I have a 93 Ranger(4.0/5spd). i bought it needing a tranny, so i never started it, but after i got done the tach just goes nuts. When you start it, it shows about around 1000 for an idle but as soon as you rev it it will start running backwards or peg forwards. Just all kinds of weird stuff. I was told a noise cap. could cause this, so i replaced the cap, and still does it. Anyone had any experience with this prob. A used gauge cluster is $40.

 

Check any ground wires that may have been diconnected while doing the tranny work. My son had to have a radiator replaced in his car and one of the ground wires, for who knows what, was accidentally left disconnected. His speedometer would do the same thing you descibed about your tach plus the dash lights would come on and off by themselves. I also have a jeep that did the same thing as your truck, The tach would go mental, It also was just a loose ground.

 

I have pulled the gauges and the dash apart twice now and have found nothing unhooked or loose and im constantly under the hood(for custom reasons,lol) and have never noticed anything off, but ill check it out anyway. thanx