98 t56 bird speedo reading twice the rpms
 

Car sat for a week. Now all of the sudden if I'm doing about 60mph, it reads 120-130mph. Wtf... Any ideas off hand? No abs light. Nothing on dash at all! Thinking bad ground... But where to start or is it something else? Hopefully someone has had this issue before and can shoot me in the right direction for a fast fix.
Thanks in advance

Also to add... It's rained a lot in FL lately also...

Tach OR speedo??

Speedo

So road speed, NOT RPM....

Ok so speedo needle is just buried at all times almost... I can't find any part kits for stepper motors like i did on my gmt800. Since they fail a lot and ebay is riddled with kits for them. Anyone know? I guess maybe junkyard and solder suck a stepper motor of a v6 cluster? Any thoughts? Or maybe just swap cluster faces with a v6? Anyone ever do that?

The speedo uses a pulsed signal from the ecm via the cross car harness (4k pulses per mile if I remember). If there were an open - it would stay at zero. If it were a dead short (can happen from the radio via speed comp volume) it would still not move. If it were a bad ground you'd see problems in the other gauges, as I believe there's only 1 ground pin on the gauge pack - I'd have to go double check the pinout.

Does this ever move at all, such as shutting down and powering up? If so, the ground to the gauge is probably good - leaving a stepper motor, or circuit problem.

It could be a bad gauge pack. I've seen a guy on the tube that does rebuilds of gm clusters. Takes like 10 mins to swap them out - is there anyone nearby that can swap as a test?