SENSORS

TPS(Throttle Position Sensor/Switch)

This is a potentiometer type throttle position sensor, it works by having a variable resistor connected to the throttle butterfly so that as the throttle opens/closes the resistor changes. By running a current through the resistor the CPU can use the output voltage to work out what the position is.

below is a chart of the voltages put out by a throttle sensor with a 5V supply

closed     3.4

1/4          2.2

1/2          1.7

3/4          1.4

full           1.1

This is an example of what the output looks like when connected to a running engine. it starts at a closed position and the high point is full throttle as you can see it works in the opposite way of the one I tested in the lab. 

When I tested it on vehicle I checked the ground which was(from the earth wire to a known ground) and it was only 14mV meaning it was a good earth if it was bad it would have a voltage above 100mV if it was this the voltage that the TPS would be returning to the ECU would be low and it would regester the throttle as being more closed than it actually is. Also by looking at the oscilloscope you can see the lines going up and down are rather even meaning there are no sudden jumps or flat spots so we know the potentiometer is also working correctly.

Above is a throttle position switch, it has two switches in it, they will be in one of three positions, idle closed and full throttle open when in idle position, both open when in between idle and full, and full closed and idle open when at full throttle.

Above is the oscilloscope pattern is the throttle position switch that was built in to the TPS as you can see it is open at idle (high voltage) and when the throttle opens it opens and the voltage drop goes to almost zero.  The problems that may occur are if the switch does not close properly the voltage will not go all the way down and you will get a similar effect if the earth is faulty.

MAP (Mas Air Pressure)

MAP test video

Above is a link to my lab test on a MAP sensor as you can see the voltage increases as the pressure increases. What is happening is there is a 5V supply and the MAP sensor works as a resistor as the pressure in the manifold decreases so does the resistance so the output signal increases. You can see an example of how it works on the below on car oscilloscope pattern. as I rev the engine the pressure decreases and the voltage increases.

At idle the MAP sensor registered 3.67V when the engine was not running and 1.41V at idle, which shows there is a vacuum at idle, and then when revved it peaked at 3.4V showing a pressure increase that the ECU interprets as load. Problems that can occur are vacuum leaks making the pressure too high(will send too hight signal will run lean) a short to earth before or after the sensor would cause it to send a signal too low and if there is a malfunction with the unit itself it may just register one signal that may be too high or too low. If the ECU  received these wrong signals the engine will run incorrectly or not at all because the ECU uses the signal from the MAP sensor to work out load, if it was unable to work out load it wont be able to accurately compute the injector timing and will run lean or rich.

MAF (Mass Air Flow)

Filiment type

This is the MAF sensor I tested in the lab the output voltage when just hooked up was 0.466V but it increased as air was passed through it. The way it works is it has a wire filament in it that as the air flows over it the resistance decreases so the more air flowing over it the higher the voltage it will put out.

Vane type

This is a diagram of a vane type air flow meter. it works by having a flap in the air intake that is pulled open as the engine sucks in more air. When it is opened the input signal is run through a different point on a variable resistor. It has two output signals(so the ECU can reference the two and know if there is a malfunction) so when the MAF is closed (low air flow) one output signal will be high on one output and low on the other and vice versa when the MAF is opened. in this case the resistance between 1 and 2 were 245-50 Ohms (closed-open) and between 1 and 3 were 98-294 

ECT (Engine Coolant Temperature)

Thermistor 

Here I am testing a thermistor it works by as it heats up the resistance decreases and in earlier cars this would go to your temprature gauge but in later cars it also goes to the ECU that uses it as a reference to know the water temp. Below are the readings I got as I heated one in water.

34°   1898Ώ
40°   1591Ώ
50°   1054Ώ
60°   808Ώ
70°   542Ώ
80°   395Ώ
90°   249Ώ
93°   204Ώ
I would of liked to go to a higher temperature but cant as a vehicles cooling system is under pressure so has a higher boiling point.

Fan switch

Like a thermistor the fan switch works on the water temperature but instead of being a variable resistance it will either have an almost infinite resistance when cold and will switch to almost no resistance at a predetermined temperature(or opposite depending on manufacturer) this is most commonly used for switching on radiator fans.

IAT (Intake Air Temperature)

The IAT sensor works in the same way as the coolant Thermistor but is measuring air temperature not coolant.

Oxygen Sensor

Above is an O2 sensor, O2 sensors are located on either the exhaust manifold or further down the exhaust. This is a zirconia sensor and has three wires two white(one being earth the other is the signal out) and the other is a wire that the car can send a voltage down to heat up the O2 sensor as it needs to be at about 316 °C to operate properly. Other O2 sensors may have a fourth wire that is aldso for the heater but this one uses the exhaust as an earth.

above is the sensor once warmed up with the engine running at about 2500RPM. The low point on the scope is when the exhaust gas has a large amount of oxygen in it and the high point is when it is low of oxygen. It works by creating a voltage when there is oxygen present in the outside atmosphere and none in the exhaust gas, the voltage can be anywhere up to one volt and averages about .5 volt and cycles about once a second. It is cycling to try and achieve lambda which is a fuel to air ratio of about 14.7:1 Air:fuel. and also cycles to help activate chambers in the catalytic converter.

Above is the O2 sensor when the engine is at idle (800 RPM) as you can see the voltage is slightly higher and is cycling slightly slower (about once every two seconds) this is because at a lower RPM the changes made by the engines ECU take a little longer to register and so the changes are made less often.

Above you can see what happens if the engine runs rich, due to the lack of oxygen in the exhaust gas the sensor will produce a voltage that stays high instead of cycling.

Above I was trying to get the engine to run lean but it would not, what it should do is at the point just after the horizontal line should dip below the normal low voltage but the ECU picked it up too quick and went back to the normal cycle, this tells me it is able to react whiten about a second.

The way a zirconia oxygen sensor works is it has a membrane of zirconia on the end of a probe mounted in the exhaust, inside the probe is air from the atmosphere that will contain a normal amount of oxygen. if there is a difference in oxygen levels between the inside and outside the zirconia produces a voltage, the more the difference the more the voltage, this is then read by the ECU that can tell how much oxygen there is in the exhaust gas by the amount of voltage. 

Speed or Crank Position Sensor Inductive

above is the oscilloscope pattern for an magnetic pickup type speed sensor. It works by having a magnetic field that as one of the reluctive tip moves towards the magnetic field it creates a positive current and as it moves away a negitive voltage if you look at the pattern the straight like is when the tip is aligned and the voltage is alternating from the positive to the negative, this is the point that the ECU reads and the amount per second (Hertz) that this is happening will determine how fast the engine is rotating.



Hall Effect

A hall effect sensor works by having a magnetic field and a chip that has a current running through it in one direction. When a magnetic field is present the chip produces a current at right angles to the current being run through it. Then there is a cutter plate that is used to interrupt the signal that will cut off the current so what you get is the above digital signal.


Optical Sensor

An optical sensor works by having a diode that produces light directed at another light sensing diode that switches open when it senses light, it uses this to earth a signal voltage so that when the diode is closed the signal is unearthed so at about the 5V signal then when the diode is open it is about 0V. It uses a rotating disk with slits cut in it to let the light come in at certain points to let the light from the producing diode to the sensing.