I found this old thread while I'm searching for something else, but I have the solution. The tach on my '91 Dak was usually at 2,000 rpm forever, but sometimes zero, and sometimes whipping around like crazy. At least with it doing that, I knew the little motor inside the tach still worked. A couple weeks ago I took the dash apart and greased the speedometer cable, and while I was in there, I popped the circuit board for the tach off to see if I could fix it. This also works on old Daytonas that have a similar setup. The board is about 2 or 3 inches square and is on top of the cluster over the tach. Just lift the back of it off the main circuit board and then slide it towards the front of the vehicle (as the cluster would be installed). Look at all the solder joints. Sometimes you can actually see a cracked joint, but sometimes you just see a tiny divot in them. Fire up your soldering iron and reflow any and all questionable looking joints on the board. Make sure you don't use acid core solder. I'm not sure what the best kind would be, but I know acid core ain't it. Install the tach board back onto the cluster and put the dash back together. If none of the actual components were bad, your tach should be back in business.