Cutting out at high rpm

[Kirk Feldkamp]

There a number of instances that I've run across with Motec ECU's that have warranted modifying the sensor output voltage so as to not overwhelm the ECU input. It was so common an issue that Motec added an oscilloscope to the MX00 series ECU's and created a few webinars to explain how to make things work for their products. Check out the webinars on ref/sync that they have on the website to better understand the issue. It is very very common to hear about the older Busa engines acting like they had a *hard* rev limiter around about 7000 rpm with the OE trigger wheel/sensor once you added a Motec. The solution was the modify the voltage inputs to an acceptable level and to filter things. It wouldn't surprise me if this same sort of thing was happening on an ECU that perhaps wasn't designed to go to relatively high RPM's like on a bike engine. Probably worth having a talk with Bosch about it.

-Kirk

[VFR750R]

Try resistor spark plugs if you don't have them installed already.

If you are using a bosch ECU, seems like they are happiest with a 60-2 wheel and one of the bosch inductive sensors (which most are relatively cheap). Our grand-am engines use a bosch sensor that is found at the bosch-motorsport website.

[wweissin]

If you increased the number of the teeth I would be very inclined to say that you are saturating the input on the ECU with the voltage. A common way to fix this is to attenuate the signal with a resistor. It can be a bit of trial and error or you can do the math. I would recommend starting around the 1k ohm mark and go from there.

I ran in to this specific issue with the new BMW SBK in AMA on a Pectel ECU. It would get close to the rev limit then just lose sync because of the crank sensor voltage. I ended up using 14.7k ohm of resistance to get the bike to run all they way to redline. And this is with a 60-2 wheel at well over 11k RPM.

[Tickers]

The ECU monitor is showing a lost sync, with the crank sensor dropping out (I haven't seen this with my own eyes, it's what the engine manager has reported)

The gap is currently 0.4mm, with the ECU manual recommending 0.8 ± 0.3. The signal wires are twisted, which is as standard on the bike and previous cars (which have no noise issues).

Alex, do you think it's likely to be voltage saturation due to increased teeth? It's what we'd initially guessed, but we're also worried about the obvious possibility that the size of the trigger wheel (and therefore teeth) is too small for the sensor to react to the passing tooth. Unfortunately the space in an R6 engine gives no room for increasing the size of the trigger wheel.

To attenuate the signal, where is the resistor placed into the circuit with the two-wire sensor?

[Kirk Feldkamp]

My understanding is that it's not the gap that necessarily has the largest effect on voltage... but more the speed with which the teeth pass the sensor (for a mag sensor... aka variable reluctance... aka "VR"). Basically, I think the voltage corresponds to how fast the magnetic field changes. I'm sure someone will correct me if I'm wrong. Larger wheel OD's and higher RPM's would then tend to produce higher voltages with mag (VR) sensors. I don't believe increasing the tooth count will have an effect on the voltage... just how many times it crosses the 0V level per revolution.

You can always chuck a trigger wheel up in a lathe and play with the sensor gap, tooth count, speed, etc. to prove this to yourself too. All you would need in an oscilliscope!

[Jimmy_seven]

How thick is the trigger wheel you're using? With lots of teeth and a small OD the sensor can have trouble distinguishing between teeth. Using a thicker disc can help. I think the only way to really find out what is going on is to get a scope on it and have a look at the signal.

[Kirk Feldkamp]

For reference:

h t t p : / / www . megamanual . com / ms2 / pickups . h t m

[VFR750R]

I forgot to mention before, make sure your speed sensor is not vibrating or moving around. Make sure your bracket is stiff. A vibrating sensor can double count teeth, and confuse the ECU.

Also, I'm not believing that an inductive sensor can't handle the speed. Most can register frequencies of 12-15kHz, which is waaay higher than what you're talking about. Make sure your tooth depth is deep enough, depending on the sensor, at least .150" but more won't hurt anything.

I agree on the thicker disk. Bosch recommends 5mm or thicker for their sensors. Don't forget that crank thrust and/or deflection may mean the wheel changes gap or axial location. I've seen dampers off the nose of a V8 engine deflect .025" radially using high speed video!

[Steeeeeeeeeeve]

This may be unrelated as we run a WR450 with a custom trigger wheel, but we had a problem where our sensor just wasn't working at a high enough frequency with our old 60-2 wheel. We went down to 9 or 12 teeth and that solved our problem with the high RPM cutout.

[Jimmy01]

We have had a similar issue in the past. R6 with CDI and Autronic ECU. Our issue was related to the timing of tooth pulses - Or sync (cam) signal was occuring at almost precisely the same time as the crank signal. The engine wouldn't rev past 7/8000 rpm but was immediately fixed when we moved the position of the crank sensor.

The other option relates to your trigger wheel dimensions. Thickness as others have mentioned may be the problem but the ratio of tooth width to air gap is also very important. This Honeywell document has info about trigger dimensions that they recommend for their GT1 sensors.
Honeywell GT1