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Camshaft Adjustment, Function - GF07.10-P-1016MRA

Engine 274.9 in model 205 (except 205.047/053/054/147/247/253) 

Engine 274.9 in model 253 (except 253.354/954) 

Function requirements for camshaft adjustment, general points 

Camshaft adjustment, general points 

Camshaft adjustment allows the two camshafts to be adjusted continuously by up to 40° CKA (C  rankshaft A  ngle) to "advanced" and "retarded". This means the valve overlap in the event of a gas exchange can be varied within wide limits.

This optimizes engine torque characteristics and improves the exhaust characteristics.

IMPORTANT Valve overlap

The intake valves open before the exhaust valves close.

The ME-SFI [ME] control unit (N3/10) reads in the following signals regarding the camshaft setting:

Function sequence for camshaft adjustment 

The function sequence is described in the following steps:

Function sequence for release of the camshaft adjustment 

Camshaft adjustment is enabled by the ME-SFI [ME] control unit dependent on engine speed and engine oil temperature.

The engine oil temperature is determined by the ME-SFI [ME] control unit using various operating data (e.g. coolant temperature, time, engine load) and a stored temperature model determined.

IMPORTANT The engine oil temperature is important, even when the oil is hot, to ensure that there is sufficient oil pressure (>1.5 bar) for adjusting the camshafts.

Enable of adjustment of the exhaust camshaft does not occur until a higher rotational speed compared to the intake camshaft. In this way for exhaust, also at a low oil pressure level, reaching the locking position against the "retard" operating reaction torques of the camshaft are secured. There is a return spring located for support in each camshaft positioner.

If both camshafts are adjusted, adjustment of the exhaust camshaft takes place after a delay (later). Oil supply problems are prevented and secure functioning of the locking mechanism achieved.

Release of the camshaft adjustment occurs load-dependent:

Function sequence for oil pressure 

The engine oil pressure is regulated via the engine oil pump valve (Y130) in order to ensure an adequate oil supply and to lower the oil pressure if necessary (a saving on fuel).

Function sequence for adjustment 

The intake camshaft solenoid (Y49/1) and the exhaust camshaft solenoid (Y49/2) are actuated by ME-SFI [ME] control unit by means of a pulse width modulated signal.

The control plungers are adjusted via the characteristics map-dependent duty cycle. The oil filling quantities for the camshaft positioner is controlled according to its position.

In the camshaft positioners the vane-cell piston which is firmly connected to the camshaft is turned by the oil pressure.

Function sequence for adjustment range 

Function sequence for start position 

The camshafts are locked in a fixed position for starting by catch bolts (locked). This start position is unlocked hydraulically at the first actuation of the intake camshaft and exhaust camshaft solenoids.

Shown is oil flows in the intake camshaft positioner 

G12933913Courtesy of MERCEDES-BENZ USA

The upper half of the illustration shows: 

Filling of oil reservoir galleries (A), oil reservoir galleries (B) open

The lower half of the illustration shows: 

Filling of oil reservoir galleries (B), oil reservoir galleries (A) open.

Function sequence for valve overlap 

At low engine speed and load, the ME-SFI [ME] control unit sets a large valve overlap in order to produce internal exhaust gas recirculation. Less fresh air is admitted, as exhaust gases with low oxygen content are still present in the cylinders. This lowers the combustion temperature and reduces the formation of nitrogen oxides (NOx ).

The intake air mass is reduced by the amount of exhaust gases present. The ME-SFI [ME] control unit shortens the injection period accordingly. The smallest valve overlap for gas exchange occurs if the exhaust camshaft is adjusted to the maximum before TDC (advanced) and the intake camshaft to the maximum after TDC (retarded). The resulting increased fresh air content produces more engine torque and engine power.

Function sequence for camshaft positions monitoring 

The camshaft positions are detected by the intake camshaft Hall sensor and the exhaust camshaft Hall sensor and communicated to the ME-SFI [ME] control unit. Acquisition of the positions takes place through detection of the positions of pulse wheels which are located at the front on the camshafts.

Shown on intake camshaft adjustment 

G12933914Courtesy of MERCEDES-BENZ USA

Shown: intake camshaft positioner 

G12933915Courtesy of MERCEDES-BENZ USA

Function sequence: diagnosis 

During diagnosis of the camshaft adjustment, the ME-SFI [ME] control unit checks whether the camshafts are in start position at engine start and whether the requested adjustment has been reached after the engine has been running for a short time. Output stage errors in the camshaft solenoids (integrated with the ME-SFI [ME] control unit) and defective camshaft Hall sensors are also detected.

Function sequence for intake camshaft valve lift switchover (for code A14 (Camtronic)) 

With the valve lift switchover, a two-stage switchover of the cams on the intake camshaft occurs depending on the characteristics map.

The intake valves close earlier due to the switchover of the camshaft to a shorter stroke. This leads to an optimization of the gas exchange in partial-load range. The valve lift switchover to the smaller cam stroke is performed in an RPM range of 1000 to 4000 RPM.

The intake camshaft valve lift switchover actuator (Y49/8) is actuated by the ME-SFI [ME] control unit by means of a pulse width modulated signal.

A coil in the actuator is energized here and a tappet moves in a corresponding curved track on the camshaft. With the turning of the camshaft and the forming of the curved track, the camshaft is moved axially and the smaller cam affects the intake valves.

A bump in the curved track has the effect of bringing the tappet back to the default position.

To reset the camshaft to the long stroke, a second tappet moves in a corresponding curved track and the reset occurs in the same manner. The position finding of the tappets is over the intake camshaft valve lift switchover Hall sensor (Y49/8b1) that sends a signal to the ME-SFI [ME] control unit for valve lift switchover.

Sectional view of cylinder head cover 

G12933916Courtesy of MERCEDES-BENZ USA
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