quote Jeff Register:
Dave is correct. The tach sees the AC pulses made by the alternator. Just be sure the tach uses the same number of pulses per revolution as some stators are built different.
This type of tachometer is essentially a frequency-to-voltage converter – it converts the frequency of the alternator (or sometimes, the breaker point) waveform into a proportionally scaled voltage. The meter mechanism itself is actually a voltmeter, which displays the voltage as a specific RPM on the meter face.
Point is, the ‘meter’ end of things is an analog circuit, and as such, should have a fair range of adjustment (for calibration). Even if the tach isn’t designed for your specific system, it’s quite often a simple job to adjust or slightly mod it such that it will work. It may be as simple as turning a pot shaft, or it may require swapping out a couple of resistors.
I recently repaired & calibrated the tach from a 1968 Jaguar E-type – which is of the same design. The Jag takes its signal off the points, though, rather than the alternator. Car alternators are belt driven, and any slippage or change in drive ratio will skew the RPM figure – plus the AC signal isn’t usually available externally, unless a ‘TACH’ terminal is available.
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