Thermistor air, thermistor water & thermal fan switch.

 Thermistor air, or thermistor water are just the same name for 2 different purpose, one senses the temperature of intake air, one measures the heat of the coolant that flows in all the coolant jacket. These heats will affect the amount of voltage signal generated by these sensors, hence when sent to the ECU, it will know what temperature the air or engine is at.

  Temperature affects conductivity of materials, gradually though. Therefore the very basic principal of these thermistors are using materials that have conductivities susceptible to change of temperature. In terms of electrical, conductivity means resistance, and hence depending on different materials, resistance can rise or sink when temperature increases or decrease.

  In that case, we know have 2 well know types of thermistor: Positive Coefficient(POT) and Negative Coefficient Thermistor.
POT: Material resistance is proportional to rise of temperature & vice versa.
NOT: Material resistance is inversely proportional to rise of temperature & vice versa.

Because resistance change means voltage drop and available voltage in a fixed reference voltage circuit can be affected as well. Hence this opens up for some type of configuration of thermistor circuit. But generally, all these configurations are again, basically application of voltage divider law.
When searching for thermistor coefficients, there are 2: temperature/resistance and voltage output/resistance. In a circuit, voltage output can either be available voltage or voltage drop, as the two complement but in the same time contradict each other, therefore its definition should depend on situation.
For example:

A positive coefficient will have resistance rises as temperature rises, as resistance rises, the voltage drop across the thermistor proportionally rises as well. This configuration allows the signal to the ECU is equivalent to the voltage drop across the thermistor.

In reality, the temperature and resistance are not always proportional.

 However, this does not alter the signal going to the ECU as well as the thermistor as a whole. Because these are figures tested in boiling water environment. In real engine condition, the heat generated by combustion is ideally constant enough to keep a heating power on the coolant, thus raising the temperature rapidly. Average engine temperature varies from car to car, but hottest is always the exhaust manifold and coolant is counted as average engine temperature.

 
Coolant temperature sensor can usually be found as a plug on the side of the cylinder head, where a hole on the head is. There is 2 wires leading away one is a signal wire(red) the other one must be ground. The 5V feed is ECU internal. This makes testing the ECT simple, just by back probing the signal wire and ground the multimeter, start the engine and the temperature will change in accordance with specification, if not, you know what to do. Simplicity in design and function doesn't mean it's not important to check the ECT when the engine is starting cold or idling warm. Because, the signal makes the ECU "thinks" of how the engine is warm or cold. If the engine is warm and the ECT reads cold, the ECU will make the mixture rich and retard the spark timing, making a warm idle for your car, for example. This of course leads to bad CO and severe fuel consumption.

  As you notice, after a while, the car's fan turns on as it reaches a certain level of temperature, blowing all the really hot air in your face. That is certainly for "cooling the coolant". Because the engine temperature and the coolant's temperature balance out, the coolant can't really cool the engine so good anymore. Hence the fan will turn on to blow air temperature in-through the radiator to balance out the coolant's temperature. This whole function is packed into what we call a thermal fan switch. Sometime it is ECU controlled, most of the time it senses temperature by itself, which I think is more reliable because when other thermistor fails it will fail this thermal fan switch from the ECU.
   Thermal fan switch is kind of "backwards". The input is a mechanical change, and output is an electrical trigger. All it does is when temperature reaches a level which will eventually close the circuit of the fan's motor to battery. When the coolant decrease back to the ideal temperature, the temperature sensed @ the coolant switch material makes the contact retracted, or opened, in a few second the fan will turn off.
   Checking this switch is simple, heat it up and check for continuity, or available voltage.

Testing MAP sensor

 We can't be talking about Mass air flow without taking Manifold Absolute Pressure into account.
Why is it called "absolute pressure"? Well I guess it is because the intake air pressure in the manifold is essential as how much power we will get when the intake valve is opened. In other words, it is the pressure that is untouched by the atmosphere (only through the throttle body) that is ready in the intake manifold to be vacuumed in.

  Air, gas - in general, has three main physical properties that we are all common about: Volume, Temperature, Pressure - and they are the 3 musketeers who can't exist without others. When pressure increases the volume will be squeezed smaller; when temperature rises the gas will likely to expand, and when trapped in a confined space, the expanding gas will build up pressure - those are undeniable gas/air properties and based on this we must have a MAP sensor inside our intake manifold. 

