and sensors, the weight of the added components would negate some of the efficiency it provides and would add cost to the vehicle. A vehicle’s multiple electronic systems could require miles of insulated wiring, splices, and terminals.

One solution to reduce the amount of wiring required is the use of a system that integrates sensors into a common wiring harness, called a bus. The bus combines all the individual systems wherever possible into a multiplexed serial communications network. An added advantage of such a system is that with less wire and fewer connections, there is less chance of dirty connections causing faults. Of course, if a network fails, it can shut down the entire network and with it the controlled devices. Also, troubleshooting a networked system requires more training and more tools than a simple digital volt-ohmmeter (DVOM).

With networking, a single (or multiple) network consisting of one or more twisted pairs of wires, with terminating resistors at the end of each pair, carries digital information throughout the entire vehicle. In many cases, there is a medium- or low-speed bus for nonsafety systems and a high-speed bus for systems such as anti-lock brakes, supplemental restraint systems, and many of the engine controls. All components “listen” to the signals carried on the network and respond when requested to do so. All networked data/information is shared throughout the vehicle. Such a system is often referred to as a controlled area network bus (CAN-bus) Figure 39-29. The thin twisted pair of wires connects all the on-board control modules to each other. Thus, the power train control module (PCM), BCM, electronic brake control module, transmission control module, and many others are networked together and communicate, much like “party lines” on old-fashioned telephone systems. Output devices that respond to commands on the network are referred to as nodes. All nodes hear all data on the network, but only respond when called upon to perform some action or diagnostic function.

FIGURE 39-29
Typical CAN-bus diagram.

The advantage of a multiplex network is that it enables a decreased number of dedicated wires for each function, and therefore a reduction in the number of wires in the wiring harness. This network also reduces system weight and improves reliability, serviceability, and installation.

Sensor data multiplexed on the network are prioritized. For example, such information as vehicle speed and engine temperature data is shared on the network for other systems such as the instrument panel cluster to access and use for their purposes. This design helps to reduce the number of sensors needed. Some information being shared takes priority over other. For example, a message related to a loss of engine oil pressure would take precedence over a lamp outage message. In such a case, the vehicle operator must be immediately alerted to the urgent fault.

Networking of data also allows greater vehicle content flexibility because functions can be added or modified through software changes. Other control units, such as those related to trailer towing, can be added as required to the system by simply connecting them into the network and updating the software to integrate it into the network. You can think of this process as being similar to adding a printer to your home computer system; plug it in, configure it, and start using it.

A diagnostic scan tool is connected to the CAN-bus network to both monitor and extract operational information to assist in diagnosis and fault finding. Factory scan tools and other so-called bidirectional scanners are used to command engine and vehicle components to operate in order to confirm their correct function.