Post one: Sensors

For the past two weeks we looked at the sensors that we have in a car today. We looked at how these sensors work. What do they do in relation to the operation of the car, and performed tests on them to see it they meet the manufacturer's specification.

Throttle Position Sensor (TPS)
A TPS sensor is an important sensor in a car as it tells the ECU how wide open or shut the throttle is. The TPS is mounted on the throttle body (in series with the throttle body valve). It converts the throttle valve angle into an electrical signal. This electrical signal is then sent to the ECU, indicating the throttle position. The signal sent to the ECU is in DC (direct current) volts. The ECU then uses this information to alter the air/fuel mixture. The more the throttle is opened, the more air is coming into the engine. To burn up all this air efficiently more fuel is required. This is why the ECU needs to know the throttle valve angle so that it can compensate for the air that is coming into the engine. The TPS sensor works on a 0 to 5 volt scale. When the throttle valve angel is low, say around 10 degrees. The voltage sent to the ECU is also low, say around 1.5 volts. The ECU will then look at this voltage and know that throttle is barely open and therefore it can then adjust the air/fuel mixture. The same applies when the throttle valve angle is very high, say around 75 degrees. The voltage now being sent to the ECU will also increase, say to around 4 volts. The ECU will look at this voltage and know that the throttle is now wide open and hence it will adjust its air/fuel mixture by adding more fuel.

Now how does the TPS convert the throttle valve angle into a voltage reading? Well this is very simple. Inside the TPS there are two major components; a resistor and a wiper arm. Imagine a simple series circuit with a 5 volt power supply and a resister. This is pretty much the what the TPS is. The wiper arm is in contact with this resistor at all times. Now this wiper arm is mechanically connected with the throttle valve, and as the valve moves so does the wiper arm. As the wiper arm moves on the resistor, the signal voltage output changes. At the point of contact the signal voltage that we are getting is actually the available voltage. This voltage then indicates the position of the throttle valve.

There are several types of TPS out there in the industry. But the one I was able to test and have a look at was the switching type TPS, or a Throttle Position Switch. This TPS was very strange to me as it would only tell the ECU the throttle is at idle or the throttle is fully open. It would not tell the ECU the throttle position in between those two points. When a car is using a throttle position switch, the ECU uses the MAF (Mass Air Flow) sensor to determine the amount of air coming into the engine. the idle position signal is mainly used for fuel cut-off control and ignition timing corrections. The full throttle signal is used to increasing the amount of fuel being injected.

The conditions of the contacts inside the switch was our first test. We checked for high resistance in both the idle and the full throttle positions. The reading we got when checked for resistance at idle was 0.1 ohms. This was a good result as it shows us that the contacts are in good condition and it also shows us continuity. The reading we got when checked for resistance at full throttle was 0.2 ohms. This was also a good result as it showed us that the contacts are in good condition and also showed us continuity. These readings were done using a multimeter (set on ohms). The E terminal was used as ground (black lead of the meter) and then the red lead of the meter was placed on the other two contacts to get the readings. The internal resistance of the meter was 0.2 ohms and this was subtracted from the resistance readings.

How does this throttle position switch work? 
Well its simple there are three contacts inside the throttle position switch. The middle contact is the one that moves between the two points. At idle when the throttle valve is at an angle of about 1.5 degrees; the middle contact is connected with the idle contact (bottom contact). This will then send the ECU a voltage indicating that the car is at idle. Once the throttle is being used and the car is no longer at idle the middle contact is not connected with either of the other two contacts. As a result no voltage is being sent out to the ECU. When the throttle valve angle exceeds 70 degrees the middle contact connects with the full throttle contact (top one). This will then send another voltage to the ECU indicating that the throttle is wide open and that more fuel is required.

