Thermo Fan Switch

Its very important that Thermo fan switch works properly on every car as it used to turn on radiator cooling fans. When vehicle driving on a highway radiator cools down with upcoming air stream, but when its in a traffic jam and only short starts and stop, that when that sensor very important. Its a few type of that sensors, one will complete the circuit when hot enough, another will do an open circuit when at certain temperature.

Here is a pictures showing that at 92 degrees TFS open circuit, so that means if engine will get to 92 degrees, fan will start working until temperature drops below 92 degrees.

Engine Coolant Temperature or ECT sensor

ECT sensor is a very important for engine cold start and an open loop running of the engine. Its a negative temperature coefficient, which means the colder the sensor the more resistance it produce, the hotter the temperature the less resistance. This sensor determine injectors cold start duty cycle.Here is a graph showing different resistance reading at different temperatures. This sensor decrease resistance value with hotter temperature. So if we use 5 volts power supply to the sensor and connect it to ECU it can see different voltage input signal with different temperatures and tell ECU when ever engine hot, cold or warm.

Intake Air Temperature sensor works the same way so ECT and IAT can be checked same way.

Vane or flap air flow sensor/meter

The most dodgy design i ever seen on cars (personal opinion no judge) found it first time on my own Toyota Levin 4AGE engine. It is actually need some force before it can start working so i basically blocks air flow in the engine. Most of the Toyota VAFM got 7 pin outs, Ground, Power, Signal, Idle, Sensor output and fuel pump switch.
That how it looks like! Big, massive, but a bit useless for hi-performance cars.





They can be tested by connecting them to 5 volt power supply and reading output voltage at different angle of the flap. Inside this sensor is a flap on pretension spring which connected to variable resistor (potentiometer) which change resistance as more air passes through and opens flap. In most cases this sensor fails if resistor track is broken. Here is some readings i took.

Mass Air Flow sensor or Air Flow Meter

I have used BOSCH air flow meter and when it was connected to power first it gave output voltage of 1.08 volts.







When I've been blowing into sensor it changed its voltage up to 3 volts but camra only could capture 1.73 volts reading (bloody camera cant work that fast)




This sensor change resistance with air mass passing it and cooling down sensor wire, inside mass air flow meter is a resistor group called Winston bridge, which measures 2 points resistance output, compare them and take a decision what to do, that thing called ECU-electronic control unit(computer easy words).

Manifold Absolute Pressure sensor (MAP sensor)

MAP or manifold absolute pressure sensor.

It measures vacuum inside intake manifold and change output voltage in relation to vacuum.


Here is voltage reading of 1.78 volts at 15 in/Hg of vacuum.










On this graph i recorded different voltage readings according to different vacuum. It shows that the MAP sensor load line is straight and for every vacuum reading its a specified voltage. Inside MAP sensor is a silicon diaphragm that increase resistance when vacuum applied so sensor output voltage decreases. So at idle engine sucking more and vacuum is high, but when it revs up vacuum is getting lower, according to this readings ECU adds required amount of fuel need.

Throttle position sensor

TPS or throttle position sensor.

There are two types of TPS sensors, potentiometer (liner resistor) type and switch type.

Usually they can be identified by the shape, square or rectangular shape indicate onto switch type and round shape indicate onto potentiometer type.

Both throttle position sensors can be check with the multimeter, switch type is easy to check turn multimeter on resistor check, place one of the leads to common ground of the switch and another to next terminal which when its close one of the terminal will read around 0.4 ohms of resistance, showing that its in close position and car runs on idle, next it needs to be fully open to read resistance from other terminal and it will show that butterfly fully open and vehicle needs extra fuel as its accelerating. Third position is when its partly open and reads no resistance on both terminals that normal driving position when vehicle traveling on the same speed on highway. So switch type of throttle position sensor is quite simple as it only shows 3 positions of butterfly, and a bit useless for good fuel economy and emission control.



This graph explain 3 positions of throttle position switch.






Next type is a potentiometer type or liner resistor type. It also can be check with multimeter, but it has to be connected to 5 volts power supply. But to find a fault in broken resistor track better to use a oscilloscope equipment. With 5 volts supply voltage, sensor changes its output voltage with position of butterfly (throttle). It can be from almost 0 volts when throttle closed and up to 5 volts when its fully open, giving all the angles and position of butterfly so ECU can get more accurate reading.



This graph shows operation range of linear throttle sensor, where reading were taken at different throttle angles.

This sensor is more common to use this days as it gives more accurate reading for ECU.