MAT Sensor
The manifold air temperature sensor is a thermistor, a resistor that changes its resistance with temperature. It is screwed into the intake manifold to measure the manifold air temperature and signal it to the control unit. The intake air temperature is used the control unit in calculating the injector pulse width.
Distributor Sensor
A sensor is fitted in a distributor which is shown on the image on the right. It consists of two main parts: a disc or rotor plate and a sensor unit which carries the diodes. The rotor has 360 slits spaced at 1° intervals. These are used to provide a light signal at each 1° of disc rotation which is used for engine RPM.
The disc has four slots at 90°intervals for a four cylinder engine, and six slots at 60° for a six cylinder engine. The signal from these slots is used for ignition and injection timing. The light signals from both sets of slots are changed to voltage pulses by the photodiode and are the directed to the central control unit.
Air Flow Sensor
The air flow sensor I’m describing is for older model vehicles, but the reason I’ve chosen to look at this design is because it is used in my BMW 318i 1998. This is surprising because they manufacture these for vehicles up to the early 1980s, so it’ll be interesting to study this design.
The type of air flow sensor that I am discussing is illustrated in the image below. In this design, a heated wire in the sensor is located in the path of the airflow and used as a sensing element. Air passing over the heated element produces a cooling effect, which reduces its temperature.
Changes in temperature are used as signals to the central control unit. The signals are used to measure the quantity of air flowing through the air sensor and into the engine.
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