on my 83 tdt the tach has a magnetic pick up . if that get real dirty / greasy then it wont work
the pick up is a round pointer located at the front of the block in the middle off to the right as you stand in front of the car it is round. clean that
next is a black round cyclinder located on the fender right side. it looks like a 35 mm film canister. take the cap off, pull the piece out of the cap place a cig butt in the cap and put it back together. that should work .
it puts preasure on the solder which causes it to work .

that is all.
john m