RFERG43

I am building a microcontroller based speedometer for us conversion guys with mechanical speedometers. It will measure the width of the VSS pulse from the PCU to determine speed. What I need to know is does the PCU start the VSS signal at certain speed, say 3 or 4 mph or does it send it whenever it detects a signal from the sensor? This question applies to '98 and up LS1 PCUs.

Thanks.