The Mercedes Benz M103 engine is a 6 cyl engine and uses three teeth on the flywheel, each 60 degrees wide, leaving a 60 degree window between them. There is a single inductive/VR sensor of 830 Ohm to sense these teeth, the sensor wire has only one conductor with a shield that also acts as ground.
After some discussion it was revealed that the trigger input is not compatible with this type of signal. The input is a zero crossing detector, while this signal requires a level crossing detector.
This was done :
- A very strong pullup to 5V using 2.7 KOhm
- 10nF plastic cap for filtering at the LM1815 chip
- 5 Volt modification for the LM1815 chip
On the input with sensor connected you should measure DC voltage to ground of 1.1-1.2 Volt.
It is unknown which edge of each "tooth" is at TDC for each cylinder, but one of them is very close. On the 4 cyl Mercedes engines with this trigger the positive signal occurs at 55 deg BTDC and negative at 5 deg BTDC.
On the 6 cyl M103 engine the TDC after the trigger was set to 78 degrees and strobing the timing with fixed advance was like looking at a still photo, awesome. The engine has been run up to 6500 rpm without any issues.
The scope image below shows the signal picked up by the sensor while cranking at about 200 rpm.
The ground of the probe is connected to the shield of the sensor wire.
More on the engine here: http://en.wikipedia.org/wiki/Mercedes-Benz_M103_engine