leading edge or trailing edge and VR sensor gap

[StevenD57]

I am just about done with my bracket to mount the VR sensor so I am
now looking at the alignment of the toothed wheel in relation to the VR sensor position.
I have looked at the drawings and other info posted on the autosportlabs.net sight
but i don't see a mention of if the alignment of the VR sensor to trigger wheel needs
to occur at the leading edge or trailing edge of a tooth (or lack there of). I am using a
toothed wheel from a Ford Taurus and the teeth are pretty course. I am wondering if
the VR sensor should be aligned to the leading edge or trailing edge of a tooth. I know
I am aligning with the missing tooth but is the system looking at the missing tooth and
timing off of where it would expect the leading edge of the tooth to be?

Also what is the tolerance on the gap between the VR sensor and the toothed wheel?
--
Steve

[dan97019]

I just lined my up by eye centered over the tooth and it ended up being 2 degrees off (advanced).

It calls for a max of 1mm gap, I would call mine a fat mm but it seems to work fine that way

[StevenD57]

I just lined my up by eye centered over the tooth and it ended up being 2 degrees off (advanced).

Are you saying then that you should have had it be lined up at the leading edge of the missing tooth and then it would have been right on?

Anyway, thanks a bunch for the reply to both questions.

[dan97019]

I just lined my up by eye centered over the tooth and it ended up being 2 degrees off (advanced).

Are you saying then that you should have had it be lined up at the leading edge of the missing tooth and then it would have been right on?

Anyway, thanks a bunch for the reply to both questions.

Im really not sure if it was because my concept of center was off or maybe I didnt have the engine at exactly TDC I wasnt that worried about it when I set it up because I figured i would just fine tune it later if I needed too, but at 2 degrees off it was close enough that I just took it out in the software and left it at that. It was all pretty much a ball park set up so I was surprised at how close it was. But all the diagrams of the setup that I have seen show the VR centered directly over the tooth so I went with that.

[StevenD57]

But all the diagrams of the setup that I have seen show the VR centered directly over the tooth so I went with that.

So the VR sensor centered over the center of where the missing tooth is supposed to be located if it was actually there?

[dan97019]

I think I went more by this diagram http://www.autosportlabs.net/Image:Edis_wheel.gif

[Spockie-Tech]

I believe its the *edges* of the tooth that the timing is reference too.. (not so) quick explanation follows

If one looks at the output signal from a VR sensor on a scope, you will see it is a sine-wave shape, curving up and down smoothly..

The peak positive and negative voltage of the wave goes up and down according to the gap, the egnine rpm, the "permeability" (magnetic conductivity) of the material used to make the trigger wheel (different types of steel), the width of the teeth and probably a few other things.

The positive wave peak occurs in the middle of the "tooth present" position where the sensor can see the maximum amount of metal, and the negative going wave peak occurs in the middle of the "tooth absent" position where the sensor feels the least amount of metal in proximity.

but, the EDIS module doesnt "know" in advance what the amplitude (size) or peak of the wave is going to look like - The only predictable voltage point on the waveform is the moment where it *changes* direction from a positive-going wave, to a negative-going wave.. as the waveform passes through the "Zero Point" on its bipolar excursions.

This wave-polarity-transition point is much easier to accurately detetermine and detect for the electronics using what is called a "Zero Crossing Detector" circuit, and the timing measured from Zero-crossing to zero-crossing will be constant regardless of other wave variations.

So while I dont *know* the EDIS's internal circuit, I would be very surprised if it didnt measure from zero-point to zero-point for its timing. This means the timing would be referenced from edge-to-edge of the teeth, not middle-to-middle as far as I can theorise

I could prove it on the bench with a dual-channel 'scope and show the VR sensor Waveform at the same time as the Spark plug pulse and see if the spark occurs on the zero-cross of the VR wave, but I'd give odds that it does and occurs on the edges of the teeth.

If its important I can do the measurements to confirm (In fact I might anyway in the next few days since I'm building another Megajolt system at the moment),

I imagine that most people arent too concerned about their timing being accurate to within 2.5 degrees (36 teeth x 2 edges - 72 edges, or 5 degrees between edges, 2.5 from tooth center to edge) of any particular *number*.

Whether your timing display vaue reads 15 degrees current advance or 17.5 degrees current advance is not important as long as its constant in relation to the engine. If you take a pre-made fine-tuned Map from one trigger sensor installation to another one with the exact same engine but the VR sensor a half-tooth out, then you would be out by (at worst) 2.5 degrees, and your map probably isnt sliced that fine unless you are a damn fine tuner chasing every last horsepower for this particular install.

I hope that made sense and didnt confuse things more

[BRUCEROE]

I agree it is the tooth edges that set the timing. I don't think the waveform description is correct, though: "maximum positive on the tooth, most negative in absence of a tooth". That would imply the voltage directly tracks the wheel height.

First, note that the voltage goes to a half way point during the missing tooth, NOT most negative. It then heads for positive peak when a tooth eventually comes along. Remember, voltage isn't induced by a magnetic field, voltage is induced by a CHANGE in magnetic field. In fact I believe The VRS puts out an increasing positive voltage as the tooth is approaching, then drops back to neutral going across the top of the tooth. Then as the trailing tooth edge comes the VRS puts out an increasing negative voltage, then drops back to neutral going across the absence of the tooth.

I had to work through this to build an electronic waveform emulator for bench testing. Bruce Roe

[DannyP]

Run EDIS in limp-home or with some type of fixed map at idle. Read the advance with a timing light and adjust accordingly. I set mine up in the center of the tooth(I think, it was over two years ago!), and it ended up being 2.5 degrees off. No problem, I just adjusted it in the v3 Global options. Of course I adjusted my maps too. Kind of a pain in the butt really, shoulda just left it alone probably!

[Spockie-Tech]

I agree it is the tooth edges that set the timing. I don't think the waveform description is correct, though: "maximum positive on the tooth, most negative in absence of a tooth". That would imply the voltage directly tracks the wheel height.

Correct you are.. Well spotted, I stuffed that up.

I typed that off the top of my head, when the last time I looked an EDIS wave on the scope was a few months back, and forgot about the VR sensor construction.

Since the VR sensor is basically a coil and magnet, it would indeed change output voltage with the *change* in magnetic field conducted by the tooth, not the absolute level of the field. Which also explains why the peak voltage rises with increasing RPM.

So, the positive going side of thewave on the leading edge of the tooth, and the negative going side of the wave on the trailing edge of the tooth...

Thinking it through, wouldnt that put the zero-crossing point in the middle of the tooth, and we're back to the center of the tooth being the timing point ?

Although I could also be wrong in my guess that its the zero-crossing-point that EDIS times from. I was thinking about what that would imply for wheels with different teeth/gap (mark/space) ratio's. A short narrow tooth with a wide gap, or a wide tooth with a narrow gap could spend more time at the zero-point at low rpm due to the flux having time to return to zero-change before the next edge came along. Not that many wheels arent 50:50 M/S Ratio, but I have seen a few tooth wheels posted on here that look like they could be 75/25 or thereabouts.

Interesting, if I find time in the next week or three, I might hack up some PWM code for an Arduino to drive the VR input with a varying MS ratio and see how the edges relate to the coil firing time at a fixed rpm on the bench with my scope.

Thanks for catching that