im assuming you have an HEI distributor in your car.
an HEI will only accept an ELECTRONIC tachometer - unsure of what you might have, but an electronic tachometer is all it will be able to use.
there is a terminal on the cap of the HEI that will have the word "TACH" on it. Your wire from the tach should go there. It will take one of these kinds of terminals
http://www.allelectronics.com/images...large/1125.jpg
you should have most likely 4 wires from the tach
1 needs to go to a good ground
1 needs to go to the TACH terminal on your HEI
1 probably goes to switched 12 volts (from your ignition switch)
and 1 will most likely go to your dash lighting so when you turn your headlights on it will also light up
if you have an HEI, and you have the tach wire attached to the terminal marked TACH on the cap, then find out if the tach is electronic
next - your volt meter
im a licensed electrician and i build big fancy buildings, and i understand DC theory, but im not an automotive expert (especially on computer controlled cars), so take this advice for what its worth, and thats not much - im just now getting back into old cars and ive been out of the hobby for a while
your car is a 12 volt system
going of memory, a fully charged battery with the ignition off should give you a reading of somewhere between 12.5 - 12.8 volts - not running.
when your car is running the altenator/generator takes over and produces electricity to charge your battery - with the engine running, the volt meter should read about 14 - 15 volts cause of the altenator. The volt meter should stay around 14-15 VDC until you put the electrical system under load (turning on the lights, wipers, heater, AC, stereo, turn signals, etc. etc.)
if when you have the car running, & you turn on any accesories and the volt meter reading falls below 12 volts, then that means your battery is providing some of the current to feed your accesories. Driving around like this for a while will drain your battery and it might be hard to start your car after you shut it off.
if the voltage is constantly below 14 volts, you need to have it checked and replace whats bad cause your charging system isnt working 100%
if it ever goes above 15 volts, you need to check your voltage regulator
hope this helps a bit
sorry for the long post
Mike