Is it my alternator?

[Cubey]

I watched this guys video above a couple times and read some things here and there and I'm sort of confused about what happens to the other wires running from the voltage regulator particularly the wire that runs to the tach sensor. Will it still work if I just cap this wire off? What am I supposed to do with that?

IDI tachs don't run from the alternator. They run from a two wire sensor on the timing cover, right next to the engine oil filler.

 

[MtnHaul]

And of course with a manual transmission a tach is just for show anyways.

 

[Cubey]

And of course with a manual transmission a tach is just for show anyways.

Econoline never had tachs as an option at all, until the 92+ generation. My 89 E350 with E4OD has the "tach" sensor for the transmission operation, but no tach.