Frequently Asked Question

Overview: 

The tachometer measures the engine RPM by determining the frequency of the tachometer signal from the engine. This information is combined with the pilot’s entry of TachP/R (EMP on very old instruments), which is set according to the number of tachometer pulses per revolution of the engine, to allow the EIS to calculate engine RPM.

In general, 4-stroke engines like Lycoming, Continental, Rotax 912, Subaru, etc, will provide a dedicated tach signal, or a something almost as good (like the p-lead from a mag). Tachometer sensing from 2-stroke engines and HKS generally require that the lighting coil be used both as a tach source, and to charge the aircraft battery via a regulator/rectifier. Some 2-stroke engines will have other signals that could be used as a tachometer source.

Lighting Coil Equipped Engines (2-stroke engines, HKS, Jabiru etc.)

The switching characteristics of the regulator/rectifier is highly dependent on its type, the design of the lighting coil, the battery and its state of charge, the electrical load on the engine, and the engine RPM. Regulator/rectifiers generally operate by shorting the power input to them (the lighting coil leads) to control charging and thus regulate. Since the lighting coil is also used as the tach source, there are situations where the tachometer signal can be affected.

Generally, if the tachometer reading is not erratic, and the TachP/R (or EMP) setting is correct, the tach will be accurate.