Manifold Absolute Pressure (MAP) sensor       
A MAP sensor measures the vacuum inside an intake manifold. The MAP sensor then sends out a voltage to the ECU indicating the pressure inside the intake manifold. The higher the vacuum inside the manifold, the lower the voltage output. MAP sensors usually work on a DC (direct current) voltage scale of 0.5 to 4 volts. The MAP sensor voltage output is highest when the car is in key ON, engine OFF position. This is when the intake manifold pressure is the highest. The MAP sensor voltage output is the lowest when the car is o deceleration with the throttle closed. This is when the intake manifold pressure is very low (high vacuum). Note this theory only applies to all naturally aspirated cars (NA) and not turbocharged cars. The MAP sensor is important because the intake manifold pressure has a direct relation ship with engine load. The ECU needs to know the intake manifold pressure to calculate how much fuel is going to be required. The higher the pressure inside the intake manifold the higher the voltage being put out by the MAP sensor; and as a result of this more fuel is required by the engine.

Example
When a car is accelerating it draws in more air. This causes the pressure inside the intake manifold to rise. Higher intake manifold pressure results in a lower vacuum. The MAP sensor then measures the intake manifold pressure and sends out a voltage to the ECU. This voltage can be 3.5volts. This is a high voltage output by the MAP sensor, hence indicating high pressure inside the intake manifold. The ECU then looks at this voltage reading and knows that the engine in under acceleration and that more air is now coming into the engine. The ECU will then supply the engine with more fuel to compensate for the more air. As a result of this engine power increases and the car experiences a smooth acceleration.   

How does a MAP sensor work?
Well this is simple. Inside a MAP sensor there is a silicon chip. The silicon chip is mounted inside a reference chamber. As a result one side of this silicon chip faces the inside of the reference chamber and the other side faces the inside of the intake manifold. In side the reference chamber there is either a perfect vacuum or a calibrated pressure. This vacuum or pressure inside the reference chamber always stays the same. It never changes. The reference chamber is a fully sealed unit. As the pressure inside the intake manifold starts to change, the silicon chip starts to flex. (The silicon chip flexes with the changes in pressure of the intake manifold). As the chip start to flex the electrical resistance of the chip starts to change. This change in the chip's resistance alters the voltage output of the MAP sensor. The ECU then see's this change in the voltage output and knows that there is a change in the pressure inside the intake manifold.

As you can tell a MAP sensor is quite important, but what if it was faulty? Say you had a MAP sensor and it is faulty and you wanted to accelerate. What would happen? As we know when a car is accelerating more air is being drawn into the engine. This causes the pressure in the intake manifold to go up. A faulty MAP sensor might not pick this up and even under acceleration put out a voltage of 1.5volts. This voltage indicates that the pressure inside the manifold is not very high. The ECU will then take this faulty reading into consideration and not supply the engine with the proper amount of fuel. Now you got more air coming in but very little amounts of fuel coming in. This will cause the car to hesitate and will decrease engine performance. The car can even stall because of this.


Vane Air Flow Meter (AFM)
The Vane Air Flow meter provides the ECU with an accurate measure of the load placed on the engine by measuring the air intake volume. It also measures the intake air temperature as well. A vane air flow meter is an old design air flow meter. The vane air flow meter works in a very similar way to the TPS sensor. The vane air flow meter has a flap which is pushed opened by the air that is coming in. The amount that this flap is opened determines the amount of air that is coming into the intake system. A wiper arm is mechanically connected to the flap. When the intake air pressure increases it causes the flap to move and open more. This movement of the flap will also cause the wiper arm to as, as these two are mechanically connected. The wiper arm just like in a TPS sensor is always in contact with a resister. As the wiper arm moves across the resister the voltage output changes. The vane air flow meter works on a DC (direct current) voltage scale of 0 - 5 volts. When the vane is shut the voltage output is high indicating that minimal air is coming into the engine. A lower voltage output will indicate a lot of air coming into the engine. 

