Wednesday, 14 September 2011

Engine Electronic Control Systems (Off Car)

The task to be done was to check all the different sensors on a modern day engine to make sure they work correctly and so that it is known how to test and diagnose different sensors when they fail.

TPS Sensors

The first sensor to be tested was the Throttle Position Sensor/Switch (TPS), there are two different types of TPS there is the more commonly used today TPS which is a potentiometer and has variable resistor in it to send different voltages to the ECU. The older type of TPS is the switch type, this type has two or three contacts that send a voltage at idle and at full throttle, in the case of the one we tested it was a two contact Throttle Position Switch. The TPS does not measure how much air is going into the engine but rather how open the throttle is, and the ECU uses this to estimate how much air is going into the engine and how much fuel is required to be injected. The TPS is always located on the opposite side of the throttle control since the TPS measures throttle angle.

SWITCH TYPE TPS

The Throttle Position Switch has two references an idle reference and a full throttle reference, and the central arm that moves between the IDL(idle) and PSW/WOT(Power Switch or full throttle) grounds one circuit that will send a reference voltage to the ECU (this reference voltage varies for different manufacturers)

The central arm is moved by using grooves in the TPS to guide the contacts to create a circuit and send the correct voltage to the ECU so that the ECU knows whether it is on idle or full throttle, as is seen above, the idle switch is mostly used for fuel cut-off and correcting the ignition timing advancements so the engine still runs smoothly. The full throttle reference is used for increasing the volume of fuel that is injected into the engine to give the engine greater power when its needed, there is however no reference voltage being sent to the ECU when the throttle is partly open so a Throttle Position Switch always works in conjunction with a Manifold Absolute Pressure (MAP) sensor so the ECU knows how much air is going into the engine when the engine is off idle but still below full throttle, the MAP sensor lets the ECU know the in between of idle and full throttle.

Faults

If the throttle position switch was to become faulty lets say the idle contact has corroded and there is no longer an idle reference, the engine would begin to run rough at idle as the ECU is not getting that reference voltage from the TPS telling it that the engine is idling so the ECU believes that the engine is running at above idle. The engine will run rough as the ignition timing would be out and the ECU could be firing the plugs to soon causing combustion to take place to soon causing the engine to knock and the ECU would be running the engine rich at idle as well since it believes that there is more air coming into the engine than what there actually is.

POTENTIOMETER TPS

The most commonly used Throttle Position Sensor in modern engines is the potentiometer TPS, this sensor has a variable resistor in which a slide contact that moves with the butterfly valve moves up and down the resistor creating a larger or smaller voltage drop as more or less of the resistor is used. As the throttle position is increased or decreased the voltage output voltage to the ECU from the TPS is increased or decreased in conjunction with the position of the butterfly valve. This voltage reference is used so that the ECU can estimate how much air is going into the engine and therefore determine how much fuel is injected into the combustion chamber. The Throttle Position Sensor works in conjunction with MAP sensors, or Mass Air Flow (MAF) sensors or air flow meters.

The variable resistor and sliding contact set-up can be seen above on the wiring diagram.
Usually the lowest voltage output from the TPS is the idle position and the highest voltage output is full throttle. 

RESULTS
The type of TPS that we got was the switch type Throttle Position Switch, to test whether the TPS was in good condition, continuity was tested to make sure that contact was being made. This was done using a multimeter that is set to ohm's and by placing the earth probe on the earth terminal of the TPS, then placing the red probe on the IDL(idle) terminal. This should give a result and the result that we got was 0.1 ohms, this is a good result as it shows that a connection is being made with the Idle switch and the resistance is very low so the ECU is getting a good reference voltage, when the ECU receives a voltage from the Idle terminal from the TPS, the ECU knows that the engine is on idle and the ECU adjusts the ignition timing and the quantity of fuel injected so that the engine runs smoothly whilst idling. Then when the throttle is opened the multimeter should read O.L or open circuit which it did for our TPS this is a good result as it shows that there is no longer a voltage being sent to the ECU so it knows that the engine is not on Idle. 
Now leaving the black probe of the multimeter on the earth terminal and placing the red probe on the PSW (power switch) terminal and leaving the throttle in the closed position the multimeter should read O.L, which it did for our TPS, this is a good result as the TPS is not meant to make a connection at this point. When the throttle is opened up to 70 degrees or more or when the throttle is basically fully open, the multimeter should get a reading our TPS gave a reading of 0.2 ohms this is a good reading as it shows that contact is being made at the right time and that resistance is very low so the ECU is getting a good reference voltage. When the ECU receives a voltage from the PSW terminal it knows that the driver wants power and acceleration so the ECU will inject more fuel into the combustion chamber and adjust the ignition timing so that the engine gets the most power.
Faults

If the potentiometer type throttle position sensor had a fault, for example there was a break in the earth, this would mean that the ECU would always get a reference voltage of 5volts(if this is the power supply) from the TPS and this could cause the engine to run rough as the ECU thinks there is a lot of air  going into the engine and so it injects more fuel as the ECU believes the driver wants to accelerate this would make the engine run a rich air/fuel mixture. This would mean that the engine would idle roughly as the ECU is injecting to much fuel into the engine, this would also mean that the engine would have increased fuel consumption.


MASS AIR FLOW SENSOR

A Mass Air Flow(MAF) Sensor works by using what is known a hot wire, this is where a voltage is placed through the wire to keep it at a set temperature.There are actually two wires on a MAF sensor, one is the platinum hot wire that is kept at a certain temperature using varying amounts of voltage depending on the amount of air coming in. And the other wire is a thermistor that measures the temperature of the air the hot wires job is to keep that thermistor at a set resistance by increasing the amount of voltage going through the wire which increases the temperature or by decreasing the voltage.  As air flows over the wire the heat is taken away from the wire so voltage is increased to keep the wire at that temperature, so as the engine accelerates more air goes into the engine, which means that more air is flowing over the wire so voltage has to be increased to keep that wire at that set temperature. The ECU can then estimate how much air is going into the engine by how much voltage is required to keep the hot wire at that set temperature. The MAF sensor is always located before the intake butterfly valve of the intake manifold.


When the engine is at idle there is less air flowing over the wire and going into the engine, so less voltage is needed to keep the wire at that set temperature. So with a lower voltage required for the hot wire at idle, the ECU sees that less voltage is required and the ECU knows that it does not need to inject to much fuel, but when the air flow increases the ECU will see a greater voltage is required and it will inject more fuel.

The MAF sensor is essentially is a voltage divider circuit so as the thermistor changes its resistance the voltage  output to the op amp is changed and this means that the transistor is more or less switched on depending on how much voltage is required to keep the thermistor at the set resistance so more or less voltage is required through the hot wire to keep the thermistor at the correct resistance.
RESULTS

This sensor has not been tested yet so there are no results.

Faults

If the hot wire became covered in sludge from the exhaust gas recirculation system, then the hot wire would be insulated from some of the air flowing over the wire. This means that the MAF sensor would require less voltage to keep it that set temperature, this would mean that the ECU would think that there is less air going into the engine and it would therefore inject less fuel into the engine than what is required and this would cause the engine to run rough as the engine could be running such a lean air/fuel ratio that the fuel cant ignite, this would most likely occur during acceleration and be lacking in power since the engine is running lean.

VANE OR FLAP AIR FLOW SENSOR/METER(AFM)

The vane air flow sensor is essentially like the throttle position sensor but unlike the throttle position sensor the AFM uses the air coming into the intake manifold to act on a flap, the AFM uses a potentiometer to send a voltage to the ECU. Whilst the engine is operating the air going into the engine act on a flap that in turn acts on the arm that is connected to the potentiometer, which then determines how much voltage is sent to the ECU, the AFM can help the ECU determine how much load is on the engine.  This is an accurate way for the ECU to determine how much air is going into the engine and the ECU can use this information to in turn inject the correct amount of fuel into the combustion chamber. When the engine is at idle there is not much air going into the engine so the flap is not pushed open very much, but as the throttle opens more, more air goes into the engine, which in turn act the flap and pushes the flap further open. The AFM is always located infront of the intake butterfly valve on the intake manifold.

RESULTS

The vane type air flow meter was wired up to a 5 volt power supply and its voltage out terminal to the ECU was measured. It showed that as the flap opened up more, more resistor was uncovered so the voltage went down.

