Sensor testing Part 2

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.

Sensor testing part 1

Sensor testing

Crank Sensor

The crank-angle sensor is located in the distributor. It detects both engine rpm and crank angle, which is the angular position of the crankshaft. With the way it is arranged a perforated disc is used, together with a light-emitting diode (LED) as a light source and photodiode as a light sensor.

                        

The dis is rotated between the diodes, and light from the LED passes through the holes in the disc onto the photodiode which acts as a light sensor. The frequency of the light passing through the holes in the disc is used as a means of sensing speed. It can also be used to sense the angular position of the disc.

Testing

We tested the crank sensor with a Multimeter set on Ac voltz, we placed the first probe on the crank and the negative probe on a good earth. We tested it under two conditions: The first condition was at Idle RPM and we got a reading of 0.80v which is a good reading for this condition. The second condition was at 2500 RPM, the reading I got was 1.57v which is a good reading for the specified condition.

MAP Sensor

The manifold absolute pressure sensor measures the absolute pressure in the intake manifold; it is also referred to as manifold vacuum. The sensor is responsive to manifold vacuum and also to barometric pressure. The air/fuel ratio, the ignition timing and the idle speed can all be modified by the control unit as a result of the MAP sensor.

Testing

With a multimeter we connected the probes output connection on the circuit bourd and the earth to a good ground. We tested the first reading at idle vacuum which read 1.10v, this is a good reading because all air pressure is drawn out at idle. The next reading was at key on engine off condition the reading was 3.73v, this is a good reading because at no vacuum manifold pressure is full.

Coolant Temperature Sensor

The coolant temperature sensor is fitted into the water jackets of the engine. It senses coolant temperature and signals this to the control unit, which can adjust the fuel mixture and ignition to suit a hot or cold engine.

Testing

We connected the positive probe to the THW connection and the earth probe to a good ground, the first condition is with the engine cold and the second condition is with the engine warmed up.  The reading we got when the engine was running cold was 3.50v and the second reading we got with engine warm was 1.8v voltz.