Blog 1 4848(off car)

Air flow/mass sensors (MAF)  

We did not conduct the experiments for the hot wire airflow meter in class due to insufficient gear, but I think these meters are a very important thing to show understanding about because most modern cars use these meters rather than the Vane and Vortex type meters due to the reduced size, weight and amount of parts used in the production of the hot wire meter.

This sensor works by measuring the amount of incoming air. The sensor has a Hot wire in which a constant current is applied to, this wire is a temperature coefficient resistor, usually of positive temperature coefficient nature. This hot wire is placed in the air stream of the incoming air, basically as air flows past this hot wire, the resistance of the hot wire is changed in accordance to the amount of air flowing in, as the amount of air flowing in rises, the resistance drops due to the heat of the hot wire dropping and because its a temperature coefficient resistor (ptc) the resistance will drop as the air flow increases.


The resistance change corresponds to output circuit, this circuit gives a reading to the ECU (electronic control unit). For example, at idle our airflow wont be very high, so the output voltage and frequency wont be very high. When the engine is at WOT (wide open throttle) our airflow will increase significantly which will give a high output signal and a high frequency. These output voltages and frequency's correspond with other components like the o2 sensor, to allow accurate amounts of fuel injection and ignition time. Sometimes manufacturers will include an internal cold wire in the airflow meter which basically measures the density of the air flowing through the airflow meter and allows the ECU to make any changes to injection rates and timing according to the density of the air.


 There is also another feature that some manufacturers have included in these sensors, as the vehicles ignition is turned off there is a large voltage spike induced to the hotwire of the sensor to allow it to burn of any carbon dust build up, if there is carbon dust on the hotwire the sensor output readings could be incorrect meaning the ECU could be giving the engine the wrong amount of fuel and ignition timing.






Here is a video to display the basic function of the MAF sensor, along with a small amount of fault finding procedures;

Test procedures;

Step 1;
We can test the MAF sensor by hooking it up to a 12v power supply, 5v reference voltage and a ground. From here we need to hook a multimeter up to the signal out pin and a good earth. Set your multimeter to voltage test. Now we need to blow air through the passage way, you can do this by simply using your mouth to blow air through but a more effective way is to use a hair drier set on a cold setting. As the amount of air flows past the heated element we should notice a drop resistance causing the output voltage to be higher. If you dont experience this whilst testing you MAF it may be faulty.



REFERENCE;

All the information included in this post was taught at unitec, aswell as http://www.autoshop101.com/forms/h34.pdf and http://www.sensorland.com/HowPage060.html

Picture: http://www.google.com/imgres?q=maf+sensor&um=1&hl=en&sa=N&biw=1680&bih=956&tbm=isch&tbnid=hf6JyTRp8wTKWM:&imgrefurl=http://www.aa1car.com/library/maf_sensors.htm&docid=S1IX39h6_NB5JM&imgurl=http://www.aa1car.com/library/maf_sensor_ford.gif&w=557&h=395&ei=leuhTo_DA8WtiQeg793DBg&zoom=1&iact=rc&dur=195&sig=102311554416164353344&page=1&tbnh=130&tbnw=183&start=0&ndsp=41&ved=1t:429,r:0,s:0&tx=96&ty=72

Video: http://www.youtube.com/watch?v=sSuL58YEH-E




MAP sensors (maximum abosulute pressure sensor)

The MAP sensor was designed to measure incoming air pressure (absolute air pressure). This measurement that the MAP sensor makes is then sent to the ECU, this information is very important because the readings given by the MAP sensor allows the ECU to make calculations towards the air/fuel ratios


The diagram above shows the pressure sensor input, the 5v input, ground and the signal out wire. The sensor has a 5v input signal which is a constant 5v. As the pressure sensed by the MAP changes, the resistance inside the sensor is changed, so when the resistance is changed obviously the output signal has to change to. This is where the signal out wire comes in, when the resistance internally changes the output signal changes to, for example when the pressure input rises, the resistance corresponds to this by rising aswell so with a higher resistance the voltage output signal will be lower.


The MAP sensor is commonly found in most modern vehicles now days, mainly EFI (fuel injected vehicles). Most vehicles use etheir a MAP sensor or a MAF sensor to determine pressure or airflow if a MAF sensor is being used.

The diagram above shows the circuit that this particular sensor uses, as you can see terminal 61 is the 5v supply terminal, 36 is the output signal to the ECU and 43 is the ground to the sensor. The squiqily line inbetween 5v supply and sensor ground is the variable resistor, the position of the sensor output line is determined by the position of the plunger which is moved by the amount of pressure coming into the sensor.
Test procedures

MAP sensors are fairly easy to test for correct function,

Step 1; Hook up 5v power supply to the power input terminal, make a sufficient ground to the ground terminal.

