Technical Description: Tachometer: Notes

The main instrument unit has an electronic tachometer which is supplied with current and controlled by the main instrument unit's control module.

The control pulses which the control module needs in order to control the tachometer are obtained from the engine management system on pin 12 (K22). The signal is a square wave pulse which varies periodically between B+ and 0 V. This happens twice per engine revolution,

At engine speeds below 1100 rpm (idle speed range), the display is damped to give a steadier reading.

The main instrument unit sends engine speed information out on the bus as Engine speed, unit rpm.

Fig 1: Identifying Electronic Tachometer

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Fig 2: Identifying Electronic Tachometer Wiring Diagram

Courtesy of SAAB-SCANIA OF AMERICA, INC.