An INTRODUCTION to SENSORS

Compared to those trundling around the UK road network, when this magazine debuted, current vehicles are computers on wheels. While it remains true that most of today’s cars retain internal combustion engines, and therefore, a degree of mechanical integrity, they are still controlled electrically. Much of this is down to necessity. With our roads becoming considerably busier, engines had to become more fuel-efficient, reliable and powerful, while being less polluting. More accurate means of metering fuel and ignition were made possible only after replacing relatively imprecise mechanical distributors and carburettors with more accurate electronic ignition and fuel injection.

Early injected cars tended to use a single computer/Electronic Control Unit, whereas current cars employ multiple ECUs that communicate together. Our investigations into both ECUs (August 2023 issue) and data networking (November 2023 issue) explain how all of these systems work. Yet, without some kind of measuring hardware, there would be little point in all of this processing power.

Enter the sensor

With the need to detect anything from movement to pressures and temperatures, a modern motorcar possesses hundreds of different sensors. As space precludes us from including all of them, this feature’s focus is to provide a basic introduction to sensors that monitor primarily the engine, fuel system and exhaust gases. Should one of these sensors develop an occasional, or permanent fault, the situation would likely cause the engine management indicator lamp (MIL) to illuminate, the presence of which is an MOT Test failure. Yet, consider that they do not operate in isolation and that sensor data is used by more than one ECU on modern cars. Furthermore, an engine management ECU, for instance, will consider inputs from other sensors that are not situated beneath the bonnet, such as those that measure wheel speeds.

As heat, vibration and moisture are anathemas to electronics, it is amazing that sensors are so reliable. One reason for this is that most of them are sealed units, meaning that they can be neither dismantled, nor repaired. Yet, they fail eventually. Corroded, or damaged wiring aside, there tends to be no way of telling whether a sensor itself is functional, or a dud, just by looking at it. So, what can you do?

Uncovering the mystery

Helping us understand the secrets of sensors is Hella – a respected brand that will be familiar to many readers. As the company supplies car factories with components, the sensors that it distributes as replacement parts are made to the same standards as those specified by vehicle manufacturers. This is important, because replacement sensor quality is a known challenge. While a cheaper copy sensor might look the same on the outside and will fit physically, the internal parts may not be to the correct specification. The result is incorrect signals being sent