Tachometer not working

[SchwarzStock]

Trying to put in a Auto Meter tachometer but it does not work.....

I've got a Mallory HyFire ignition box so the tach should be connected to the yellow wire from the box. I have power to the light in the tach and the tach needle jumps when you start the motor. I called Holley/Mallory/MSD and they said I need to check the input wire (yellow) from the box. The guy I spoke with said I could use a simple tester like you use in the house. That does not work! I am supposed to have 12 volts square from the yellow line, suggestions how to test that?

[SchwarzStock]

I was able to get it on an oscilloscope today and the yellow wire is dead. So, I started to really look closely at the Mallory box. It is connected with several wires to the the solenoid but it is not connected to the distributor or coil. Looks like the box is totally inactive. Next move will be the extract the box out of the wiring.... I have to wonder who put the box in and why the installation was not completed. Someone took a lot of time to wrap all the wires up in black tape making it really hard to know which wires are going where.

This afternoon I called Holly/Mallory/MSD and they concurred with me; the box is not doing anything as currently installed. Looks cool though....

Oh, the tach, I will attach that to the coil in the usual manner. I expect that will go trouble free.

SS