Tachometer

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Tachometer and hour meter.

A tachometer is a measuring instrument that indicates how fast the engine is running. This speed is indicated in revolutions per minute (rev / min). The tachometer can be connected to the engine in different ways, namely electronic and mechanical.

Mechanical tachometer.

With a mechanical tachometer, a cable runs from the tachometer to the motor that drives the tachometer. This cable is usually connected to the camshaft of the engine. The camshaft of the engine turns half the speed of the crankshaft of the engine, so the speed at which the cable is driven remains within limits. Because the camshaft has half the crankshaft speed, this is corrected in the rev counter. The impeller pump is usually also mounted on the camshaft, if available on the engine.

Tachnometer rev counter.

A tachometer uses pulses and is a so-called induction sensor that registers the passing clamps of a gear via a magnetic path. This tank wheel can be the starting ring around the flywheel of the engine. Yanmar uses this system on several engines. Volvo Penta also used a tachometer for the MD11, but used the camshaft for this. In the top part of this text you could read that the camshaft and flywheel do not have the same peripheral speed and this is corrected in the meter, this means that when purchasing another meter it must be adjusted to the pulse number of the sensor. There is a formula for this, but in practice this means that you can count down the meter on the back of the meter with an adjusting screw so that it is equal to the measured values of a manual tachometer. When replacing a tachometer or sensor, it must always be calibrated by means of a manual tachometer.

Tachno sensor from a Volvo Penta MD11.

Manual tachometer.

A tachno sensor can lose its sensitivity over time. By moving the sensor closer to the gear it can work again, cleaning the gear can also help to get the tachometer running again.

Tacho sensor with gear.

You can measure a Tachno sensor.

When the motor is stationary, the two connections must be measured between 1200-1700 Ohm, but first disconnect the wires from the sensor before measuring. You can also measure when the engine is running, an AC voltage of at least 1 volt must be measured.

Generating a signal can also be done by running a file along the tachno sensor without putting pressure on it. The file teeth give an asymmetrical sine wave that will yield tens of mVolts.

Test with meter with metal file.

Connecting the Tachno sensor.

Tachno sensor is connected to the G and W connection of the Tachno sensor, the polarity does not matter.

Dynamo tachometer.

Most modern diesels have an alternator with a W connection. The W connection of a 12 volt alternator has an AC voltage of at least 8 volts, the frequency of which depends on the speed and the number of pole pairs of the alternator. Most alternators have 6 pole pairs and each revolution of such an alternator produces 6 pulses. The tachometer registers the pulses and then calculates the speed of the motor based on this.

W contact.

Hour meter.

A boat does not have an odometer but an hour meter that registers the running hours of the engine. As a rule of thumb, it is stated that 1 operating hour equals 100 car kilometers. This is a rough approximation because a marine engine generally runs at a constant speed. The hour meter may only work when the engine is running and must therefore be connected with the plus to the 30/15 contact of the diesel ignition, but if the engine is turned off and the ignition remains on, the counter will continue to count. Another way of connecting is to connect the hour meter to the D + contact of the alternator.