I think you've pretty much narrowed it down to the unit itself, if I'm reading you correctly. You might be able to get a speedo shop to fix it. Your other choice seems to be a new unit.
Took speedo off my bike and put on buddies bike, and it worked.
Took speedo off my buddies bike and put it on my bike, and it didn't work.
If your speedo worked on your buddy's bike, there can't be anything wrong with his sensor, cable, ECU or wiring.
DISCLAIMER: All observations made in this post are mine and based solely on my own anecdotal experiences, and may contain large doses of facetiousness. YMMV, of course. You are "on your own", and I take no responsibility if someone tries anything in this post and gets into trouble with the law, damages their person or property, or goes blind. Take everything you read or hear "anywhere" butt especially on the Web with a large dose of salt.
cpmodem screwed with this post 05-27-2011 at 10:19 AM