Alternator W terminal

[superheat6k]

I would expect the resistance power value to be minimal power because the tacho is not presenting a load, just counting the pulses. For the capacitor I expect similar value to a car suppression capacitor would be about right. Now mine are set up they seem reliable enough at speed, but are a bit erratic around idle, settling between 600 - 900 RPM.

 

[William_H]

I would expect the resistance power value to be minimal power because the tacho is not presenting a load, just counting the pulses. For the capacitor I expect similar value to a car suppression capacitor would be about right. Now mine are set up they seem reliable enough at speed, but are a bit erratic around idle, settling between 600 - 900 RPM.

Don't quite know what a "car suppression capacitor" but presumably around the 1 microfarrad. I think this would if connected on the tacho side of the resistor swallow all your pulses. For noise suppression I would try around .001 microfarrad.
Theory
Capacitive impedance is given as 1/2 piFC so 1/ 2x3.14 x freq (220 to 660) x 1 microfarrad would look like an impedance of around 241 ohms (at 660pulses per second) So .1 microfarrad would give impedance of 2410 ohms. Now the impedance looks to AC like a resistor so if it follows a resstor of eg 24kohms you have a voltage divider giving 1/10 output
With a 24k ohm resistor and .01microfarrad (impedance 24kohms) you get a divide by 2 divider. So I would suggest .001 microfarrad to lose 10% in the divider. (90% out)
(A voltage divider is where if you connect 2 resistors across eg 12v then the voltage measure from negative to the join of the 2 resistors relates to the ratio of the bottom resistor to the total resistance. (further affected by current drawn) So a resistor from W terminal to the tacho will not affect result because little current flows. But when you put a bypass capacitor across the tacho input you lose some of the pulse (AC) voltage. Of course the purpose is to cause much more loss of high frequency interference.
Here endeth the lesson ol'will (yes I am old and have probably got the maths wrong somewhere)