Detecting the Position of the Camshaft (Camshaft Timing Changes)

Detecting The Position Of The Camshaft (camshaft Timing Changes)







In order to detect the position of the camshaft (camshaft timing) in relation to the crankshaft, the engine control module (ECM) uses the signals from the engine speed (RPM) sensor (the position of the crankshaft) and from the camshaft position (CMP) sensor (the position of the camshaft). The control module uses these two signals to determine the position of the camshaft in relation to the position of the crankshaft.

Detecting the camshaft flanks







The camshaft is divided into four flanks per revolution of the camshaft (flanks 1-4). The flanks are detected by the camshaft position (CMP) sensor. The signal affects the shape of the camshaft rotor.

The teeth on the camshaft pulse wheel for camshaft flanks 1 and 4 are shorter than the other teeth. This enables the control module to determine the position of the camshaft. The control module is able to establish which combustion cycle the cylinders are in.

Detecting the reference positions of the camshaft







The crankshaft has four reference positions, one for each camshaft flank. The reference positions are predetermined points on the flywheel. The reference positions are detected using the signal from the engine speed (RPM) sensor.

The camshaft turns at half the speed of the crankshaft. This means that two reference positions are detected for each turn of the crankshaft. Therefore two engine revolutions are required to detect all flanks on the camshaft. The positions on the flywheel are designated degree CA (Crank angle). 0 degree CA = Top dead center cylinder 1. Flanks 3 and 4 are detected during the second revolution of the engine.

Detecting the position of the camshaft in relation to the position of the crankshaft







Each camshaft flank aligns with pre-defined positions on the crankshaft when the camshaft is in its 0 position. These positions on the crankshaft are called reference positions for the flanks. The illustration shows how the signals relate to each other when the camshaft is in its 0 position (the camshaft is not deployed).
A: Engine speed (RPM) sensor signal.
B: Camshaft position (CMP) sensor signal. From high to low signal when the teeth on the camshaft pulley leave the camshaft position (CMP) sensor.
C: Low engine speed (RPM) sensor signal because of the holes in the flywheel/carrier plate.
D: Top dead center (TDC) cylinder 1, 0 degree CA (84 degree CA after hole "C" in the flywheel/carrier plate).

1: Detection of flank 1, reference position 47 degree CA "D1".
2: Detection of flank 2, reference position 227 degree CA "D2".
3: Detection of flank 3, reference position 407 degree CA "D3".
4: Detection of flank 4, reference position 587 degree CA "D4".

If the flanks do not correspond to the reference positions on the crankshaft when the camshaft is in the 0 position (not deployed), the engine control module (ECM) will store the difference. There may be a difference from the camshaft 0 position if the timing belt is incorrectly seated or the camshaft are not correctly set for example. A mechanically damaged camshaft reset valve may prevent the camshaft moving to the 0 position when the engine control module (ECM) stores the adaptation value for the deviation of the camshaft. This may result in high deviation and a diagnostic trouble code (DTC) being stored.

The deviation can be read off from Vehicle Information and Diagnostics Application (VIDA).

Regulating the camshaft position
The engine control module (ECM) controls the camshaft reset valve steplessly. The valve controls the flow of engine oil to the continuous variable valve timing (CVVT) unit which is affected by the oil pressure that builds up. This allows the CVVT unit to change the position of the camshaft.

When deploying the camshaft by for example 10 degree CA (the engine control module (ECM) deploys the camshaft), the detection of the camshaft flanks will be offset by 10 degree CA from the reference positions for the crankshaft.







The illustration shows how the signals relate to each other when the camshaft is controlled (deployed camshaft), D1 - D4 is reduced. The engine control module (ECM) is then able to calculate the degree CA (crankshaft degrees from top dead center (TDC)) that the intake valve opens and the exhaust valve closes for each cylinder. This is because the opening and closing angles are fixed and predefined in relation to the flanks on the camshaft rotor.

The closing angle of the intake valve and the opening angle of the exhaust valve can be read off in Vehicle Information and Diagnostics Application (VIDA).

The following applies when the camshafts are in their 0 position (no deployment):
- the opening angle of the intake valve 27 degree CA
- the closing angle of the exhaust valve is 26.5 degree CA.

The following applies when the camshaft is fully deployed:
- the opening angle of the intake valve is -(minus) 23 degree CA
- the closing angle of the intake valve is -(minus) 3.5 degree CA.

The total of the closing angle of the intake valve -(minus) the opening of the exhaust valve = the valve overlap.