I wanted to return my tacho from my autometer 5” to a stock functionality with the least fuss. This mod should work for any Nissan/Datsun direct input style (NOT induction) tachometer reading from a 5v square wave tach input eg. aftermarket EFI, CDI ignition module.
In the context of this write up I am using 280z tach internals in a 240z housing and the signal is generated from a MSD 6AL. I haven’t included dissembly/assembly of the housing in this write up, just mods to the PCB.
In the case of the 280z PCB, the input is a 12v feed and typically reduced to around 5V peak internally by resistors. Putting a 5V source through this will reduce the voltage even further (Ohm's law) and will not be recognised. All you have to do is locate this internal resistor and remove/bridge it out of the input signal line. In my case I tinned up from 8g wire and bridged out the resistor. Make sure there are no other forms of resistance between the tach peg and signal source and you should be right to roll.
There was a potentiometer in my tacho which was set to half way and I did not need to adjust.
If someone has covered this before please delete. I had a quick search through the forums and couldn't spot anything previous.