I do have a milling machine that will run about twice the speed but no easy way to chuck up the wheel. In any case I ran it on the car and saw absolutely no improvement. But since I had the leads out to hook up the diode I measured the sensor output with the load of the speedometer installed. It appears that with the speedometer installed I loose about 25% of my voltage. That means when I clip the signal at 12 V I will still have peaks at 12V and at 6V. So it appears I need to clip at 6V or less. I don't have problems below 60 mph but I would like to get some margin on this and clip at 3V.That's encouraging.
Would a drill press give a higher speed?
I should be able to get away with a threshold of 1.2V since the speedometer doesn't register until 10 MPH. Or higher if I decrease the gap. I will try the zener diode trick since I have spares. So the way I understand it is to put the zener in series like in the picture I posted in post #26. Which side does the diode stripe go to.The noise pulses are ~0.4V amplitude, so a circuit which responds only to signals above, say, ~0.6V should eliminate the noise problem. Try simply connecting a forward-biased diode (one from the 1N400x series, or even a spare zener diode of any voltage rating) between the sensor output and the speedo input. This might not work, though, since noise amplitude will increase as the revs go up. A circuit with a noise threshold which increased with rpm would be better, but a good bit more complex than you might want.
A Hall sensor would almost certainly avoid the problem. Mercedes may be more comfortable with the older technology, and there would be re-tooling costs to consider if they changed.
I have assumed that too. From your scope and meter readings it seems the pullup resistor is internal to the sensor, but if it's not it would be simple to add externally.I assume that everything in the blue dotted box is what is already inside the hall effect sensor
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by Gary Elinoff
by Gary Elinoff