CAN under control

Ever faced with a wiring harness that looks like a cross between a plate of spaghetti and a two-inch manila rope? We all have, and with increasing electrical system content in modern vehicles, it can be a nightmare tracking down even the most innocuous system faults.

Want to simplify? The automakers did, too, which is why Bosch developed CAN. Now adopted widely by major automakers (and many other industrial and commercial OEMs) CAN stands for Controller Area Network, and is the automotive equivalent to the LAN (Local Area Network) that connects computers.

The idea is simple: hook all the sensors and actuators into a loop, or “bus” and let them share the same wire, each sending and receiving signals coded to mean, for example, “open sunroof” or “honk horn” or perhaps, “manifold pressure low”.

Bosch estimated a weight savings of 25 percent over ‘discrete” wiring and a cost savings of 17 percent. What’s in it for us in the repair aftermarket? CAN eliminates large numbers of wires, splices and connectors, which in turn increases reliability.

Generally there are several CANs in a modern high-tech vehicle, with low and high-speed versions used for the appropriate systems. A typical vehicle might have:

High Speed CAN for Engine/Transmission Control, ABS/ASR and Electronic Stability Program (ESP)

Multimedia Bus controls and transmits audio, video signals and information such as navigation and wireless services.

Firing Bus controls the deployment of front, side and rear airbags

Low Speed CAN Bus for Body Electronic Units such as window lifts and climate control

How does this change the way you troubleshoot CAN-equipped vehicles? Much of your intuitive capabilities apply here too, but keep a few facts in mind:

CAN provides sophisticated error-detection and error handling mechanisms as well as high immunity against electromagnetic interference.

This means that inductive pick-up from other electrical devices is unlikely to scramble the signal from a sensor or actuator and shouldn’t cause an uncommanded action.

Any erroneous messages are automatically retransmitted, and temporary errors are recovered.

Signal dropouts ort glitches shouldn’t affect real-time operation of the system, since it sends a signal again if it doesn’t get the commanded action the controller wants.

Permanent errors are followed by automatic switch-off of defective nodes.

This feature helps you find bad controller devices or actuators, since a seriously intermittent unit will be “killed” by the controller.

Signals are prioritized to keep them from “colliding”.

If your issue occurs when multiple systems are switched on, it’s unlikely that the network is confusing the signals. A power drain or dead short in an actuator circuit is a good path to follow.