Wheres my tach. sensor located on my 91 gmc 1500 sierra?

the tach is doing its own thing,engine running fine. thanks for responding

Now that we have that out of the way , it's time for you to 'make nice' with a parts guy at your local GM dealer and see if you can get some info from him or his favorite tech in shop to find out any common issues they had with the tach or signal circuit (possibly a printed circuit board issue) Either way , given the age of the truck , and if it's running good ,I'm not sure how much the tach would concern me (relative to expense of repair). You could try removing and un-plugging then plugging the instrument cluster back in check all pins for corrosion.might just be worn contact after 21 years. good luck! hope I've been a little help.