TDC Sensor

Piston Dead Center Sensor

The Piston Dead Center Sensor is responsible for confirming that the engine piston has reached the maximum height point in its travel, known as the dead center. It is a vital part of an engine's synchronism since with this data a correct start can be made.

What is the Piston Dead Center Sensor?

sensor tdc

The piston dead center sensor itself is an electrical component that is designed to send a voltage signal to the computer to tell it what position the first piston is in when the engine starts. It has a circuit diagram similar to the CKP sensor where it has three connections in its electrical circuit diagram: a power cable, the ground cable, and the signal terminal.

The TDC is an impulse sensor, this kind of device is usually placed on a phonic or toothed wheel, discs with sectored windows, or on rings made of ferromagnetic material. They detect the variation of the magnetic flux generated by the teeth and valleys of the wheel or the reliefs of the cam. This reading is converted into an electrical signal.

How does the TDC sensor work?

The piston dead center sensor is very important in engine ignition. The TDC sensor detects the piston position in the cylinder, specifically the top dead center or extension of the piston. But it cannot make contact with the piston, since it is a closed cavity. So, it references the camshaft and its position. By sending the signal to the computer, it is possible to synchronize the correct moment to generate the spark at the spark plug, which must be precisely before the piston of the first cylinder reaches the upper stroke limit.

If the engine is a diesel engine, since the fuel ignition is a compression ignition, there is no spark at the spark plug. In these engines, the TDC sensor helps to synchronize the fuel injection.

The Function of the TDC sensor

Engine cylinders always have a Top Dead Center (TDC). The TDC is a reference point that relates to engine timing, which indicates when the spark plug should fire. It is the top location of the cylinder on the compression stroke. At this position, the exhaust and intake valves in the cylinder head are closed. In turn, the fuel mixture is compressed in the cylinder.

The TDC sensor will monitor a reference point on the camshaft that signals the location of the top dead center of one of the cylinders, usually cylinder number one. When it senses the position, it sends a signal to the ECU which in turn communicates and sends a spark to the cylinder at its top dead center. The fuel is ignited by the spark and the power stroke begins as the piston is pushed down.

The ideal time for the spark plug to fire is just before the piston reaches TDC. So the sensor sends an impulse to the ECU to ensure that the ignition is strictly timed. In the case of diesel engines, since there is no spark plug, the sensor moderates the timing of the fuel injectors.

Types of Piston Dead Center Sensors

The TDC Sensor has no major variants since they all capture an impulse. However, we can mention two types:

Inductive TDC Sensor.

This type is a coil and is placed very close to a small wheel composed of one or several toothed discs. The magnetism generates the voltage signal that is sent to the computer. This signal is proportional to the speed variation detected by the sensor.

Hall model

Here there is a semi-conductor, connected to an electrical circuit and a magnet. This allows greater durability because it will be able to withstand voltage peaks. With this Hall sensor the signal travels perpendicularly since when the magnetic field is created by the magnet, the energy or voltage is induced through the direction of the semiconductor wire.

TDC Sensor Faults

Being an electrical part, it is prone to failure. If this occurs, the timing for the spark at the right time is lost, so the engine will either not start or run erratically.

Symptoms of a Damaged TDC Sensor

One of the first symptoms you will see is the check engine light on. This indicates that the ECU has detected an anomaly and an error code will be generated.

If the sensor damage is total, the engine will not start, because the ECU does not receive the camshaft position signal and has no reference or starting point to send a response to start the spark at the spark plug. This action of the ECU is given as a protective measure to prevent further damage due to lack of synchronization.

A third symptom is a pull or false starts. There, the sensor is sending the signal but erratic or the pulses are not constant.

How to check the TDC Sensor condition

You can test the TDC Sensor with a multimeter. Here are the steps:

  • Raise your vehicle at the front or side where traction is generated.
  • You must disconnect the ignition coil.
  • Set your multimeter to the DC or direct current option.
  • Turn the ignition key to the ON position, without trying to start it. With a lever on the crankshaft, the wheel tries to turn it. There the multimeter should register a reading of 5 volts each time you turn it.

Interference of the Piston Dead Point Sensor on other systems

In combustion engines, a very precise synchronism is required to achieve the best performance. Therefore, the TDC sensor is in charge of signaling the exact point or position of the piston and acts synchronized with the crankshaft position sensor.

At certain times, it can even replace the signal from the CKP sensor to generate the starter and allow the engine to keep running. This condition occurs only in some vehicles.

Among the most common piston dead center sensor codes, which are registered in the ECU are the following:

  • P0348: Indicates voltage above 10% allowable.
  • P0365: Low voltage fault.
  • P0369: Intermittent voltage. Possible electrical circuit fault.

How to clean the Piston Dead Point Sensor?

Sensors, whether electrical or electronic, have a life span. The best thing to do is to check the connections and wiring from time to time along the lines of the diagram.

The connector sometimes gets sulfated and you can clean it with an electronic cleaning spray. If you find a damaged cable, you can also repair it. If it has badly deteriorated, a total replacement of the sensor may be the best option.

Where is the TDC sensor located?

This component is located in the timing system. It is not as widespread and visible as other sensors. But already a large number of manufacturers have placed it in the system of modern vehicles, as driving conditions vary. You can find out the exact location of the TDC sensor by acquiring the car's electrical diagrams.


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