I am swapping a 56A 4 speed for a 5 speed on a 79 B120
The 56 has a top gear switch fitted and wiring connected.
The 60 has no top gear switch
I think this switch does something to the timing?

When I fit the new box with no switch, should I leave the circuit open or join the wires?

I'd need to read the wiring diagram to work it out Mal..

sounds like a pogostick kinda question though... lol

Just leave it disconnected. If you connect it, it reduces your timing in top gear (bad for performance).

thanks DD

are the thread and pitches the same for both switches?

yes threads are the same
but dont try to use a 56A top gear switch as a reverse switch on a 60A

Yes, one is normally open, the other is normally closed. So don't swap them unless you test 'em first.

how does it affect timing? i.e. by changing what and how?

There is a solenoid in the vacuum advance line. When top gear is engaged, the solenoid turns on and leaks air into the line. So you get zero vacuum advance. Less timing = less performance. But also less emissions. This is the Nissan TCVAS system.

It is an evil system as far as performance and fuel economy go. New cars also reduce timing at highway cruse, but they modify the timing advance electronically which is a much better way.

I always thought it would be a good idea to wire up the 5th gear switch to a sign in the back window that reads...'hammer time'. Also, you could wire it in series with the automatic kickdown switch on the accelerator so when you have both flat to the floor in 5th.. the sign lights up.

I grew up before I had a chance to make the sign..