Step 2; Set your multi meter to DC voltage test, attatch the positive lead to the output line and the negative lead to the ground line on your MAP sensor (the terminals can be determined by etheir reading the corresponding manual or doing a little bit of research on the internet).

Step 3; Aquire a pressure pump to pump pressure into the pressure input of the sensor, as the pressure is raised, the voltage reading should drop, if this happens with the voltage decreasing/increasing smoothly this  then your sensor is working with-in specification. If the voltage out-put is quite jumpy then you may have short circuits through-out the resistor meaning that you will have to replace the sensor.

Throttle position sensor

The throttle position sensor measures the angle of the throttle position, this sensor is one of the most important sensors for correct drive-ability because it corresponds with other sensors to allow the ECU to make accurate decisions towards fuel injection, timing and spark ignition. 

This sensor uses a reference voltage which is usually around 5v, a signal out voltage and a ground. As voltage is applied to the reference circuit it goes through into the sensor where there is a variable resistor, ground and output signal. The output signal is the signal that is sent to the ECU, this signals value is completely determined by the position of the signal output wire on the variable resistor. The position of the output signal wire depends on the drivers intent, as the driver steps on the accelerator pedal, the rotor(spindle) inside the sensor moves (the variable resistor) so as this rotor is moved, the resistance also changes which means the signal output changes accordingly. 

At idle the TPS will give a low signal to the ECU, this just lets that ECU know that there is no load on the engine and it needs to keep the engine at a constant low idle speed. As you can see by the chart, as the throttle shaft changes angle so does the resistance, it drops. So basically that means the voltage output rises. When the output rises the ECU knows that the engine is under load, from here the ECU can make calculations as to how much air/fuel needs to be injected etc.

Test procedures;

Step 1; Test the supply voltage to the sensor by locating the sensor, unplugging the sensor plug and attatching the red lead of your multimeter to the reference pin and attatching your ground wire of the multimeter to the ground pin of the plug. Make sure your ignition is on but the cars not started up. After following these steps your multi-meter should read 5.0v without any hesitation and jumpyness with the signal. If it does not show 5.0v constantly you may have a problem with reference voltage supply.

Step 2;
Now we are going to check the signal voltage to the ECU from the TPS. First of all, start by plugging the TPS plug back into the TPS. Now we are going to back probe the signal terminal using a pin or safety clip, what ever we have handy, we also need to back probe the sensor ground aswell, but be careful not to let the two probes touch as it will cause a short which could damage the sensor. Now we connect the red lead on our multimeter to the signal back probe, and we connect our black terminal to our ground back probe. We have to start the engine for this one, once the engine is started move the tps manually, smoothly move the tps angle and carefully monitor the voltage on the meter, the voltage should start off at 1.0v without any angle on the tps, when angle starts to be applied the voltage should move up smoothly towards 5.0v without any hesitations and jumpy signals. If you notice any jumpy signals and hesistations in the signal output you may have a faulty MAP sensor, this means you may have to replace the sensor.

Step 3;
This test is similar to test 2, backprobe and connect terminals as said in test 2, but you will not need to have the ignition on or engine on because this is a resistance test. Basically adjust the angle of the tps manually as we did for test 2 and note the OHM reading on the sensor watch the resistance go up and down as the angle is adjusted. If there is no huge jumps in the resistance when the angle is changed the sensor itself may be fine. If there is you may have to replace the sensor as they are not repairable.







 

Knock sensors

Knock sensors were designed to detect pinging (detonation) going on inside engines, detonation can cause serious damage to an engine if it is carried on for long periods of time. Detonation is when a secondary flame front inside an engine cylinder ignites the fuel before the spark ignition system does, which of course stuffs up timing and injection rates etc.

The knock sensor is a pretty basic set-up, it has a signal out wire, it does not need an earth because it earth's it-self through the block in which it is placed in, although some do have an extra wire for earth depending on the area the knock sensor is situated it. The knock sensor uses the piezoelectric effect, this law states that when a crystal is vibrated or pinged it produces its own voltage. That voltage is sent through the signal line to the ECU to let it know that it needs to make changes to the timing to allow the detonation to stop.

Here is a quick video that physically shows how the sensor works;

Test procedures;

Step 1;
The best way to test these sensors is to use an oscilliscope, Basically because they are alot more acurate because the capture a pattern where as a multimeter just captures the voltage. By seeing a pattern it is easier to determine faults. Hook your oscilliscope up the signal out line of the sensor and find a decent ground on the sensor. Now we need to tap the bottom with something that will allow for a decent shockwave, something like a coin of metal rod, anything the has abit of weight behind it. As you tap the sensor, you should see the pattern of the oscilliscope jump up really quickly. This shows the sensor is working properly. If you are having trouble getting a signal, it could be how your osilliscope settings are set but usually it shows that your knock sensor is not functioning properly. This means you'll need to replace it.
Step 2;
Really the easiest way to test one of these sensors is to use a scan-tool to read any fault codes, if your knock sensor is faulty the scan-tool will bring up a code number, this number can be looked up in the user manual of the vehicle, the code will explain what is wrong with the sensor.

