HOW TO SETUP YOUR TACHOMETER
The correct installation of the tachometer and the determination its correct settings are crucial to get an accurate indication of the Engine Speed.
This paper provides guidance about how this value is calculated and how the settings are affecting this measurement by indicating the methodology for the correct pulses per revolution factor calculation based on the different installations of the sender.
CALCULATING THE ENGINE SPEED
The tachometer is basically a frequency reader.
The tachometer measures the frequency coming from the RPM source (f) and calculates the Engine Speed (RPM) through the Pulses Per Revolution factor (PPR) with the following formula:
RPM = ( f x 60 ) / PPR
The PPR factor must be determined by the user and set on the tachometer.
The calculation of the PPR factor depends on the which source is used for the tachometer input.
Let's see how to determine the correct tachometer settings, case by case.
Inductive senders contain a magnet with a coil around the magnet.
When a tooth of the flywheel passes in front of the magnet, a small current is generated in the coil, and this "impulse" is sensed by the tachometer to calculate the RPM.
Therefore, the number of pulses per revolution in case of an inductive sensor equals the number of teeth of the flywheel.
PPR = # of flywheel teeth
The number of pulses per revolution in case of a generator sensor is is fixed and provided by the sensor manufacturer.
PPR = # of pulses
ALTERNATOR “W” TERMINAL
The output frequency of an alternator depends on the number of poles of the alternator and the engine/alternator pulley ratio.
The number of poles can be read on the data plate of the alternator, or requested to the alternator/engine manufacturer.
PPR = ( # of poles / 2 ) x ( ØA / ØB )
If specifications are unknown, then it is often possible to connect the tachometer to the W-terminal on the alternator and adjust the PPR until the known idling RPM is displayed on the tachometer.
IGNITION COIL “1” TERMINAL
PPR = ( # of cylinders x 2 ) / # of strokes
or it can be obtained by the engine manufacturer.