RPM-/Reference Mark Sensor
Previously, it was called the TDC-transducer. It's basic principle has not changed. However, it is sometimes more favourable for the controlling-electronics, if marks, other than the TDC are shown. Thus, nowadays it is
called the reference mark sensor.
Please take a look at the diagram (figure 3). Below the injection signal diagram, one can see that of the combined RPM- and reference mark sensor. Thereby, the even deflections serve the determination of the RPM
and the constantly recurring, larger oscillations, the identification of the reference mark. Through this, e.g., the injection-starting-point can be regulated.
Should both sensors be combined, then the scanned gear-rim- or the aperture-plate must feature a gap. If this, as in the case of the starter-gear, is not possible, then two sensors are necessary. For the second one,
there is then an additional stud in the flywheel. The reason why the flywheel is often used, is because that way, one receives a particularly high signal-resolution, i.e., from the numerous individual signals per rotation,
the respective position of the crankshaft can be determined to within a few degrees.
The principle of concluding the function of the sensor through the number of cables, has also been changed. Previously, two connecting leads indicated an inductive pick-up, and three, pointed to a Hall effect sensor. Nowadays, a voltage supply can exist with only two cables, which excludes the possibility of using a resistance
measurement. We have also come across inductive pick-ups with three connections.
The engine RPMs can thus, be determined in the front- or at the rear of the crankshaft. The passage of the reference mark as well, however, to ascertain this, an additional sensor on the cam-shaft is needed, to indicate
whether a certain cylinder is in ignition- or in overlapping TDC position. Where previously, almost exclusively in fact, inductive sensors were used, we now increasingly find Hall sensors, because, e.g., these can
measure also very low wheel-RPMs. Thus, even the slightest reverse movement of a vehicle is detected. 03/10