<VV>Tachometers

[Padgett]

>However, the third
>option was listed as "complete with sending unit" and was only seventy five
>dollars.

In another like, I used to repair tachometers and the GM circuit used until 
the HEI ignitions came out in 1972 was simplicity itself, totally passive 
and powered by the coil. Used a zener diode for clamping. RLC circuit 
converted pulses from the primary coil into an analog signal that drives a 
milliampmeter (moves the needle). Aftermarket tachs were touted as having 
"transistor circuits" and were considerably more accurate (though the GM 
can be quite accurate if calibrated).

Easy way to tell the difference is that the passive tach just has one wire 
(if there is a power lead it is for the illumination) while the transistor 
tach also requires 12v to operate.

The difference is in the input circuit with a different capacitor needed 
for 4, 6, and 8 cyl engines.

Passive GM tachs are not compatible with transistor ignitions since they do 
not have points and reverse the coil signal. I have posted on my web page a 
one transistor circuit to comvert a passive GM tach for use with a 
transistor ignition that still has a coil, it is the same circuit GM used 
with the Delcotronic ignition (1962-1966).
http://padgett.performanceresearch.us/cars/ttach.htm

When the tach says it needs an "adapter" that used to mean the tach was in 
two pieces, with the movement/needle/dial in one assembly and the 
circuitboard in another. Reguardless, as long as the tach is designed to 
work with a six cyl points- ignition engine, it should work in the Corvair.

Padgett