GenBoard/Manual/VrSensor/Polarity (2006-03-16 04:43:08)

This page is about Variable Reluctance sensor (VR) polarity

Pretty much any VR sensor can be used that came from an automobile. Ford, GM, and basically all manufacturers applied millions of them. VR sensor is a passive component, which means it does not require supply voltage. Resistance is

However there are some restrictions for the wheel and polarity must be correct to be usable as the input trigger signal.

The stock opel wheel is "missing tooth" ( not missing gap). Picture of the stock opel wheel:

opelwheel.JPG.

Good polarity: if you measure positive voltage (DVM, DC 200mV mode, relative to GND) when approaching a ferromagnetic object (like a wrench) to the sensor. (and negative voltage when moving away).


[Picture of stock VAG 60-2 missing tooth wheel]

Good polarity for missing gap: if you measure negative voltage (DVM, DC 200mV mode, relative to GND) when approaching a ferromagnetic object (like a wrench) to the sensor. (and positive voltage when moving away).

To use this type of wheel with a v3.2 or earlier, a series resistor might be required in series with c38. v3.3 already has it for more tolerance on peaky tooth amplitude after the missing tooth.

If the VR is connected in reverse polatirity

it will be

That is a problem, since the usual practice is applied to trigger on tooth-times > 1.5 averaged tooth-time.

(Undesirable) Effects


Indication

Currently, there is no indication on LCD or MegaTunix if the VR sensor was connected with wrong polarity (other than the wrong polarity connection might prevent triggering altogether at certain conditions).

What info could be used ?


Waveforms

LM1815 generates the pulse on the negative-going zero crossing

Looking at http://www.picotech.com/auto/waveforms/crankshaft_sensor_inductive_running.html they probably swapped the wires.

Note that both EDIS and LM1815 triggers on the negative-going (falling) edge of the signal, around 0mV. But the [picture of Jaguar crank waveform] is measured with wires swapped.

However the [other waveform seems OK] for triggering on falling edge

If the tooth-heights are non-uniform, InputTrigger/RunOut can happen.

By looking at a scope it should be pretty obvious, but everyone does not have a scope. Unfortunately polarity is often poorly documented by the manufacturer.


How the polarity should look like around the missing tooth:

vr-output.JPG

But the peaks around the missing tooth is bad. But the solution can be found at InputTrigger/MultitoothProblem.

Note: I had to connect the sensor with different polarity than on the stock ECU. pin 48 on the motronic is labeled CAS signal. And 49 CAS earth. But I had to connect 48 to gnd and 49 to signal on the genboard.


See also