It’s the red thing in the lower left hand corner of your image with the 10A fuse in the top. Check the fuse to see if it’s burned.
Oops you said you did that my bad
Okay other things that could be wrong. Your connection from your rpm pickup to your tach could be corroded. There is a video on YouTube of a guy finding this break on an 84/84 SD. I think the plug that was bad is right behind the valve cover. Trace the wiring from the sensor on the bell housing (right near the oil filter) on up eventually to the dash. I’ll se if I can dig up a video.
ETA - ok, it wasn’t a vid. It was a guy I helped on FB with a bad tach on an 84. He had a broken wire here. It was the sensor wire coming up from the bell housing pickup.
Check this. This wire should eventually find its way up to the tach. It’ll probably get routed through the EGR computer where there are also some potentially bad connections.
