The Engine Coolant Temperature Sensor (ECT) is used by the on-board computer system to determine engine temperature. Engine Coolant SensorThe on-board computer uses this information to calculate proper fuel delivery and ignition timing. On some vehicles, this information is used by the on-board computer to activate certain emission control systems or to engage the engine cooling fan. The ECT is a thermistor. The electrical resistance of the sensor decreases as the temperature increases. The ECT sensor is screwed into the side of the engine where it is exposed to the engines coolant. The on-board computer sends a reference voltage to the sensor, usually 5 volts. As the engine heats up the resistance accross the sensor decreases. The computer then determines the temperature of the vehicle by reading the voltage accross the sensor. The voltage across the sensor can range from 4 volts when the engine is cold to less than .5 volts when the engine has reached operating temperature.


An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman. The original sensing element is made with a thimble-shaped zirconia ceramic coated on both the exhaust and reference sides with a thin layer of platinum and comes in both heated and unheated forms. The planar-style sensor entered the market in 1998 (also pioneered by Bosch) and significantly reduced the mass of the ceramic sensing element as well as incorporating the heater within the ceramic structure. This resulted in a sensor that started sooner and responded faster. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other vehicles. Divers also use a similar device to measure the partial pressure of oxygen in their breathing gas.
Scientists use oxygen sensors to measure respiration or production of oxygen and use a different approach. Oxygen sensors are used in oxygen analyzers which find a lot of use in medical applications such as anesthesia monitors, respirators and oxygen concentrators.
There are many different ways of measuring oxygen and these include technologies such as zirconia, electrochemical (also known as Galvanic), infrared, ultrasonic and very recently laser methods. Each method has its own advantages and disadvantages.


Is located in the air stream next to the electronic throttle valve. Measuring the pressure and the temperature of airflow into the engine, it is capable of determining the volume of air being used by the engine as well as the engine load and the intake temperature (outside air temperature). In the Cooper S, it determines the pressure differential across the supercharger in cooperation with the MAP sensor. 


This sensor is mounted on the left side of the cylinder head and measures intake vacuum from a line connected to the supercharger supply ducting. It changes voltage as intake pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. It performs essentially the same job as the vacuum advance diaphragm on an old fashioned mechanical distributor that changed the ignition timing by rotating the distributer under heavy manifold vacuum. WHen the ignition is turned on, it measures atmospheric pressure. Once the engine is running, it measures the absolute manifold pressure which is the barometric air pressure minus the vacuum created by the pistons.


Mounted on the electronic throttle body, there are actually two sensors here, a primary and a backup. As the MINI uses a 'drive by wire' throttle, (see here) this approach provide a fail-over in the event of a problem. The throttle position sensor (TPS) changes resistance as the throttle opens and closes. The computer uses this information to monitor engine load, acceleration, deceleration based on throttle position. The sensor's signal is used by the to compute how much to enrich the fuel mixture during acceleration, as well as to retard and advance ignition timing. 


Located in front of the crankcase at the flywheel end of the engine the crankshaft position sensor serves essentially the same purpose as the ignition pickup in an electronic distributor. It uses a Hall effect circuit and "sees" the presence of a reluctor ring bolted to the end of the crankshaft and as it rotates, there are 58 targets and two missing "teeth" on the ring. It generates a signal that the ECU can use to determine the position of the crankshaft and the piston position in number one cylinder. This information is necessary to control ignition timing and for the accurate operation of the fuel injectors.


This is located in the front of the cylinder head just under the valve cover and again uses a Hall effect circuit with a reluctor ring to determine the exact position of the camshaft. Viewed in correlation with the crankshaft position sensor this is used to determine injector timing in both the open and closed loop and has a clever design that controls the four injectors completely individually. So that if one injector circuit fails, the other three can continue to provide power, albeit limited. 


The knock sensor detects engine vibrations that indicate incorrect fuel detonation is occurring. If this occurs, the computer can momentarily retard the ignition timing. It is actually an acoustic or vibration sensor like a small microphone and listens for unusual vibrations typical of partially or spontaneously ignited fuel - called pinging. It is bolted to the crankcase just below the intake manifold.
There are many other sensors at play in this complex and highly evolved system. Many feed other subsystems and their appropriate control modules which in turn feed data back to mission central - the engine control module. Pt 2 of this OBD article will look at the signals coming from the OBD connector and evaluate their contribution to a smoothly running, well tuned and happy MINI engine.


The Mass Air Flow (MAF) sensor directly measures the amount of air flowing into the engine. The sensor is mounted between the air cleaner assembly and the air cleaner outlet tube.
The sensor utilizes a hot wire sensing element to measure the amount of air entering the engine. The sensor does this by sending a signal, which is generated by the sensor when the incoming air cools the hot wire, to the PCM. The signal is used by the PCM to calculate the injector pulse width, which controls the air/fuel ratio in the engine. The sensor and plastic housing are integral and must be replaced if found to be defective.


Exhaust gas recirculation


The EGR valve helps your car more efficiently and completely burn fuel by recirculating a portion of your exhaust and running it through the combustion process again. This results in a cooler, more complete burn of the fuel which decreases you car's noxious emissions by prohibiting the formation of some harmful gases.
  • The EGR valve is vital to your car's emission controls.
  • When the EGR valve goes bad, it must be replaced.
  • EGR Valve = Exhaust Gas Recirculation Valve.
  • The EGR valve controls formation of noxious emissions.
  • Rough idle and poor acceleration can be caused by a faulty EGR valve.
Guide Review - EGR Valve
The EGR valve, or Exhaust Gas Recirculation valve, is a vacuum controlled valve which allows a specific amount of your exhaust back into the intake manifold. This exhaust mixes with the intake air and actually cools the combustion process. Cooler is always better inside your engine. The exhaust your EGR valve recirculates also prevents the formation of Nitrogen related gases. These are referred to as NOX emissions, and are a common cause for failing emissions testing. Unfortunately, your EGR valve can get stuck, causing NOX gases to build up. You'll know if your EGR valve is stuck or malfunctioning because your car will experience symptoms like rough idle and bucking on accelertaion.


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