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The TDC sensor (Top Dead Centre sensor) is an important and clever electronic sensor which works in conjunction with the car's ECU, playing a crucial part in enabling the engine to run on time and smoothly. Information gathered by the TDC sensor is sent to the ECU which uses it, along with information from other engine sensors, to determine and control the ignition and fuel injection timing of the engine, transmission changing and variable valve timing. 

The TDC sensor can be found in the engine compartment, positioned on the gear box above the fly wheel, or mounted on the top of the transmission bellhousing at the left-hand end of the cylinder block. It can be easily seen when the air box is removed. 

It is common for an internal combustion engine to use a four-stroke action.  The intake stroke, which is the first stroke of the pistons, draws an equal amount of air from the inlet manifold into each cylinder, delivering the air via the air intake valve. For the next three strokes of the engine the air intake valves are shut while compression, combustion and exhaust processes are occurring. Then the intake valves open again and once more the air is drawn through into the cylinders from the inlet manifold. The spark plugs should fire, igniting the fuel in the cylinder, when the piston is at top dead centre. When the piston is in this position, the air intake valves are shut while compression, combustion and exhaust processes are occurring. The TDC sensor lets the ECU know when a cylinder is at top dead centre and the ECU then instructs a spark to be sent to the correct cylinder at the correct time for ignition to occur. The spark is produced via the spark plug which provides an electrical current to the combustion chamber in a cylinder in a petrol engine. The spark produced ignites the engine enabling the air/fuel mix to burn in the cylinder and combustion to occur. A defective TDC sensor can literally stop the engine from starting. If the engine does start, a fault with this sensor may cause the engine to run poorly. 

As is often the case with electrical components, the TDC sensor can fail because of damage caused by corrosion or it can simply wear out. A faulty TDC sensor may cause the engine control unit to miss a correctly timed signal and the spark may not ignite in the right cylinder at the right time, which could lead to the engine operating badly or ceasing to work. Correct operation of the TDC sensor is vital to the running and timing of the engine. It is wise to consult with your mechanic when experiencing problems with ignition and the engine, to ensure the correct fault is found and fixed as soon as possible. When this electronic sensor goes wrong, it is important install a compatible replacement part. The TDC sensor offers complete compatibility, specifically manufactured for the make and model of your vehicle.