Tip & How-To about Mercedes-Benz E-Class
3.2) CMP - Cam
shaft Position (sensor)
What is it?
This electrical device is very similar in concept to the crank position
sensor (above) in that it detects the position of the rotation of a shaft, in
this instance the cam shaft. It is the
cam shaft signal that indicates to the ECU that cylinder one is at top dead
centre at compression and in the process provides the ECU with the timing reference
for fuel injection. The cam shaft sensor
provides extra information, to that of the crank sensor, to fine tune timed
events such as injection and spark delivery.
Where is it located? The location of this
device depends on the design and age of engine.
On many modern overhead cam shaft engines the cam shaft sensor is
located either on the end, or to the side of the end, of the cam shaft housing. On older pushrod engines the cam sensor is found
where the distributor would once have been located. This type sensor assembly makes use of the
distributor shaft meshing directly, at its bottom end, with a gear on the cam
within the engine block. In this respect
the distributor shaft is just an extension of the cam shaft. On the top end of this distributor shaft is a
magnetic armature. The cam shaft sensor,
that measures the movement of this armature, is located in the cover and uses
either magnetic or 'Hall effect' pickup modules.
How does it work? The sensor detects the position of a magnet
or set of teeth on the gear on the end of the cam. An electrical waveform output signal is sent
to the ECU as the cam turns. The cam
sensor tells the ECU that cylinder 1 is at TDC on its compression stroke prior
to ignition. The cam sensor ensures the
correct timing of the fuel injection cycle.
If the timing belt ever jumps a tooth on the cam the lack of agreement between
the respective signals from the cam and crank sensors is indicated as an error condition
by the ECU.
Symptoms of faulty cam sensor
Associated OBD2 error codes DTCs: P0341 - P0349; P1345
Note The cam sensor body often
protrudes into the cam shaft gallery and when it's 'o' ring seal becomes age hardened, engine oil may leak
back into the sensor electrical socket plug connector causing errors due to
interruption of signal.
How to check? It is not technically easy
to determine that the CMP is at fault and, like the crank sensor, may
require the use of an oscilloscope. If
it is part of a sophisticated on board diagnostics system its failure may be
logged as a specific error code in the ECU. Due to the variability problems
with engine running are often experienced well before any error codes are set
by the ECU.
pin socket (magnetic - sine wave output).
One pin is 'ground' the other is 'signal'. A voltmeter set to 2 volts AC should measure
a signal in the 0.2 to 2 volts range on the 'signal' pin.
pin socket (Hall effect, magneto - square wave form output). One pin is 'reference' (5volts), one pin is
'ground' and the third pin is 'signal'.
A DC voltage should be detectable in the 0.5 to 1.5 volt range from the
It is important that
the voltage measurement take place at start up when the engine is cold and
again, 20 minutes later, when the engine has fully warmed up to operating
temperature. Often the voltage output from faulty sensors declines with
How to fix?
If the CMP failure is due oil leakage into the
electrical connector then a thorough clean and a renewal of the oil seal on the
CMP may correct the situation. It can be
a wise precaution to change the connecting cable as oil migration beneath the
insulation may possibly contaminate components at some distance to the CMP.
If the CMP has an
internal fault then it must be replaced.
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