Outer Limits 4x4 Board

hey guys bought a coil cab ute with a 6.2 chevy diesel in it already on the cheap and the tacho has never workd could anybody tell me or give me any info to why it wouldnt be working were its run from etc. i havnt started looking into it as i have no idea were the feed comes from etc.. any help and information is greatly apreciated and hopefully it will get me on the right track..

I run mine from a tacho sensor on the top off the oil pump drive , some set up use this as the spot for the vacuum pump thought. others run from the back off the alternator, and the later 6.5's have a pick up from the from of the crank in the front cover.
I have my tacho recuperated to suit the v8.