[BHP231]

Hi all. I know there’s a lot written about this but I don’t see exactly what I need to know.
1985.1 car with the engine replaced with the same early type.
Since engine replacement the tachometer doesn’t work.
The mpg gauge does, the lights around that gauge do.
I’ve taken the instrument cluster out and removed the tachometer and cleaned it all up already.
So it seems the signal isn’t getting through or the tachometer itself is broken. (The needle moves slightly with the gauge in my hand so it isn’t frozen).
Any thoughts on how to trace the problem? Thanks.