4848 Blog number 1

OFF CAR ELECTRONIC SYSTEMS

This blog will be an in depth focus on  some sensors I have covered in our of car practical work book. I will be choosing 3 sensors and comment on their function and intended use. I will also go in depth to explain the various types available and state the reason while older versions have been replaced. The 3 sensors I have chosen are.......

TPS (throttle position sensor)

MAF (mass air flow sensor)

MAP (manifold absolute pressure)

Throttle position sensor(TPS)

A TPS sensor monitors the angle of the  butterfly valve so it can relay your throttle response to  the ECU. The further you press the accelerator pedal the wider the throttle butterfly valve opens. The ECU uses this info to alter the vacuum, Ignition timing and fuel injection timing so it can adjust  the fuel ratio accordingly to meet performance demands. It can also alter other parameters depending on how sophisticated the system is.

Location
In most cases it is located in the butterfly spindle so that it can directly monitor its position. Its box and wiring harness are usually located on the outside of the throttle body by the air intake hose.

http://zhome.com/ZCMnL/PICS/hesitation/hesitation.htm

It will detect the speed at which the angle changes. A common scenario of sudden changes in throttle response could be when  when your warming up your car in neutral, or simply revving it to show off. 

Types of throttle body sensors.

There are a few different types of throttle position sensors out there but they mainly fall under 3 categories. The  contact type, potentiometer type and the non-contact  type which work on the hall effect principle.  Most run on a 5v supply and give the ECU a variable voltage signal depending on the angle of the throttle valve. The higher the voltage the further open the butterfly valve. A select few use the opposite approach by starting with a high voltage that deceases the further it opens (just being plain difficult).

1) Contact type  

Fairly out dated and doesn't monitor the full throttle position  range (which leads to poor fuel economy).  Uses many contact points and prone to wear and tear. As they are outdated and becoming very uncommon I wont be doing much research on them.

 2)Potentiometer type

Found in most of today's used cars. The potentiometer gives a variable resistance reading and uses a sliding contact. Prone to wear and tear from mechanical contact parts.

3) Non contact hall effect types 

Used in today's modern cars and have been met with great success as they last much longer due to having less mechanical parts. They are equipped with a magnet and a hall effect sensor

As the potentiometer is the most common type to date (and Ive done some work on one) I will be focusing primarily on this.

Potentiometer type TPS

The potentiometer type TPS uses a type of variable resistor (Rheostat) inside the sensor. It adjusts the voltage signal output depending on the position of the throttle butterfly. A low voltage means low throttle while a high voltage means higher throttle.

Common readings

An output signal of around 1v-1.4v would mean the vehicle is idling and the butterfly valve is closed.




An output signal of around 4.5v-5v means the vehicle is quickly accelerating (or being revved in neutral) and now has a wide open throttle (WOD)

http://adamkerr4826.blogspot.com/2010/08/ttec4826-sensors-throttle-position.html

http://adamkerr4826.blogspot.com/2010/08/ttec4826-sensors-throttle-position.html

Potentiometer throttle position sensor operation

Mass air flow sensor(MAF)

http://qcwo.com/technicaldomain/iat-sensor-information-on-1996-2002-ford-taurus

MAF or mass air flow sensors are used to determine how much air is entering your fuel injected engine. The ECU will need this critical information so it can adjust the fuel ratio. During normal driving conditions your ECU tries to keep the fuel 14.7 to 1 so fuel can burn at an optimum level and create as little emissions as possible.If the car is facing other conditions it wont actually try to keep this fuel ratio. When the car is warming up or driving up a hill it will actually make a richer mixture  to deal with these situations. If the car is decelerating (going down a hill etc) it will actually make the mixture leaner as it doesn't need to burn excessive amounts of fuel.

How they work?
The two most common MAF designs are the vane meter and the hot wire. Both are usually powered by a   the ECU and communicate with it via a 0-5v signal (in some rare cases it uses a pulse-width  signal).  Some older MAF designs use a frequency instead of a voltage reading to relay messages to the ECU.

Common readings?
The normal voltage reading from a MAF sensor while in idle is around 0.8v-1.2v while at wide open throttle you would be looking at 4.4v-5v.  A select few operate differently by stating at 5v and decreasing as the air flow increases. One must make sure to obtain the MAF sensor specifications as they change significantly from model to model and have have their own allowable tolerances to determine whether they are faulty.

Vane airflow sensor

http://www.aa1car.com/library/vaf_sensors.htm

 How they work?
A spring loaded air flap/door is connected to a  potentiometer that varies resistance as the door opens and closes. The vane will move depending on the proportion of air passing it. A voltage signal is created in proportion to the amount it moves. Vane meter systems are usually equipped with an intake air temperature sensor (IAT sensor) and a pressure sensor to help record air volume. They need a IAT sensor to record the temperature of the incoming air. Air becomes denser when the temperature drops so the ECU needs to take this into account. Because of their size and cost of production most manufactures have replaced them with with hot wire set ups.

Mayjor drawbacks

Airflow is somewhat restricted. This limits airflow

Finding a suitable place to mount it can be problematic in a tight engine bay

Has to be placed in a suitable place in regards to gravity and air intake

Numerous mechanical and electrical contact parts are subject to wear and tear

Hot wire MAF sensor

An electrical component that communicates with the ECU via an analogue voltage signal. 

http://www.aa1car.com/library/maf_sensors.htm

How does it work?
The analogue signal is dependent on the hot wire resistance,which in-turn is proportional to the temperature of the hot wire. The hot wire warms up to a predetermined temperature and tries to stay there. As air passes the hot wire it begins to cool down (proportional to the air passing it) lowering the resistance. As less air goes past it begins to warm up to its original base temperature and the resistance increases. The hot wire can also determine air temperature as colder air will cool down the hot wire quicker. Also cold air is denser then warm air which means less is required for the air fuel ratio.

Benifits of a hot wire vs a vane meter

Very little airflow restriction

Allot easier to mount in your engine bay as there small and compact 

Less expensive to to manufacture


Improved reliability due to far less mechanical parts.


Quickly responds to change in air flow and temperature.


Does not require additional temperature and pressure sensors.

The two common drawbacks with hot wire meters are

Oil and dirt contamination. This leads to the hot wire deteriorating and giving inaccurate readings.

Installation requires a laminar flow across the hot-wire

Conclusion

Due to all the mechanical parts and the cost of production vane meters are being replaced with hot wire setups as there more cost effective, compact and reliable. Also they don't require separate pressure and temperature sensors  to determine air mass and density.

Manifold absolute pressure sensor(MAP)

http://fordfuelinjection.com/?p=27

http://fordfuelinjection.com/?p=27

A manifold absolute pressure sensor is used to determine the absolute pressure in  the maniflod. They are typically found in fuel injected cars.  While common in many vehicles during the 1990s most manufacturers have replaced them for MAF sensors as they give better fuel milage , but rarely ever use both (though they are found in rare cases). They are used to measure the amount of air pressure in the manifold instantaneously, which is then relayed back to the ECU. The data is used to calculate the amount of air coming in and its density, which in turn will determine the amount of fuel needed for optimum combustion.

How does it work?
The map sensor uses a piezoelectric (pressure-sensing) disc to measure intake manifold vacuum, which is then relayed to the ECU. Instead of using a variable voltage output signal like a MAF sensor, it uses a frequency output instead. As the vacuum increases the frequency increases. This signal allows the ECU to read its data bank to determine its air/flow ratio, EGR flow, ignition timing and altitude compensation.

MAP sensor VS. MAF sensor

While most manufacturers use MAF sensors both are designed to accomplish the same thing. They simply do things differently, and both have their pros and cons. A car equipped with both is more reliable  as both can give airflow readings if the other one fails. The pros and cons as follows....

MAP sensor

1. If an intake pipe blows off or there was a vacuum hose leak the car would still run same, as the actual manifold pressure will not be any different. MAF sensors cant detect any leaks past it in the air intake assembly.

 

2. The MAP sensor in OBD2 can determine if the EGR (Exhaust gas recirculation valve) is working  as manifold pressure should increase as exhaust gasses enter the manifold.

3. Sensor reads actual load not just the air intake. This eliminates guess work for the ECU

MAF sensor

1. The amount of air entering the motor is VERY accurate when paired with other sensors, even if the airflow is low. These precise readings create better air/fuel ratios which result in better fuel economy.

2. Most come with a built in AIT( Air Intake Temperature ) sensor. This helps  the ECU monitor air intake temps prior to it entering the combustion chamber.

MAP sensor readings

A MAP sensor can instantaneously record pressure in the manifold send it to the ECU in a matter of micro seconds. This in-turn lets the ECU quickly know how much air is available so it can refer to its data map and adjust the fuel ratio accordingly. 

http://fordfuelinjection.com/?p=27

http://fordfuelinjection.com/?p=27

MAP sensor data can be converted to air mass data using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. The MAP sensor can also be used in OBD II (on-board diagnostics) applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines.