Re: Stock Tachometer Wiring and HEI
There are two types of tach's.
There are ones that work by measuring voltage pulses at the coils (-) terminal, and then there are the ones that measure the current pulses flowing into the (+) terminal.
The 6 and 8 cyl tachs are the current pulse type.
Power from the battery flows through the ignition switch, through the tach, in one red wire and out the other, then it flows down to the (+) terminal of the coil, through the primary winding, out the (-) terminal to the module or points, and to ground.
That power only flows when the points are closed. This means that the current, which is a measurement of the flow of electrons, is pulsing every time the points (or module transistor) close and complete the circuit.
The tach uses these pulses to measure the RPM.
Now when you switch modules you might not be getting the same type of pulses. If the module does multi spark then it's producing several pulses for every one pulse the old module produced. If new module may also take current for its own internal circuits which the tach counts as a cyl firing. The width of the pulse (dwell) may also change which could account for the errors in the readings.
The new modules may also change how the coil gets its power. If the coil is getting power directly from the battery by passing the tach circuit then there are no current pulses to measure. If the module is filtering the power to coil there won't be any current pulses. If it's a capacitive discharge unit there won't be any pulses.
Some of the modules provide a simulated tach input but those are usually designed for the voltage type tachs which sence the (-) terminal voltage pulses.