Tach Signal Filter for Motorcycle

Thread Starter

Murizg

Joined Mar 23, 2018
49
Hello everyone!
Now, I have a situation with a Kawasaki VN 2000 from 2010. We installed a aftermarket Tachometer as the motorcycle doesn't comes with one and the customer wanted it. Installation was rather simple, tach was grounded to chassis, and with this diy adapter, I was able to get it to show expected RPMs. Photo attached below, credit goes to 4strokes.com for schematic.
Now, I do have a problem with the "bouncy" needle on the tach. The thing is, on low RPM's, especially when engine is working on idle, needle is quite bouncy as it would be expected since this is a V2 engine with almost 2000ccm. However, customer really doesn't likes it and I would need to smooth that out. After some research, I found out that I would need to either purchase or make my own Signal Filter. If I understood correctly, this is just a low pass RC filter consisting of one resistor and one capacitor. Signal from the coil gets in the resistor which is in series with the tach (acts as a load), and capacitor is in parallel with it. Please, do correct me if I'm wrong at this point.
However, what values should I know to try to calculate values for resistor and the capacitor in this circuit? Or does anyone else has a better solution?

Every info is very much appreciated.

Thanks
 

Thread Starter

Murizg

Joined Mar 23, 2018
49
And btw...
I just saw a guy on YouTube that just put the 100kO resistor in series with the signal wire from the coil to the tachometer and the needle works like a charm...
I'm really confused now...
 

crutschow

Joined Mar 14, 2008
23,312
With 100kΩ in series it may be using the intrinsic input capacitance of the tach to do the filtering.
It all depends upon the input impedance of the tach.

If you use a RC filter, the resistor is in series, with the capacitor going from the tach input to ground.
If you measure the input resistance of the tach, that would tell you how high a resistor value you can use.
Then you will likely need to experiment to get the right filter value.
Too low an RC value and you get needle bounce.
Too high and the tach will be sluggish in following the engine RPM changes.
 

Thread Starter

Murizg

Joined Mar 23, 2018
49
Thank you very much!
Do, you do recommend me to use a RC filter, not just a resistor in series?
 

Thread Starter

Murizg

Joined Mar 23, 2018
49
Will do. I will deal with it on Tuesday and I'll get back with the results, it might be helpful to someone...
 
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