The blue wire is actually the coil "control" wire.  The ignition module in the footwell makes and breaks the electrical circuit to the coil.  That is its main purpose, not tach sense.

You can just unscrew all of the wires at the module. Some of them (two of them?, the blue one for sure) are hot with the key on, can't remember which.  So don't bundle the ends together, keep them separate and tape them up.  Or check the wiring diagram and isolate the hot ones. 

7 hours ago, Zed Head said:

The blue wire is actually the coil "control" wire.  The ignition module in the footwell makes and breaks the electrical circuit to the coil.  That is its main purpose, not tach sense.

You can just unscrew all of the wires at the module. Some of them (two of them?, the blue one for sure) are hot with the key on, can't remember which.  So don't bundle the ends together, keep them separate and tape them up.  Or check the wiring diagram and isolate the hot ones. 

Wouldn't that disable the blue wire or would that just allow the tach to work without interference?

The blue wire will still be connected to the tach.  It just won't be connected to the old ignition module which seems to be interfering with the new ignition module's purpose.  The wire branches in to two after it leaves the coil, one branch to the tach, one branch to the old ignition module.

6 minutes ago, Zed Head said:

The blue wire will still be connected to the tach.  It just won't be connected to the old ignition module which seems to be interfering with the new ignition module's purpose.  The wire branches in to two after it leaves the coil, one branch to the tach, one branch to the old ignition module.

Gotcha - I'll give it a shot and let you know tomorrow if it works.

There is something odd about your situation though.  I think it's why I got confused.  The blue wire also branches to Pin 1 on the ECU to know when to open the injectors.  Is he running carbs?  An EFI engine should not run if the blue wire is not connected to the coil negative terminal.

1 minute ago, Zed Head said:

There is something odd about your situation though.  I think it's why I got confused.  The blue wire also branches to Pin 1 on the ECU to know when to open the injectors.  Is he running carbs?  An EFI engine should not run if the blue wire is not connected to the coil negative terminal.

That's odd.  Nope, standard L-jet is connected.

Maybe somebody has been in there.  Anyway, the old module should still be disconnected.

Here's some reading.

http://www.atlanticz.ca/zclub/techtips/wiringdiagrams/75_280z_wiring.pdf

More.

19 hours ago, Zed Head said:

More.

Disconnecting the blue wire from the ignition box worked.  Should we leave the other wires connected or remove the box in full?  All wires except the blue are still connected and the car runs with the tach.

Look at the diagram and do what makes you feel comfortable.  If the Crane module fails it will be easier to reconnect the old one.

If the blue wire is disconnected, the rest of the ignition module is just along for the ride.

It'll still be reading the input signal and switching it's output stage, but that output stage will not be connected to anything, so it will have no effect.