The mass airflow sensor’s purpose is to measure the mass amount of air entering the engine. The mass sensor is usually located on the intake inlet running to the intake manifold. Sometimes if there is a problem with it you can tap on it gently in the electronics area, and the engine should surge. There are multiple types of mass airflow sensors. Hot wire airflow meters use a current to keep a wire at a constant temp. The heat is carried away from the wire dependent on the airflow and temp. The ptc resistor varies with temperature change, as the temp drops so does the resistance. As the resistance changes the current has to increase to keep balance. This is sent to the ecu in a form of a voltage change. Another system is the hot film. It is very similar to the hot wire mass airflow meter. It heats a wheatstone bridge board, The air moves across the board and is measured on the back of the board. The thermistor measures the change in temp and thus the airflow change. The intake air temperature sensor also contributes to this equation being that humidity can slightly throw off the density reading. A vane airflow sensor simply rotates a flap as air comes in.
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A knock sensor is generally used on turbocharged or high compression performance engines. The knock sensor is generally screwed into the engine block for all cars. To test the knock sensor, generally you can tap a wrench on it as it runs. The engine should change idle speed as the timing retards. If the idle speed doesn’t change, try again at a slightly higher rpm. If the speed once again dosen’t change the knock sensor probably doesn’t work. High performance engines are alot more prone to detonating when they are under boost. The timing on a turbocharged engine should retard when the boost comes on. If the boost would go too high the timing will be too advanced for the higher boost and will detonate. If the engine detonates it builds very high pressure waves. This can damage the engine’s pistons, bearings and crankshaft. The knock sensor senses the very high frequency pressure waves ocurring inside the cylinder. If the knock sensor is tightened too tight or is too loose it can effect the measurable frequency band. The tightening torque depends on the knock sensor specification for that engine. If it is incorrectly tightened it will sense other vibrations in the engine and incorrectly believe that there is a problem. The knock sensor works by simply retarding the timing until the detonaton quits. The driver should feel the significant decrease in power as it retards. It saves the engine from detonation which will break pistons and burn up head gaskets.
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The crankshaft and camshaft sensors back each other up. If one does not work it could still run with the other one. If only one is present it wouldn’t. This gives the ecu or pcm away to know the timing of the engine. In older non injected cars the only timing needed was for the distributer to supply the engine with spark at the correct time. This was done mechanically and was usually assisted by vacuum at lower rpm to advance the spark timing curve. Then throttle body injection came along and the ecu needed to know when in the rotation to open injectors and which ones to open. The throttle body injection used a pulse off of the distributor to fire the injectors each time the engine made a revolution. Early injection systems used bank fire systems which injected fuel into certain cylinders one revolution then into the others on the next revolution. Newer systems are much more complex and need more information. That is where the camshaft sensor comes in. It will rotate twice for every one rotation of the crankshaft. By doing this it knows when each cylinder is coming up to top dead center on the compression stroke. This way it won’t fire on the opposing stroke to waste any fuel that could drop out on the cylinder wall, wasting fuel. It also now controls the spark ignition timing to each cylinder. The ecu now distributes the spark instead of the distributor. The coil packs are now on top of the spark plugs instead of a single coil being inside the distributor.
Both these sensors have a similar function but are slightly different in function. A barometric or baro sensor tells the absolute atmospheric pressure. The map sensor uses a relative vacuum measurement and has a different range. The map is important for many reasons and usually every fuel injected vehicle will have one of these. The baro sensor is used as a secondary system so that even slight atmospheric changes can be monitered by the engine. Air density changes, as the pressure increases the engine will go lean. The sensor tells the ecu, which adds fuel to correct the lean condition. If your map sensor seems to be malfunctioning you can test it. First check for a loose plug or a bad vacuum hose.
The sensors are what makes a fuel injection system work. The throttle body setup is not complex and dosen’t need many sensors to work. The multipoint or multiport setup is far more complex and needs alot more sensors. The sensors tell the ecm or ecu (computer) how much fuel to inject into the engine and where the timing sould be at. Certain sensors enable different small maps to be enabled. Such as the coolant sensor calls the start up or enrichening map when a certain engine coolant temp has not been reached. Most sensors send a 0 to 5 volt signal back to the ecu. From this value the computer knows where the sensor is operating.
The coolant temperature sensor checks how hot the engine’s coolant is running. The coolant temp sensor is usually located on the intake manifold or the water inlet to the engine. This is usually close to or in the thermostat housing, which is at the end of the hose coming from the radiator. The sensor tells the ecu if the engine is cold and has not been warmed up. This activated the enrichening maps in the computer. It is essentially a “choke” for the engine. When engines are cold they need more fuel to run properly until they’ve heated up to a certain temp. Usually the normal fuel maps will be used when the engine reaches within 20 or 30 degrees below where the thermostat opens. That is why it’s important to not lower the thermostat temp or it could keep the engine running rich all the time, wasting fuel. If you have a problem with your coolant sensor you can test it. This test also works for a MAT (manifold air temp) sensor. First check the plug to make sure the wires look good and the plug is tight. Run the car for five minutes. Shut off the car and remove the sensors plug. You will need a volt/ohm meter to check the ohms. Set the ohm meter on 20k ohms. There are three types of temperature plugs. With a single wire sensor, touch the one wire and use the other wire to touch the brass or metal on the sensor. Two wire sensor, just touch the two wires. Three wire temp sensor, touch the center and one of the outers. The ohm value should be higher than 500 ohms or it is not good and needs to be replaced.
The air temp sensor simply checks the temperature of the incoming air. The sensor is usually located on the air box (filter area) or somewhere in the air inlet tube. The purpose of the sensor is to help with calculations of air density coming into the engine. The information is fed to the computer. If the air is warmer it’s less dense and requires less fuel for the engine to run at it’s correct air to fuel ratio. The engine needs more fuel at colder temps due to the denser air. The colder denser air gives the engine the ability to make more power, needing more fuel. The air temp sensor changes resistance with heat changes and will send the new voltage to the ecu. The car’s computer then changes the amount of fuel injected in the engine. If you have a problem with your air temperature sensor, you can test it. First check the plug to make sure the wires look good and the plug is tight. Run the car for five minutes. Shut off the car and remove the sensors plug. You will need a volt/ohm meter to check the ohms. Set the ohm meter on 20k ohms. There are three types of temperature plugs. With a single wire sensor, touch the one wire and use the other wire to touch the brass or metal on the sensor. Two wire sensor, just touch the two wires. Three wire temp sensor, touch the center and one of the outers. The ohm value should be higher than 500 ohms or it is not good and needs to be replaced.
