Testing Throttle Position Sensor
The first sensor wave pattern that was tested was the TPS (Throttle Position Sensor) this was tested to see whether there is a flat spot during acceleration that could cause the engine to run rough as the engine is running a lean air/fuel mixture. This is because the TPS is telling the ECU that the throttle is open less than it actually is so less fuel is going into the combustion chamber. The TPS is used so that the ECU can estimate how much air is going into the engine and therefore adjust how much fuel is injected into the combustion chamber.
To test the TPS and other sensors we used the boards that are wired up to each sensor and then place the probes for the multimeter or oscilloscope into the tester connections for the individual sensors, however in the real world there are no boards to use to test each sensor, so back probing is used. This is where a small metal pin or something similar to a pin that is used to slide down the side of a wire and goes down and touches the terminals of the connector for that sensor, this is done so that no damage occurs to the wire or connectors but allows for testing to take place.
Problems that can occur are flat spots in the TPS that mean the engine could run lean. Another fault that could occur is that the TPS gives to high voltage when the engine is at low RPM's or idle this would mean that the engine would run a rich air/fuel mixture, this is because the ECU believes the engine is under acceleration or the throttle is more open than what it actually is so the ECU thinks there is more air going into the engine, this could make the engine run roughly if there is to much fuel going into the combustion chamber and the fuel could flood the spark plug causing the spark to go through the fuel and jump across the gap to earth, this would mean that the engine would misfire and run roughly and there would be a lot of unburnt gas going out the exhaust creating high exhaust emissions. These are some of the problems that can occur with the TPS and how it would affect engine running and how you can test the TPS using the back probing method.
MAP Sensor
The next sensor to be tested using the oscilloscope is the MAP (Manifold Absolute Pressure) sensor, this is used to check how the voltage behaves in relation to the air pressure in the intake manifold so that it is known whether the MAP sensor is sending the correct signals to the ECU, this means that as the throttle opens up more air pressure will increase so voltage from the MAP sensor should increase, this is another method used with the TPS so that the ECU knows how much air is going into the engine. So when the engine is at idle the sensor should be sending a small voltage to the ECU but as the throttle is opened up more the air pressure should increase and therefore the voltage from the ECU should increase as well. When the throttle is closed suddenly the voltage should drop down quickly as air pressure decreases quickly then the voltage should drop below idle voltage then return to normal idle voltage this drop below idle voltage is because the throttle is now closed but the engine still is revving this means that air is still rushing into the engine quicker than what air pressure can restore behind the throttle valve so there is a greater increase in vacuum and a greater decrease in air pressure temporarily than what it is at idle.
A MAP sensor can be tested by setting a mulitmeter to DCV (direct current voltage) and can be checked using the back probing method to test how a sensor is operating by reading its voltage outputs to the ECU. But an oscilliscope can also be used to see how the voltage wave patterns behave to get a more accurate reading of what signals the MAP sensor is sending to the ECU, this can also be checked by using back probing to check the sensors condition.
If there is a problem with the MAP sensor in that it could be sending to low of a voltage to the ECU because it could be blocked up with dust and other materials that are entering the throttle body, this will mean the MAP sensor gives the wrong information to the ECU and the MAP sensor is telling the ECU that there is less air going into the engine than what there actually is. This means that the engine will be running a lean air/fuel mixture this could mean that the engine will run roughly and could as there is to much air this could cause pinking or detonation this is when combustion takes place before it is meant to and causes a knocking sound mostly when the engine is under load and this could put holes in pistons as the combustion is not taking place smoothly and pushing down on the piston instead pinking or more so detonation is like hitting the piston with a hammer and it the combustion takes place to early so the flame front hits the piston as it is still coming up on its compression stroke. This is how damage can occur to the motor and the engine will be lacking power with the leaner air/fuel ratio. The MAP sensor could send to high of a voltage to the ECU this would make the ECU think that more air is going into the combustion chamber than what actually is this would mean the ECU would inject more fuel into the combustion chamber and there would be a rich air/fuel mixture this would cause an increase in power but a decrease in fuel economy. These are some of the problems that can occur with a faulty MAP sensor.
Crankshaft (RPM) Sensor
The next sensor to be tested using the ocsiloscope is the crank shaft sensor which is located in the distributor this sensor is also known as an RPM sensor, the purpose of the crankshaft sensor is note engine speed and crankshaft rotation so that the ECU can know which stroke a piston is on so that fuel can be injected and the spark can be created at the correct time to allow combustion to take place. The sensor works by spinning iron passed a magnet the spinning part of the sensor is known as the reluctant and has 4 iron pieces attached to it at even intervals there are 4 pieces of iron as there are 4 cylinders there is one piece of iron for each cylinder. The iron spins past a stationary magnet and as it spins passed a magnetic field is created and voltage is induced to the magnet which has a winding of fine wire around it. Voltage will drop below zero at a time this is because there is a change in polarity, there is also a point where no voltage is induced as the magnetic lines become parallel and the magnetic lines have no affect and therefore no current can be induced into the winding. The ECU measures the frequency of this wave form to calculate engine speed so that fuel injectors can opened at the correct time and a spark can be created at the spark plug at the correct time.
This sensor can only be accurately checked using a oscilloscope as a multimeters refresh rate is to slow to show any correct readings. The crankshaft sensor can also be checked using the back probing method and setting up the oscilloscope making sure that the volt divisions are about 1 volt and the time per division is around 20 milliseconds as these wave formations occur very quickly and if the time divisions are to long then an accurate reading cannot be sourced.
If a problem occurs with the crankshaft sensor then the engine will most likely not start as the ECU does not know what stroke the engine is on and it therefore cannot open fuel injectors at the correct time nor can it fire the spark plugs at the correct time. So the ECU will most likely not fire the injectors or the spark plugs and the engine simply will not start. This is the only affect of operation that a faulty crankshaft sensor will have as the sensor will most likely send no information to the ECU.
Fuel Injector Testing
The next component to be tested using an ocsilloscope is an output from the ECU and it is the fuel injector. The fuel injector works by using a transistor to ground the circuit so that current can flow to ground this means that current flows through a magnetic winding which pulls up a small pin on the end of the fuel injector and fuel sprays out into the combustion chamber, then the transistor is ungrounded the magnetic field collapses and the pin moves back up and the fuel injector is off again and no fuel is being sprayed into the combustion chamber.
The wave pattern that occurs with a fuel injector is one where there is an initially high voltage of around 12volts when the voltage suddenly drops away to zero the the transistor has grounded the circuit and the voltage is therefore being used up to open the injector, when the injector shuts off the voltage increases above 12volts in a voltage spike this is because a magnetic field was created to lift up the pin that opens the fuel injector that allows fuel passed the injector when the circuit is suddenly shut off the magnetic field collapses the collapsing magnetic field induces current and creates a voltage spike.
Problems that can occur with fuel injectors are faulty transistors that mean that the injector cannot ground properly i.e a transistor that grounds the fuel injector the entire time so that it could be on 100% of the time and so it therefore never stops spraying fuel into the combustion chamber. Or the transistor does not ground the fuel injector and it therefore never sprays fuel into the combustion chamber. Other faults that can occur with fuel injectors are blocked injectors which that there is not enough fuel getting into the combustion chamber.
These are some of the diagnostics you can do using an oscilloscope to measure the wave patterns of the sensors and outputs of the ECU to see whether they are in good condition or not.
Reference:
Image of Back Probing : http://www.google.co.nz/imgres?imgurl=http://www.crownvic.net/ubbthreads/ubbthreads.php%3Fubb%3Ddownload%26Number%3D18221%26filename%3DMAF%2520voltage%2520measured.jpg&imgrefurl=http://www.crownvic.net/ubbthreads/ubbthreads.php%3Fubb%3Dshowflat%26Number%3D2086626&usg=__Lpv1EZIlASdhiGAiuDlFzLfkdaY=&h=480&w=640&sz=87&hl=en&start=154&zoom=1&tbnid=r0Gh8Gicn-f4yM:&tbnh=124&tbnw=167&ei=gSkNTtvbNKyNmQWHr83iBw&prev=/search%3Fq%3Dback%2Bprobing%2Bsensors%26um%3D1%26hl%3Den%26sa%3DN%26biw%3D1024%26bih%3D574%26tbm%3Disch&um=1&itbs=1&iact=rc&dur=330&page=11&ndsp=15&ved=1t:429,r:10,s:154&tx=32&ty=37
image for map sensor wave form : http://www.picoauto.com/waveforms/images/zoom/map_sensor_analog.png
Image of TPS waveform : http://www.aa1car.com/library/tps_waveform.gif
Image of crankshaft sensor waveform : http://www.picoauto.com/waveforms/images/zoom/crankshaft_sensor_inductive_cranking.png
Image of fuel injector waveform : http://thm-a04.yimg.com/nimage/76fed3dafc2b29e4
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