Vane opening:0%: 3.89v,    20%:3.10v,      40%:2.93v,      60%:1.9v,      80%:1.18v      100%:0.35v
these results show that the sensor is working fine as there is more of a voltage drop as more of the resistor is exposed, however the positive and negative power supplies could have been wired up the wrong way round as most potentiometer sensor increase voltage as it is opened more, this sensor shows a decrease in voltage as the flap opens more or as more air is coming in. And most air flow sensors will give more voltage as the air coming into the engine increases.
Faults

Since the vane air flow sensor uses a potentiometer like the TPS this means that similar faults can occur on the track of the resistor, for example dirt build up on one part of the track. This would cause high resistance as the arm moves over this part of the potentiometer and therefore would cause a low voltage output to the ECU.  The ECU would see this as less air going into the engine, so less fuel would be injected into the engine, this would cause a hesitation in the engine as the arm moves over this high resistance area of the potentiometer as the engine is not getting enough fuel to maintain good power. And if the track is that badly damaged it could trick the ECU into thinking there is less air going into the engine than what there actually is and the engine could stall.

THERMISTOR  

A thermistor uses heat from the engine to act on a variable resistor that it has. As the engine temperature increases the resistance from the thermistor decreases, this type of thermistor is known as Negative Temperature Co-efficient (NTC) so as temperature increases resistance decreases and as temperature decreases resistance increases. The other type of thermistor is the PTC (Positive Temperature Co-efficient), this type of thermistor is less frequently used in engines but works that as temperature increases so does resistance. The thermistor sends a reference voltage to the ECU so that the ECU knows what temperature the engine is, this thermistor is other wise known an engine coolant temperature sensor, and the ECU uses the thermistor so that it knows the temperature of the engine and there fore the ECU knows how much fuel to inject, since when the engine is cold the ECU injects more fuel into the combustion chamber this is because when the engine is cold the fuel condenses on the cold surfaces of the combustion chamber and the fuel looses its explosive properties as it is no longer a fine mist. This means that more fuel must be injected to compensate for the fuel that is condensing, so the ECU uses the voltage signal from the thermistor to know whether the engine is cold and whether the engine needs more fuel injected to keep it running. The thermistor is usually located on the cylinder head near the thermostat. The thermistors resistance can range from 400 ohm's when the coolant temperature is at and engines running temperature 85-90 to 10,000 ohm's.

Another type of thermistor that is used as a sensor is Intake Air Temperature (IAT) sensor. This sensor measure the temperature of the air coming into the intake manifold. It works much the same as the ECT and is a NTC thermistor, so with colder temperature air coming into the engine there is high resistance so the ECU receives a small voltage, so the ECU knows what the temperature the air coming in is. Since air is denser when it is cold this means that there is more air going into the engine as the air particles are much closer together on a cold day, since there is more air the ECU needs to inject more fuel into the combustion chamber to keep the correct air/fuel ratio. Although the difference in fuel injected into the combustion chamber is only slightly more, it is needed to keep the engine running as efficiently and smoothly as possible. When the air temp. is much warmer the air is less dense as the air particles are further spaced apart, so there is less air going into the engine so the ECU would receive a higher voltage from the IAT and the ECU would know to inject less fuel to keep the correct air/fuel ratio. The IAT is located infront of the intake butterly valve and reacts to the incoming air temperature and either produces a high or low resistance depending on the air temperature.
RESULTS

The thermistors where tested in water and water temperature was increased using a hot pan as this was the easiest way to test them. Both the ECT and IAT sensors got around the same results, as both sensors are NTC the thermistors started off with a high resistance of around 2500 ohm's, this resistance initially had a quick decent as the temperature decreased, till the water temperature reached about 65 degrees then the resistance drop began to slow down. When the temperature of 85 degrees was met the final resistance was taken both where around 280 ohm's. These are good results and show that the thermistors are working properly and with the voltage supplied to the thermistors, they would be sending the correct voltage to ECU so that it knows the intake air temperature and the engine coolant temperature or engine temperature.
Faults

If the thermistor had a break inside the resistor then the ECU would only receive zero volts due to the first resistor using all voltage since the thermistor is wired in a voltage divider circuit,  this would mean that the ECU would think that the engine is very cold as zero volts would be indicated with very high resistance. This would make the engine run very rich all the time as the ECU would think that the engine is not warming up, if the ECU received zero volts from the IAT then the ECU would inject more fuel into the engine as it would think that the air is very dense even though it may not be. This most likely would not affect the fuel consumption very much as the difference in fuel consumed between hot and cold air temperature is very small.