I chance to look at one of these vane type air flow meters. I had no idea how they worked as i have not seen them on a car before. I was curious as to how does it work. I then looked at the wiring diagram of it and saw that it had a wiper arm in contact with a resistor just like a TPS sensor. I then had some idea of how it worked. I then saw another vane type air flow meter similar to the one I was going to test. But this one had the top cover taken off and you could see the wiper arm move. When i moved the flap open i noticed that it was also moving the wiper arm. I then knew that the flap and the wiper arm were mechanically connected just like in a TPS sensor. I then went back to my vane air flow meter and did voltage out readings. A multimeter was used for this. I first had to hook it up to a power source to get the 5 volt voltage supply. After words I opened the flap little by little and took down the voltage outputs. At 0% open the voltage reading was 3.89volts. At 20% open the voltage reading was 3.10volts. At 40% open the voltage reading was 2.93volts. At 60% open the voltage reading was 1.90volts. At 80% open the voltage reading was 1.18volts, and at 100% open the voltage reading was 0.35volts. 

These reading clearly show us the relationship between the amount of air pressure coming into the intake system and the voltage outputs given by the vane air flow meter to the ECU. The higher the air pressure coming into the intake system the lower the voltage signal sent to the ECU. And the lower the air pressure coming into the intake system the higher the voltage signal sent to the ECU. The ECU will then use this voltage readings to determine how much load is the engine under, and to decide whether it needs to add more fuel or lower the amount of fuel supplied. As mentioned earlier the vane air flow meter also has a IAT sensor (Intake Air Sensor). What this does is that it measures the temperature of the incoming air and sends a voltage signal to the ECU. I will talk about this later on in my blog. 

The vane sir flow meter also has a By-Pass Passage. This passage is used to channel the air through the meter when the car is on idle and the flap is closed. The amount of air coming into the engine via this passage can also be adjusted by using a Idle Mixture Adjusting Screw. This screw is located at the very end of the meter. 

Because the vane air flow meter has a mechanical flap that measures the flow of air into the intake system; it runs a good risk of going wrong. As we know all mechanical objects have a life span and it is only a matter of time before they go wrong. Its the same story with the vane air flow meter. As i have learned they have a tendency of going wrong. For example over time dust and dirt particles can develop around the flap area. This can cause resistance to the flap movement. This means that when the car is under acceleration, the high air flow into the car wont be able to open the flap fully or mite take more time to open the flap. This can potentially cause restriction to the air flow into the vehicle. As a result of this the car will experience lack of power, and can under perform under acceleration. 




Mass Air Flow (MAF) Sensor 
A Mass Air Flow sensor measures the amount of air being drawn into the engine. It is located directly in the intake system, between the air filter and the throttle body. As all the other sensors mentioned above the MAF sensor works on a DC (direct current) voltage scale. It sends out a voltage signal to the ECU depending on the amount of air being drawn into the engine. The ECU then uses this information to calculate the engine load. This is important to determine how much fuel needs to be injected. Unfortunately i wasn't able to do tests on a MAF sensor in class, but I have been doing some research on it to know how it actually works.

The Hot Wire MAF sensor now is the most common sensor used today to measure the air intake volume. It consists of three primary components. A platinum hot wire, a thermistor and a electronic control circuit. The thermistor measures the temperature of the incoming air coming into the engine. The platinum hot wire and the thermistor are exposed to the incoming air. Now the platinum hot wire is kept at a constant temperature in relation to the thermistor by the electronic control circuit. As the car accelerates and the engine begins to draw in more air. This will cause an increase in the air flow. This increase in the air flow will cause the hot wire to lose heat. When the hot wire begins to lose heat the electronic control unit will compensate it by sending more current through the wire. The electronic control circuit then measures the increase in current flow and put out a voltage signal (in proportion to the current flow). This voltage signal is then sent to the ECU. The ECU looks at the increase in the voltage signal and knows that more air is now coming into the engine. As a result more fuel is required and injected into the cylinders. A simple way to look at this is the more the air coming into the engine the higher the voltage signal put out by the MAF sensor. This high voltage will then indicate to the ECU that more air is coming into the engine. The ECU will then compensate this by injecting more fuel.