  
 Basically, the MAP sensor does the same thing as a MAF sensor - measuring how much air coming in. You might thing the MAP is so much different from a Hot-wire MAF because it measures the volume of incoming air, and MAP is measuring pressure, how? The ECU can't feel the pressure, it's micro-processor is just a 1 dimensional being, the only thing that makes sense to it is "0" and "1". But, because we use the event of massive volume of air coming in or increased air pressure is in the manifold that we turn them to affect our sensors to create electrical signals, the ECU will now receive something different that it can process on. And, think about this: both MAP and MAF give the same type of analog, voltage signal to the ECU, the only difference is the way they operate. 

  The MAP sensor utilizes a semi-conductor material (it can be silicon) which will become pressurized under high pressure and will start conducting electricity. Using a 5V Reference from the ECU, the Voltage output fed to the ECU will change after the ability to conduct of the material, and it depends on the pressure it gets.

 (autoshop101.com)

So basically this is similar to a rheostat configuration, and a Vc in, Voltage signal as a divider's output, and an earth.

 How do we test a MAP sensor? Aftermarket MAP sensor means the ECU is modified, hence we won't be able to find it inside the intake manifold as we usually do. But normally, there are 2 methods that we can use to test the MAP is Off-car and On-car.

On-car test is simple: find the MAP, identify the wires(Vc; PIM; E2) and then back probe. As the ignition is turned on not the engine, we will be able to retrieve the MAP's maximum reading. This is because 1 bar of atmosphere is able to get inside the manifold through by pass air control, without encountering any vacuum being created because the intake valves are not moving, the cylinders are not moving at all. 
Other than that, try turn on the engine and the PIM reading will eventually drop down to minimum readings for idling. This is because maximum vacuum is created by the cylinders hence the absolute pressure inside the manifold is minimum. There are still atmospheric pressure blown in but not enough for the vacuum. 
As we open more throttle, more pressure hovers inside the manifold and if we maintain this pressure level we can still have maximum pressure applied to the MAP sensor.


Off-car is even simpler with the vacuum tube removed. Therefore the reading on the MAP is always maximum 1atm anyway. So if we block the hose by a vacuum tube and apply vacuum to it, we can see the voltage reading decreases, just like closing the throttle and let the engine idle.

 So overall, MAP sensor and MAF sensor can be either alternatives or complements, depends on the configuration of the engine. They both measure air by sending voltage signal telling the ECU of how much air coming in, just different by the way they utilize which properties of air. 
In which case, MAF & MAP sensor is no more useful when it comes into forced-induction engine, where air intake pressure and volume exceed way beyond what of a naturally-aspirated induction. A MAP sensor will change its way into a boost sensor or pressure gauge, measuring pressure which is much higher than atmospheric, and minimum pressure exceeds higher vacuum level. At that point, idling and wide open throttle needs to be monitored more powerfully differently.

TPS testing

In engine operations, there are 3 main phases: Off, Idle, Throttle. The throttle butterfly is an inevitable component that decides anywhere the engine is between Idle and Full-open throttle, and that control is mainly in the hand of the driver, and partially the ECU. 

Electronic engine management has become so importantly accurate, therefore the ECU knowing exactly how wide-open the throttle butterfly does is a MUST, so it can manage the Rest of the subsystem to operate in the satisfaction of this throttle position.

There are 2 main sides of the throttle body: Mechanical and Electrical. 
The mechanical part is usually a coiled/spring butterfly vane that is always coiled to be at closed position, and linked to the gas pedal. The throttle is inside the throttle body and is mounted right before all the intake valves of the cylinder head - for best accuracy and air efficiency.
Beside the throttle butterfly, when idling @ closed position, there needs to be an auxiliary air passage called by-pass air, this will be addressed in bi-pass air control system.

There are 2-types of electrical insides the throttle position sensors, based on what type of ECU-feeding signals they give: analog and digital, hence hereby we have Linear(analog) TPS and Switch-type TPS(digital)

    In linear: The electrical part consists of where the mechanism of the butterfly is linked to the variable resistor part of the position sensor circuitry. The variable resistor works on the same principal as the Vane type Mass airflow meter, as a voltage divider where the output signal that simulates the throttle position feeding to the ECU is variable along the side of the rheostat. 
Here is an example:

 (autoshop101.com - Toyota)

In expansion, there are many others modification can be done to the TPS circuit. For example, because @ Idle, the engine needs minimum fuel, therefore a fuel-cut switch can be integrated with the wider slider of the rheostat when it is at closed position so it pushes the fuel switch open - similar to the fuel switch of the vane airflow meter. From this, We can see that the Mass airflow meter and the throttle position sensor are kind of alternatively functional, since the kind of signal when mass air flows through the sensor as well as the signal when the throttle is open are proportionally, and sequentially related. In other word, the ECU can't receive a 4.5V signal of mass air without 4.5V signal from the wide opened TPS, and even if, this has got to be faulty.