MAP Sensor

The Manifold Absolute Pressure (MAP) sensor measures the air pressure in the intake manifold and sends this information to the ECU so it knows how much air is going into the engine. There is a silicon chip inside a perfect vacuum in the MAP sensor, this silicon chip moves in correspondence to the air pressure in the intake manifold, the diagram on the left in the image below shows high pressure in the intake manifold, since there is a vacuum in the MAP sensor this high pressure pushes on the silicon chip and the chip changes its resistance with air pressure, so with high air pressure or a lot of air coming into the engine like if the engine is at full throttle, then the silicon chip provides a small resistance so there is high voltage sent to the ECU so it knows there is a lot of air coming into the engine. But if the engine is at idle then the silicon chip looks more like the diagram on the right, this is because there is low air pressure or high vacuum in the intake manifold since the pressures at idle are more equal the silicon chip lies flatter, the silicon chip at this point of engine idle produces a high resistance so there is a small voltage sent to the ECU so that the ECU knows that there is only a small amount of air going into the engine, and therefore can inject a smaller amount of fuel into the engine. But if the ECU receives a higher voltage from the MAP sensor then the ECU knows that there is a high amount of air coming into the engine and the ECU will inject more fuel into the combustion chamber. The MAP sensor is always located on the intake manifold as it measures the air pressure inside the intake manifold.

RESULTS
The MAP sensor was given a supply voltage of 5volts and the voltage output to the ECU at atmospheric pressure was taken, the result was 4.8 volts. This is a good result as there should be little resistance to voltage input when there is high air pressure. When the MAP sensor was exposed to a low vacuum of 5 InHg(inches of mercury) the output voltage decreased to around 4 volts, then as the vacuum was increased to 20 InHg the voltage output from the MAP sensor was 1.3 volts. When the maximum vacuum was obtained of 27 InHg the voltage output from the MAP sensor was 0.2 volts. These are a good result as it shows that voltage increases as more air is allowed into the intake manifold. Since at Idle the air is rushing away into the low pressure zones in the cylinder the air pressure cannot equalize as the throttle is closed, this created a high vacuum, but as the throttle is opened more and more air is allowed into the engine, the air pressure is more equal so the vacuum is lower. So as more air comes into the engine, the MAP sensor sends more voltage to the ECU to tell it that more air is coming into the engine.
Faults

If the MAP sensor had a corroded positive terminal this would mean that the MAP sensor would not get its full 5 volt power supply as some of the voltage is being used up to push through this corrosion. This would mean that the MAP sensor would not send enough voltage to the ECU, so the ECU would always think that there is less air coming into the engine than what there actually is so the ECU would not inject enough fuel into the engine. This could make the engine run roughly as there is not enough fuel to ignite properly, this problem would be the worst during acceleration or when the engine is off idle as there is a greater air requirement but not enough fuel to allow combustion to take place so this is why the engine would run roughly


KNOCK SENSOR
The knock sensor is used to measure engine knock or if the fuel is igniting incorrectly or at the wrong time either igniting to early or to late. Engine knock also known as pinking or detonation occurs when the fuel is ignited at the incorrect time due to hot spots of air and fuel igniting. The knock sensor picks up on this knocking and sends a voltage to the ECU, the ECU will then adjust the ignition timing of the engine to combat this problem. The knock sensor produces own voltage by using an element that picks up on a specific engine vibration, when a pressure or vibration acts on this element, the element produces its own voltage which is relayed to the ECU. The knock sensor is usually located near the combustion chamber on the side of the engine block.


RESULTS
The knock sensor only produces a very small voltage and this can be seen on a oscilloscope, the knock sensor can be tested by wiring it up to an oscilloscope that is set to a time per division of 500 micro seconds (uS) and a voltage per division of 50 mV and then lightly tapping the knock sensor on a table and measure the voltage output. The maximum voltage obtained from the knock sensor was 0.012 volts. This is still a good reading as the knock sensor only outputs a small voltage and it only output that voltage when it was tapped on the table, this shows that the knock sensor is working fine.

