Many regulars have commented about their perceptions of electronics, mainly negative, and this is predominantly due to ignorance of what they do and how they do it. This is merely a guide to those who do not understand them, but wish to gain an understanding of how they work, and many will find they already know more than they previously assumed.
To begin with the basic engine, it consumes fuel, burns it, and produces power, all engines need electrics to run. A basic engine needs a low tension circuit, a high tension circuit, and a constant power supply for its ignition to operate its spark plugs. Likewise a basic diesel needs power to keep the fuel cut off valve open, and both need power to operate the starter motor, and diesels with glow plugs use electrical power to initially heat these glow plugs.
All vehicles have certain instruments, the two basic are a fuel gauge and a temperature gauge, and most people understand how these work, they both have a positive power supply to the gauge and a negative feed which goes to earth via a sender unit. The function of the sender unit is to provide a resistance, and our two senders are operated either by heat altering the sender resistance, or a float which wipes a wiper across a resistive track to alter its resistance. We can conclude that these items operate by applying a variable resistance to the earth wire, many know testing them is merely a case of removing the wire and placing it on a good earth and the gauge rises to its maximum as it has now has no resistance to earth, and is a common way of finding a defective sender unit.
In an electronically controlled engine there are two predominant sensor types, one is the resistance type, the other is the pulse generator type. Those who know how a basic engines temperature sender, or fuel gauge sender works are already halfway there in respect of modern engines sensors.
Variable resistors are used in many applications, and often have different names to those in traditional engines, but are exactly the same item, remember the techies of the day like fancy titles and terminology which baffles the layman and makes them sound superior. In a temperature application the old engine has a temperature sender, in a modern electronically controlled engine it is called a thermostatically controlled resistor, or abbreviated to thermister, but it is exactly the same thing as the old temperature sender, it does the same job, but now has a fancy name.
Resistors are used in a variety of applications in a modern engine, the predominant ones are temperature sending for the gauge and the engine management, throttle position switches which are merely the same as a fuel tank sender, basically a switch operated by a spindle on the throttle body which varies resistance according to the throttle position. Older air flow meters use a shaped (usually L shaped) rotating segment across a defined arc which moves in accordance with the amount of air sucks into the engine, this is connected to a variable resistor to show its exact position, and obviously the fuel gauge which we have covered.
In essence we can conclude by saying all these sensors are nothing more then variable resistors which are activated by one means, either heat or mechanical motion.
Pulse generators are a sensor which produces a variable pulse, a variable voltage, or a combination of the two.
Pulse generators use an electromagnet and a toothed sprocket or similar to produce the pulse and/or voltage, and work very simply. As the tip of the tooth is in very close proximity to the sensor it produces this pulse by passing through the electromagnetic field the sensor produces, yet the valley of the sprocket or toothed item is too far away to effect it, so the rotation of the sprocket only works when the tip is passing it, it is this which generates the pulses, and the measurement of the pulses allows us to measure the rotation speed of an item.
Pulse generators are used to measure rotational speeds, and are predominantly used to measure engine speeds, gearbox output speeds for electronic speedometers, and modern ABS systems as they can measure rotational speeds of many items such as wheels and note any difference from a locked wheel (slower) and a slipping wheel (faster) than the rest.
By measuring the amount of pulses and any voltage produced we can make comparisons if more than one pulse sender is used, if we compare the pulses of a pulse generator located on an engines crankshaft by using the flywheel or front pulley camshaft belt drive they should be the same, if we use one of these and another located on a camshaft the camshaft sensor will produce exactly half the pulses.
One derivitave of the pulse sensor is the speed/position sensor, this uses the number of teeth to give the speed, but has one tooth missing which it uses as a reference point for its actual position. If we assume a flywheel has 180 teeth each peak of our pulse is exactly 2 degrees apart, pulse/position sensors basically count these pulses and compare them to the missing tooth and from this can tell exactly where the position of the engine is on each rotation, it is this precise measurement which allows such accurate timing and fuel measurements, and allows them to operate variably, but at the optimum time.
Piezo sensors are mainly used as a listening device, they basically listen to the combustion in each combustion chamber as the engine fires and convert this into a pulse which relates exactly to the combustion sound, this is really their only application, but all electronically controlled engines have them.
Bi-metal strip sensors are nothing more than a strip which expands and changes shape in relation to heat, this motion is used to control on/off switches, and are mainly radiator electric fan thermo switches.
If we look at modern vehicles we know they are all electronically controlled, even the most basic engines used have to have them, but we now know how the sensors controlling them work.