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Recently, I asked a question about the speedometer, how it works and how accurate it is. But that got me thinking... how does the rest of the dash actually work?

I understand that the RPM measurement is a measure of the rotational speed of a mechanical component (the crankshaft) with respect to time (in minutes), partly thanks to this source from Wikipedia.

How does the tachometer actually work?

  • Is there some sort of sensor in the crankcase that can count each rotation?
  • If so, how does that work?
  • How does the data recorded from whatever it is that records the data get displayed on the dash? Electrical or mechanical signal?
  • How is the tachometer needle calibrated to move the right amount and point to the correct number (rather than just spin round in a circle or not move at all)?

Note: Answers can exclude fancy dashboards such as the new Audi "cockpit" dashboard because that's clearly just a computer running bespoke software on an unusually shaped computer monitor. I want answers to explain information about the traditional, needle based tachometer found in the majority of cars (as far as I am aware).

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While i'm not sure if there are mechanical tachometers like speedometers electronic tachometers are quite simple.

An electronic tachometer works like an old analog volt meter.

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The speed of the engine is converted to a voltage. The voltage is fed to the moving coil. The coil creates a magnetic field. That field of the coil tries to align itself with the magnetic field of the magnets. This causes coil to move against the spring deflecting the needle. By combining input voltage, the number of turns in the moving coil, the strength of the magnet and the strength of the spring a ratio is achieved that the needle will deflect a number of degrees per volt.

Electronic ignitions turn the ignition coil on and off in pulses to create the spark. These pulses directly correlate to how fast the engine is turning. For example a V8 will have four pulses per revolution. These pulses are converted to a voltage with a frequency to voltage converter. There are discrete circuits or stand alone chips that can preform this action. For example GM HEI ignitions commonly tie into the signal wire to the ignition coil to get the RPM reading.

Those pulses were, like in the GM HEI, generated by the ignition module from input of the pick up coil in the distributor. In these systems the timing is controlled an entirely in analog fashion. Fly weights and a vacuum advance control the timing in the distributor.

As ignition systems became more complex the pick up coil was replaced by a crankshaft position sensor and the fly weights / vacuum advance were replaced by a computer. Eventually instead of getting the pulses from the coil the signal came directly from the computer. Even to this day the basic structure and operation of the physical needle in the dash has remain effectively the same, the difference being only where the signal comes from and how the signal arrives at the dash board.

Since the engine computer knows the RPM digitally, in digital dashboards the engine computer sends the information to the dashboard over the network.

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