Now a MAF sensor is very sensitive to dust particles, and because of this it needs to operate in a clean intake system for it to work properly. Otherwise over time the dust particles can stick to the hot wire and cause the MAF sensor to send out inaccurate voltage signals. I had an issue with the MAF sensor in my car. What had happened was that over time the dust particles in the air had started sticking to the platinum hot wire. This started to then started to block the surface area of the hot wire which is exposed to the incoming air. This meant that less air was being detected by the MAF sensor. This had a bad effect on the performance of the my car. At idle my car kept on jumping between 600revs and 700revs. Over time this changed to between 500revs and 1000revs. The car just wouldn't idle anymore. It even stared to stall. Even when I pressed the throttle the car would hesitate first before moving, but once it was moving it seemed fine. Now what had happened was at idle my MAF sensor wasn't detecting the proper amount of air that was being drawn into the engine and as a result a lower voltage signal was sent out. The lower voltage indicated to the ECU that not a lot of air is coming into the engine and as a result the ECU wasn't injecting the proper amount of fuel needed by the engine. This caused my car to idle rough. Once the car was moving the ECU took in consideration my other sensors too like the TPS and the oxygen sensor and that is why it ran fine. The bad MAF sensor also caused my car to be low on power as the ECU never really knew how much air was being drawn into the engine. 


Thermistors (IAT, ECT)
There are many different thermistors in a vehicle. Even though they all do different tasks they all work in a very similar way. They also send out a DC (direct current) voltage signal to the ECU, which then determines the temperature. As the temperature of the sensor heats up, the voltage signal that it puts out decreases. The decrease in the voltage signal is caused by the decrease in the resistance.

Think of the thermistor as a voltage divider circuit, with a fixed value resistor and a variable resistor (which is the sensor). The voltage signal out is placed between the two resistors (therefore measuring available voltage). Now the voltage drop over the fixed resistor will be determined by the resistance of the variable resistor (sensor). When the temperature is cold so is the variable resistor (sensor). When the sensor is cold its resistance is very high. Now since the variable resistor has a much higher resistance then the fixed resistor, the voltage drop over the fixed resistor will be minimal. Therefore the voltage signal (available voltage) will be high. This high voltage indicates to the ECU that the thermistor is sensing cold temperatures. Now as the temperature stars to heat up, so does the temperature on the variable resistor (sensor). This increase in temperature will cause the resistance of the variable resistor to drop, causing a higher voltage drop over the fixed resistor. This higher voltage drop over the fixed resistor will cause the available voltage (voltage signal) reading to also drop. This drop in the voltage will then indicate to the ECU that the thermistor is sensing warmer temperatures. 

 Intake Air Temperature (IAT) Sensor
The intake air temperature sensor detects the temperature of the incoming air. The ECU needs to know this information to determine how much fuel is going to be required by the engine. Cold air is more dense, meaning the particles are more closer together. This means that in the same amount of space we have more air particles. More air particles will require more fuel particles to burn efficiently. This is why when the IAT senses cold air coming into the engine, the ECU will inject more fuel. Now on the other hand warmer air is less dense. The particles are more spread apart. As a result of this in the same amount of space you now have less air particles. This means you now require less fuel to burn them off efficiently. When the IAT senses warm air coming into the engine, the ECU injects less amount of fuel. 

Engine Coolant Temperature (ECT) Sensor
The engine coolant temperature sensor measures the engine coolant temperature. The ECU uses this information to know the average temperature of the engine. This knowledge of the engine temperature is important for the ECU to know. Example: As we know that fuel in a combustion chamber only explodes and burns when it in vapor form. Liquid fuel will not cause combustion. For the fuel to remain into vapor form the engine temperature needs to be warn. When the engine is cold more amounts of fuel is injected because the ECU knows that some of that fuel will condense and turn into liquid form. Now is the ECU wasn't able to tell the engine temperature, on a cold morning the car would hesitate to start and would idle rough when cold. This is because the ECU won't inject more amounts fuel when the car is cold. This is why is is important for the ECU to know engine temperature.