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Old 11-22-2019, 09:46 AM
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Tachometer Calibration

Is anyone able to offer a method of recalibrating VDO/MB electric tachometers so that they will correctly display engine speed from a source other than the one that they were originally intended for? For instance, how to recalibrate a 5-cylinder tach (original source OM617) to read correctly when driven from a 6-cylinder source (M110).

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Old 11-22-2019, 10:13 AM
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They count crankshaft revolutions, does not matter how many cylinders the engine has. I put a 123 tach in my diesel Blazer, I made a bracket to hold the sensor close to the harmonic balancer. (it has a small groove which causes a magnetic disruption which the sensor, well, senses)
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Old 11-22-2019, 10:47 AM
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For gasoline engines the signal source is the ignition system; the signal is called the "TD" pulse. When the signal source is via a magnetic pickup at the crankshaft (as in the example above) the signal frequency is 1 pulse per revolution of the crankshaft. When the signal source is the ignition system of a gas engine it generates a frequency that is a function of the number of cylinders in the engine. A four cylinder gas engine will generate 2 pulses per revolution, a six cylinder 3 pulses per rev, an eight cylinder 4 pulses per rev, etc.

As noted above, obviously a tach displays crankshaft speed.

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