Faults

The knock sensor could fail as the element breaking since it is very sensitive to vibrations, if the knock sensor failed and stopped relaying a voltage to the ECU. The ECU would never know when the engine is pinking so the ECU would not adjust the timing to stop the detonation taking place. If the engine is allowed to keep pinking or knocking then the engine could be damaged as combustion could take place to soon and the flame front from the combustion would try push down on the piston even though it is trying to come up to Top Dead Centre. This could put a hole in the piston over time and that cylinder would then loose compression and this would cause the engine to run rough. Also since the knock sensors element is so sensitive to vibrations, it could pick up on engine knock even though there might not be any knocking taking place and start sending a voltage to the ECU. If this happened then the ECU would reduce the performance of the engine as the ECU increases the amount of fuel injected into the engine but retards the ignition timing, this would lead to increased fuel consumption and reduced power as the ECU tries to put the engine into "anti-knock mode".

IGNITION COILS

The ignition coil is made up of two coils, one coil is a thick winding which is known as the primary winding or coil and the other is a secondary winding or coil which is connected to a high tension lead which is sent to either a distributor or spark plug, this coil is made up of a thin wire(thinner than the primary winding) with many more windings than the primary coil. The ignition winding works by sending a voltage through the primary winding this is done by grounding the primary coil, this voltage causes a magnetic field to be produced. When the primary coil is ungrounded the magnetic field rapidly collapses, as the collapsing magnetic lines move over the secondary winding a very high voltage is induced on to the secondary winding, this high voltage then grounds itself via the spark plug, this causes a spark to jump across spark gap to the base or ground. This is how a spark is produced to ignite the air/fuel mixture in the combustion chamber.

RESULTS

Two different coils where tested, this testing was done by testing the primary and secondary windings on both coils(there where only specifications for one of the coils). The first coil to be tested was CIT-118, this was the only one with specifications.To test the primary winding you set a multimeter to ohm's and then place the red probe on the positive terminal of the coil and the black probe on the negative terminal of the coil(this can be seen on the image on the left below) or the low tension terminals. The result for the primary winding should be 1-1.3 ohm's. The result that I got for the coil was 1.3 ohm's so this is a good result. To test the secondary winding you place the red probe on the positive terminal of the coil and the back probe in the high tension terminal or where the high tension lead is placed. The specifications for the secondary winding is 8.5k-9.5k ohm's and the result that I got was 9300 ohm's, this is a good reading and shows that the first coil is in good condition. The last thing to be tested was an earth leakage test, this is done by placing the positive multimeter probe on the negative low tension terminal and the black probe on the body of the coil. The multimeter is set to ohm's and the result should be O.L(open circuit), the result for this coil was O.L so the overall condition of this coil is very good.

The second coil to be tested had the part number C6R500, this coil did not have specifications. The primary winding had a resistance of 1.5 ohm's and the secondary winding had a resistance of 14,490 ohm's this high resistance was caused by corrosion on the high tension terminal. The primary winding had a good result, but the secondary winding had a very high resistance, this is a bad reading as there is to much resistance, the problems associated with this high resistance will be explained under faults.  The earth leakage test came up as O.L so this is a good result. So the overall condition of this coil is not good due to the high resistance on the secondary winding.

Faults
The high resistance in the secondary coil is not a good thing as this is where the spark for the spark plugs is produced. With this high resistance means that a lot of the voltage is used up trying to push through the high resistance this would mean a weak spark or even no spark at all. This high resistance could mean that no spark is produced because the spark does not have enough amperage and voltage to jump across the gap to earth. If there was a weak spark this would make the engine very hard to start especially with rich mixtures on cold start as the spark is not strong enough to ignite the fuel. When the engine is running it run roughly as the engine could be missing a cylinder from time to time or that is not firing on one of the cylinders from time to time. If there was no spark being produced especially if the spark is sent to a coil then the engine would not start as there is nothing to ignite the air/fuel mixture. 


This is how to test and diagnose faults with the different sensors in a modern day car and how their faults could affect how the engine runs.




Posted by at

1 comment:

Subscribe to: Post Comments (Atom)