Testing an analog TPS: 

We can see that this potentiometer is capable of delivering variable signals. As we move the slider at any various position, we get different voltage output signal from VTA. For all the test procedure to be successful, it is important that we determine the location of the potentiometer on the throttle body - in this case it is the black box on the left of the throttle body as in the pictures. After obtaining the part numbers and determining the TPS type, we have to get the correct wiring diagram to show us what the 4 pins with wires coming out are. This is a fairly simple circuitry to we can generally break them down as this standard wiring diagram, though different manufacturer have different layouts.

 The diagram shows the fuel cut-off switch for IDLE position as the Slider closes it will push the IDL1 terminal closes, this should be an alternative or complement to the fuel pump switch.
Similar principal to the Vane MAF meter, we can also have a resistor check. As we put the Ohms meter between Vc and E2 and move the slider, we will experience a constant resistance, because we check the circuit from Vc, through the whole rheostat NOT the VTA terminal.
Between VTA and E2, we can see that the resistance changes decreasingly from large, as we open the slider. This means that the resistance is largest @ closed position so we get minimum voltage signal, and smallest @ wide open so we can have large (not exceed 5V) voltage signal. Remember this rheostat and VTA is just like a voltage divider, and resistance is inversely proportional to Output voltage signal. Therefore this explains the result obtained:

For the IDLE switch, @ closed position we will have continuity between VTA and IDL1, as we slightly move the slider away that connection disappears.





Switch type signal: 

There are several reasons for the Throttle Position Switch to be developed along side as a complement for the analog signal, and yet they are quite important. The IDLE fuel cut-off switch control mentioned above on the analog TPS is an important part of Switching TPS function as a whole, and yet it is one of the reasons. What about @ full throttle? What would we want the engine to run? We want full-power, and yet all the fuel pump switch is on, ignition timing is advanced and injector frequency get higher - another reason for this switch type. Those 2 are the main visible reason. But the truth is in order for the ECU to recognize clearly and reliably fuel cut off control and ignition timing corrections, the feed should be the solid and reliable digital signal, not the precise varying analog signal.
Here is a casual layout for any switch typre TPS: 

 

The output signal doesn't care how many degree the throttle is at, it only cares when the slider is in a certain range where either the IDLE switch or the PSW switch is STILL in contact, in this case 0-1.5 degree and 70-wide open, anywhere in between neither the IDLE fuel cut off nor the Full throttle fuel pump is activated. 

Therefore, continuity check is the only thing that maters since there are normally only 3-4 pins coming out and we know is PSW, Common(E), IDLE and maybe a 5V Vc feed.

What happen when the slider is @:

 IDLE: this is where we do nothing to the slider, it's already @ IDLE position. So 5V from ECU is grounded, therefore all the voltage drop will be across a resistor inside, hence we will have near zero volt coming out. There is continuity and the resistance over this is readable(1k Ohms)

Anywhere in between: We tilt the slider, and our V meter reads "5V", @ both PSW and IDLE we get 5V signal this means the 5V circuit is not completed, the resistor will not consume any of the available voltage hence we have 5V out of the pins.

PSW: similar to IDLE when the slide is in range.

For the record, position sensor is the biggest and most important "Commander" for the ECU while it's acting like a CEO. Position sensors represents the driver's demand of operating modes/loads for the engine, so it is very important that the ECU receive both analog and digital signals.

Testing Vane Airflow Sensor

This is the component normally mounted between the throttle body and the end of the intake manifold passages. It's purpose is to have a configuration where we can set a sensor up that will react with what's happening through the whole air intake tube to feed the ECU with important information about how to run the engine. 

No argument, ECU is a computer, so the signal that this particular type of MASS AIRFLOW SENSOR or ANY type of M.A.F give has to be in Voltage signal. It's called VANE or FLAP MAS because the basic concept is: aerodynamic/mechanical reactions of the flap/vane according to how big the flow of air is, will be turned into voltage signal feeding to the ECU by a mechanical link between flap movement and a variable resistor in series with the line where Vsignal is coming out. So if we have a variable resistor, in a configuration where voltage divider applies, it will control the outcome voltage, not necessarily Ohm's Law this time!

There is a small circuit inside this little device, well every sensor needs their cute little circuitry obviously. In this particular Vane MAFS, you will see there are 7 electrical pins coming out of the plug of where the square little black plastic box sealed on the flap body. Those 7 pins represent the relationship of this circuit with "the rest of the car's circuit". Well of course in this case "the rest" is referred to ECU because this sensor seems to be simple enough that it only needs to answer to the ECU.
So this circuit is not complete, but it will run by itself like any other circuit if we just supply a little bit of voltage through the right pin. And NOT every part of any of these circuitry that appears inside the little box.

