VR Crank Sensor Question 1

[MICR0]

I have a VR type crank sensor and I'm not sure how to wire it to the harness. Its a Bosch sensor typical on Mercedes cars. I assume one side is ground reference from the ECU then the output or signal is wired back to the appropriate input of the ECU. I don't know the polarity of this sensor. How do you determine which wire is the signal out put on a sensor like this? The wires coming from it are one green and one yellow and there appears to also be a single yellow wire (same shade)in the same pigtail that goes to the oil pressure switch If that's any help.

 

[NickJ67]

It's basically just a coil of wire and generates a signal as the trigger wheel teeth pass which ever way you connect it. It's more a question of which polarity is your ECU expecting to see.

My quick and dirty method when getting my Megasquirt and Ford EDIS based system working working was to try it and see which polarity was giving a cranking signal on the monitoring software. If you have an otherwise working engine you could just try it and see which one starts it.

Cheers

Nick

 

[MICR0]

Thanks.

 

[theoldwizard1]

VR sensors do not have "polarity". Their output signal looks like a sine wave and aligns with the "tooth" of the trigger wheel at the point where the voltage crosses from positive to negative. There is usually a "zero crossing" circuit in the PCM that detects this and then generates a square pulse (one shot) for the uP.

 

[bmw5002]

it is my understanding from working with the megasquirt system and vr sensors that they do not necessarily have a "polarity", however the polarity of the two wires coming from the sensor DOES make a difference. That difference lies with the zero crossing circuit. If the polarity of the sensor is flipped, the signal is inverted, and it can throw off the zero crossing circuit. The timing will then be incorrect, or the ecu will refuse to even run the engine. The wiring needs to be correct for what the ecu is expecting. The following page will explain in greater detail:

http://www.megamanual.com/ms2/pickups.htm

 

[theoldwizard1]

mw5002 - Good reference link !

Hall effect seems to have fallen out of favor. My guess is that VR is cheaper even with the zero crossing circuit.

Biggest problem with VR sensors seems to be epoxy "creep". Expansion and contraction of the sealing epoxy breaks the fine wires. I have seen it happen in a short as a few dozed thermal cycles on prototypes. Make sure you know what kind of epoxy your vendor is using and that he sticks with it !