If you have problems touching the signal cable then you must step down it's impedance with a pull up or down resistor.

In other order, i think Denis Kuppens' idea is good. I was listen it before and he says that works well.

The alternator cuestion is: I listen that in every alternator there is a point that gives you pulses. As the alternator is directly arranged to the motor you get pulses with frecuency proportional to the RPM. The only thing is to obtain this proportional constant.