Tachometer question

[Highway by the Sea]

I bought a factory tachometer from a junkyard 74 Charger and installed it in my old Charger.  The mechanic that helped me hooked it up directly to the coil and it worked fine.  Since selling my Charger I installed the same tach in my Plymouth, but the (wiring) mechanic told me not to hook it directly to the coil (citing feedback issues).  So the tach is not hooked up until I buy the new tach harness on EBAY, or I hear that is is safe to hook it directly to the coil.  Any thoughts from electrical experts?

[Nacho-RT74]

is the only point to hook it up!

except with MSD

[Highway by the Sea]

The wiring diagram says to hook positive up to the fuse block and and negative to the coil (which I cannot do right now).  So I am wondering if it is safe to hook both positive and negative directly to the coil until the day that I can do it the diagram way.

[Tilar]

The positive on the coil does not have enough voltage to run it. You need 12 volts to the pos on the tach and the pos at the coil is only 5 to 6 volts due to going through the resistor.

You could probably tie into the ignition side of the resistor and be ok.

[Nacho-RT74]

use the accesories source on engine harness...

[Highway by the Sea]

Nacho, are you referring to the thing on my fuse box marked "ACC"?.  I do not know what you mean.  I think that I need to buy the tach harness on EBAY because it appears to have a required diode-thing on it that my home-made harness does not have.

[Tilar]

Anything that gives you 12 volts when you turn on the ignition switch will work.

The only reason I could imagine you needing a diode in the line is if you are trying to wire directly to the alternator, and I would advise against that.

[Nacho-RT74]

acc source on fuse box will work, but there is also source on harness, around Radio area with Tan wire and yellow rubber plug with three female bullet terminals inserted. Tha's the tipically used and protected by fuse on fuse box. ACC source on fusebox is not fuse protected, so will need to attach one on wire.