my tach is intermitten, i swapped it out and the new one is internitten

i know it runs off the alt, is this a common problem? if so is it usally w/the tach or the truck, never know when you have gotten a bad used part

 

It is ALWAYS with the alt. The tach gets its sig from the alt, when your alt goes bad your tach stops working.

 

so, will it start showing an alt problem before other signs do

can you swapp the brushes/voltage reg?

 

Yes you can rebuild the alt.
When the tach doesnt work then the alt is not charging at all and you are running off the battery alone.