but dont you think it is ironic that the rpms are exactly 2/3 of what they should be? this simple fact leads me to believe that the rpms have to be getting pulses or at least some timing. and therefore couldnt that timing be adjusted by more/less resistance? ...
It's not ironic - it's entirely predictable. I suspect it's totally beside the point as well ... :thinking:
It seems to me the real issue is how (i.e., on what basis) the tach unit converts the incoming sensor signal into a measurement of rpm's to be displayed on the tach face (by moving the needle).
If the rpm's were 'computed' solely on the basis of the amount of voltage on the input line per se (e.g., 1200 millivolts = 1200 rpm's; 3500 millivolts = 3500 rpm's; etc.), then yes - I could see how manipulating the amount of voltage would affect the tach reading.
But I don't think that's what's going on here ... It seems to me the calculation is based on number of pulses - each one of which is recognized as a *change in voltage*, not an amount of voltage.
Changing the overall 'voltage throughput' on the incoming line shouldn't have any effect on the number of changes in voltage detected on the line (at least not until you reach some point where the modified voltage is such that the changes defining a 'pulse' become impossible to 'read').
To use a very crude analogy ... It seems to me that adding resistance might make the incoming signal 'weaker' (voltage-wise), but do nothing to change the rate at which the modified voltage fluctuates. 1200 'weaker' pulses would still be 1200 pulses counted (with whatever equivalent reading being displayed on the tach face).
I don't claim to know exactly how the circuit works, and I'd be happy to stand corrected ... At this point, though, I still can't see how modifying the line voltage is supposed to affect the number of 'pulses' (which I'm pretty sure is what's being used as the basis for the rpm reading).