Thermo Fan Switch

Thermo fan switches are designed to switch the fans on when the temperature of the engine coolant reaches a certain level. The sensor has a constant signal to it, but its not always grounded.

The blue strip you can see in the diagram above is a Bi-metallic strip. This Strip can allow the circuit to complete and let the fans have power so that they can cool the engine. The bi-metallic is made up of two different metals, one of the metals expand at a higher rate than the other so as one metal expands it forces the other metal to bend, and if you look at the diagram if the bi-metallic strip bends enough it will allow there to be a connection through the sensor to earth the fans. So basically when the engine coolant reaches a high enough temperature it will cause the strip to bend and allow the circuit to complete.

Usually this circuit doesn't provide earth to the fan directly, it usually earth's a relay to allow the relay to provide power to the fans, because the fans are already hooked up to an earth source.

Test procedures;

Step 1;

If the fan switch is off the car the procedure is quite simple, find a pot fill it with warm-hot water around 60 degrees c (no higher as this experiment wont work properly). Monitor the water temperature with a thermometer, now we are going to hook a multimeter (using resistance measurement) up to the input and output terminals of the switch, after this is done we need to submerge the switch into the water (at this point the multimeter should be reading O.L). Now we need to heat the water with an element, we have to carefully watch the temperature and the multi-meter because a correctly functioning switch should switch at around 80 degrees c (some thermo fan switches will switch higher or lower depends on the manufactors spec). As the heat comes up 80 degrees or so we should see that the multi-meter changes from O.L to a small resistance. This tells us the switch is functioning correctly, if you dont see this reading at around 80 degrees you may have a faulty switch.

02 sensors

The aim of every vehicle manufactor is to achieve the perfect air-fuel ratio. An air/fuel ratio which gives the lowest amount of emissions/economy along with the best amount of performance.

The air/fuel ratio is achieved obviously by air and fuel, but how is the mixture determined to achieve perfect air/fuel ratio?

The 02 sensor is the answer. There is one 02 sensor which is usually placed just after the exhaust manifold, this senses what the air/fuel reading is, then the sensor works in-sync with the catalytic converter. The converter strips any unwanted emission gases that are flowing through the exhaust so that the atmospheric air is not polluted. Once the gases have flown through the cat, there is another 02 sensor, this sensor now reads what the air/fuel ratio is after the catalytic converter. The second 02 sensors signal is the one which matters the most because if the mixture is not right after the catalytic converter we can polute air or potentially damage our vehicles. The second 02 sensor reads what the air/fuel ratio is and sends this signal back to the ECU, the ecu then reads the voltage sent to it and makes changes to the ignition timing injection timing/rate etc. 

So this basically means that the 02 sensors jobs are to help the ECU to achieve the perfect air/fuel ratio for the conditions.

The sensor is very useful for all conditions the vehicle may come under. For example, a vehicle is travelling along a straight road and its running perfectly, the air/fuel ratio is reading a steady 14.7:1 (lambda 1) the driver approaches a very very steep mountain, this mountains height is huge. As the vehicle goes up in the mountain the altitude to ground level goes up, as altitude is increased the density of the air decreases, so therefore the engine wont run exactly as it did on the straight road. The 02 sensors will then sense the air fuel ratio, they'll see that the engine is running slightly rich as the intake of air to the combustion chamber will be less dense meaning less oxygen molecules thus the engine is running rich. The 02 sensor will let the ECU know that the engine is running rich, the ECU will make changes to the injection rate/ignition timing accordingly. 

There is a couple of main types of 02 sensors, There is the standard narrow band sensor this operates on a 0-1 volt scale, 0 being lean .5 being stoich and 1 being rich. These sensors are usually used on cars which are standard from the manufactor, meaning there is no aftermarket modification. The voltage is then sent to the ECU which lets it know if the engines rich lean or stoich (stoich means good air/fuel ratio).

There is the wide band, this operates on a 1-5 volt scale, Obviously this is a much larger scale, this is good if you have modified the vehicle, because it gives a more acurate reading to the ECU, instead of telling the ECU if its rich lean or stoich, the wideband can let the ECU know the exact amount of air or gas its reading which is good for high performance tuning, the more acurate your measurements are the better performance output you'll get.