In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT) side of the ignition coil, while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output. This is a special circuit inside the alternator to convert from rectified sine wave to square wave, and the electrical potential difference is directly proportional to engine speed. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) also exist - usually on simple diesel-engined machinery with basic or no electrical systems. On recent engine management systems found on modern vehicles, the signal for the tachometer is usually generated from an engine ECU which derives the information from either the crankshaft or camshaft speed sensor.
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