measuring tach signal

How do you measure the tach signal with a multimeter.  I gotten a wire that has a 12v when the vehicle ignition is on and then a 6v that bounces when the car is running.  But on the ac side all i get is constant 13.2 volts. 

I'm trying to figure is the ground of the multimeter goes to the ground on battery and positive to tach wire.  Do i use the meter in DC or AC to determine if that's the correct tach wire.  What reading should i see once i do connect to this wire.????

With voltmeter set to AC, its positive lead applied to 12V+ and the negative lead attached to the target wire, the meter should indicate one volt or more (AC). With the vehicle running, the voltage should rise as the accelerator is pressed. What vehicle are you trying to find a tach signal for ??