Car motor tachometer question

[GWTW-000]

I need some help on a tachometer question. Are all tachs equal and you just buy the one that fits your range and is waterproof. Or do you need to match the tach to the distributor. Just a little background, I have a HEI distributor on a dd 500 caddy. Last week I had some wiring short out and burnt some insulation back to my bilge switch at my instrument panel. Now when I run it I show about 1000 at idle and when I rev the engine the tach go to 0. Any help please?

 

[bravo1218]

I would check the ground wire for the tach. Take a jumper wire from the ground terminal of the tach to a known good ground and see if that helps. If not check the voltage at the + terminal of the tach. Should be al least 12 volts. Check all this with the engine running above an idle (when the tach reads 0).
Now to answer you question. Most marine tachs have a multi position switch on the back of it to adjust for # of cylinders. Instructions will tell you where to set it for a 4 stroke V-8.
Tim