No I'm afraid not, unless I've misunderstood you,
If we imagine the tachometer as a brush connected to the tachometer output wire, the ticks are registered every time it touches one of the 4 charged output plates around the circumference of the motor, the problem is, these plates are only charged when the motor is powered, so if the motor is controlled with PWM the power on the plates changes between high and low, therefore giving false readings. I need to provide a constant power to these plates so that my tachometer doesn't fluctuate and give false readings.
Hopefully what I say is starting to make sense now.
Thanks
Chris