There are 3 main parts represented by those 7 pins: The thermistor circuit; The Main vane voltage output signal circuit, and the Fuel pump switch circuit. Yet they seem physically connected but their operations in terms of circuitry are separate. 

The thermistor circuit, convenient to locate close to the signal output circuit, because when we built a sensor to measure how big the volume of air flowing through per second, we thing why not include air temperature measurement too: Temperature-Volume-Pressure are inseparable when talking about air/gas.
So in the wiring diagram, we can see that this circuit only needs an Earth for itself (E2), and everything comes out is temperature, because there is another sub-materialized circuitry inside the little nob that sticks out inside the air passage as well. Temperature will have geometric and volumetric effects on the material, and hence turn into a mechanical effect- and finally electrical signal is outcome. 

Forget the fuel pump switch and the T.H.A, the Mass air flow signal is Vs; Vb; Vc; E2. Where Vs is supply of 12V, Vc is input signal from ECU of 3-5V; and Vs is output signal that will NEVER/SHOULDN'T be greater than the ECU's input signal. But like I said, this circuit can run fine with only 5V to Vc. We can see that 5V from Vc can easily make it to earth, with a variable resistor in it's way which will act like 2 resistors with it's adjustable needle pointing out for the signal output. Which acts kind of like a voltage divider circuit with 2 resistors and an output leads away between those 2 resistors (Vs). 
This variable resistor is configured to be Maximum at Vane CLOSED position, so when Vc of 5V comes through the rheostat to Vs we will have minimum signal, because most of the voltage drop is for the BIG selected resistant part on the rheostat, only a little comes out for example 1.26V. 
As the vane opens wider(got pushed in by more air) the rheostat becomes smaller, hence Vs leads a bigger output signal, just like how a voltage divider works as we change the value of its resistors. And of course, we can't have a voltage divider where all the supply comes out the output terminal while none goes to the following resistor to earth, that would make NO SENSE of a voltage divider anymore. That's the reason why the signal must not be or be bigger than the ECU input. But, when you have Vb hooked with the 12V+ battery, 5.08V(actual @ max open position) might as well be what you have.
Current is not a concern in this type of circuit, where only Voltage and Resistance matter. So we can test the operation of this VANE MAFS by 2 ways through resistance meter check and available voltage check. 

Put the black lead on the E2, @ Vs varied between 80-900 Ohms in total and this is fairly between specification. This only tells me that the rheostat is in the voltage divider range in order to have a desired voltage signal. E2-Vb & E2-Vc both possess the same amount of resistance ,this tells me that the maximum resistance of the rheostat and the Vb resistor are similar.

Vane angle - Voltage out relationship
It behave seemingly like exponential. But as Ohm's law applies: V=IxR, this relationship should be linear and directly proportional because vane angle represents proportions of the rheostat being selected.

Fuel pump switch: a smart mechanical link between the rheostat's needle @ zero angle position and the fuel pump switch. 

 So the fuel pump switch is actually a spring that is set to be closed, then the rhestat reset spring is coiled oppositely pushing on the Fc switch when it is at zero position. When the flap is moved, the fuel pump switch closed and that coordinates to give more fuel when more air comes in.

Disadvantages: 
There are several types of air flow device besides this vane they are Hotwire Mass air flow sensor, Karman Vortex air flow sensor. And the most common is hotwire mass airflow sensor. The reason this Vane device is not called "sensor" because it is actually a meter, which convert analog mechanical movement into electrical signal. Other sensors convert heat energy into voltage signal, or photographic changes into voltage signal, which are more likely to be called "sensor" than "meter".
The Vane type mass airflow meter consists of a moving mechanical flap, hence the weight and recoil force of the spring setup exert on the flap can  restrict the flow of air, hence restrict performance.
In consistent and frequent operation of the intake air, a complex of mechanical components will eventually worn out, and dirt deposits on components. Which may lead to hard starting when the engine needs the vane to actually opens up a little bit when throttle is not depressed.
Beside, the spring composition can be out of service too, which will terminate all the voltage output function of this vane type mass airflow meter.
Comparing to the Hotwire, the vane MAF takes more room and weights much more heavier than a tube of plastic with some wire in it, hence replacing a vane with a hotwire is a much better choice. In fact, most cars use hotwire MAF sensor.

-Large & heavy
-Performance restricting=> lessen fuel economy
-Not as reliable as the hotwire MAFS
-Not as accurate as the hotwire MAFS