I am most impressed, BB! But I have questions....
I assume this is going on your 80-framed, 79 series pickup. Are you an electrical engineer? A genius polymath? Are you contemplating reverse-engineering the 79-series instrument cluster to the point where you can make that tach and all those myriad trouble lights work? I stare at the wiring diagrams for the 80 and just kinda give up after a while, especially when it comes to the ECU. The 75 series wiring diagram is a lot simpler, but I have not looked at the 79. From what I can see of the instrument cluster it looks much more advanced than what I have been dealing with on the 75 series.
All of this hits quite close to home. I've got a petrol 75 troopy and pickup and a 75 diesel, none of which have tachs. I was just talking about this with a friend down in Abu Dhabi. His view is that I should not need to know the RPM. I, on the other hand, struggle some times depending on how noisy the environment is, and I really want to add at least a tachometer onto all three vehicles. Two of them have the sub tank binnacle, one does not, but I could use a binnacle housing to add a digital tach. I'd much rather have the tach next to the speedo and be of a consistent style, just like you have accomplished.
I've never considered trying either to use a cluster that has the tach or adding a tach to a 75 series cluster. I would love it if you would go deep on the electrical engineering aspects of this. I know the 75 is much more primitive, electronics wise, but I'm sure I would learn a lot from you.
Thanks for posting this thread!