Introduction:
Combustion engines can be equipped with one or more camshaft position sensors. These position sensors measure the pattern of the reference disc (rotor) which is mounted on the transverse side of the camshaft, or the pattern of a section along the longitudinal direction of the camshaft.
The signal from the camshaft position sensor is, for the engine management system, a supplement to the crankshaft signal:
- the crankshaft position sensor is used to determine engine speed and crankshaft position;
- based on the signal from the camshaft sensor, the engine management system can determine which stroke the piston is in that is moving from BDC to TDC.
The camshaft sensor is often a Hall sensor and receives a supply voltage and ground from the ECU. The signal, in the form of a square wave, is sent via the signal wire to the ECU.
Advantages of an engine with a camshaft position sensor:
Not all engines with an engine management system and computer-controlled injection and ignition are fitted with camshaft sensors. However, camshaft sensors make the following extensions possible:
- Individual control of injectors and ignition coils: without a camshaft signal, individual control of coils and injectors is not possible, because the crankshaft signal is not sufficient: each power cycle consists of two crankshaft rotations and only one camshaft rotation. In engines without a camshaft sensor, a DIS coil is used which fires all spark plugs every crankshaft rotation and uses group injection;
- Variable camshaft timing: to obtain extra torque, or specifically for regeneration purposes (diesel engines with a particulate filter), the ECU must be able to read the position of the camshaft in order to correctly control the camshaft adjustment;
- Fault detection: in the event of problems with valve timing, a deviation in the crankshaft and camshaft relationship is detected. A fault description may be: “incorrect installation combination” or “ratio of crankshaft and camshaft signals out of tolerance”.
Location of the camshaft position sensor in the cylinder head or the valve cover:
There are several ways in which the camshaft sensor reads the position of the camshaft:
- by a cam pattern on the camshaft: in this case the sensor is usually located on the valve cover, at the height of the timing belt, or bolted to a separate housing of the camshaft, as shown in the drawing below;
- there is a trigger wheel on the camshaft (other possible names: reference disc, position disc, rotor with notches and recesses of different sizes). In that case, the camshaft sensor is screwed into the cylinder head.
In the following image a BMW engine can be seen on which a new timing chain with guides has been installed. The trigger wheels (reference wheels) are separate discs that are clamped against the camshaft with the central bolt. The trigger wheels have multiple notches and recesses of various sizes. Both camshaft sensors (positioned directly below the trigger wheels) read the pattern of the notches and recesses of the trigger wheels.
Using the pattern of the trigger wheels, the engine management system can determine within a single camshaft revolution which cylinder is starting the compression stroke. The system can then set the injection and ignition to make the engine run. The engine will start after a short cranking time.
Measuring the camshaft sensor signal with the oscilloscope:
If a camshaft sensor is defective, a DTC (fault code) is stored in most cases. With the aid of an oscilloscope we can measure the camshaft signal while starting or running the engine. Usually we measure the camshaft signal simultaneously with the crankshaft signal in order to check the valve timing. This procedure is explained on the page about the crankshaft position sensor.
Related pages: