In the last 20 years, there have been more advances in electronics than at any other time in history. So much so, that many of the newly qualified technicians out there have never actually had the experience of testing a coil or standard relay, let alone an ignition condenser. This technology has largely been replaced by sealed, solid-state componentry – which, these days, requires an entirely different and extremely complex set of diagnostic skills. So, what has changed? Let’s start with something simple like an indicator/turn signal. In the case of older technologies, it’s fairly simple: the indicator incorporates a mechanical switch, which, when activated, completes a left or right circuit, thereby energising the flasher unit relay and lighting the exterior lamps as well as the green repeaters on the instrument cluster. This circuit is fused and usually has a rating of no more than 10 amps.
With newer technology, things are far more complex. The modern switch, while still usually mechanical in nature, has a much lower-current rating, and sends activation (left- or rightturn signals) to a Body Control Module (BCM). Note that different manufacturers use different acronyms, but the concept remains the same. The BCM will usually be responsible for turn signals, interior lighting functions, and most other non-specialised interior electrical functions. The BCM is also connected to a Controller Area Network (CAN) bus.
The bus is a communication system used to link various modules (computers) together, often using two wires. These wires are generally twisted together in the harness to make identification easier and to reduce inductance and interference from neighbouring wires and components. The system links all the control modules connected to the bus – almost like a telephone conference call. The information transmitted by each module on the bus is available for other modules to interpret and respond to. Each module has a unique electronic identifier, and the ones that need the information use it, process it, and then transmit their confirmation or corresponding response onto the bus. The packets of information being sent on the bus are prioritised, so a more important message will be transmitted before a less important one. This protocol is incorporated into all the modules connected to the bus so that each module “understands” its role, and the order in which it must be processed.