Tachometer: Description and Operation

ENGINE TACHOMETER

The tachometer displays the engine speed in thousands of revolutions per minute (RPM). The noise suppressor filter sends ground pulses from the ignition system to the tachometer. The tachometer responds to the frequency of the ground pulses, which increase with the engine speed. The purpose of the noise suppressor filter within the circuit is to round off pulses and remove the voltage spikes that noise in the circuit causes.

When the fuel level is low, the resistance in the fuel level sender is high. Current flows through coil F and coil E to ground at G103 because current always seeks the path of the least resistance. Because the length of the E coil winding is twice the length of the F coil winding, the magnetic field that the E coil generates is twice as strong as the magnetic field that the F coil generates. The magnetic field therefore pulls the pointer to the E position.

As the fuel level increases, the resistance in the fuel level sender decreases. Since current always seeks the path of the least resistance, more current begins to bypass the E coil and travel directly to ground through the fuel level sender. The coil F magnetic field then becomes stronger than the coil E magnetic field,causing the pointer to move toward the F position.