Looking for some help here hooking up the tach; without changing out the whole under dash harness. I have pulled this from another '67' but the dash wiring was beyond use or able to trase. I know the red wire goes to hot and the black goes to the coil to make the tach work. But, I am not sure if there is a ballast resistor needed to get the correct reading out of tach since it was never in the factory harness? Also, will this affect the starting of the car if I wire it to the back of the ignition switch lug? I had a '67' years ago and the tach went bad, so I remember most of this from way back when.........
I do not want to fry the tach...... If there is a resistor required, what size and if anyone knows where they could be purchased at this time?
Any help on this would be appreciated!!

 

Hiya
Ford had a very weird tach setup indash and rallypack.
A tach from a multimeter or aftermarket 2 wire would pick it up from coil- as you describe.
Aftermarket HEI have a single wire output.

but the ford factory tachs wire INLINE !!
basically this thing (if original ford dash tach) hooks between the ignition switch and the resistor wire.
there is a disconnect that you can use about 2" after the ignition switch that you can use ...

 

Now we know why your other tach got fried. The wiring connections you suggest guarantee smoke will come out of your new tach.

The resistance in the circuit was a function of the pink wire in the under-dash harness. All Ford cars had this, regardless whether they had a tach or not.

The ignition switch has a connector on the ign wire which plugs, very close to the switch, to the pink resistance wire. Disconnect at this point and connect the ends to your tach plugs. Getting the oil warning light to work will require a change of engine sensor. The ALT warning light will require a new wire run to the voltage regulator.