My troubles with a Sigma BC800 wireless comp came down to a weak battery. The clock worked, the buttons worked, but it would not register anything. I would try a fresh battery first.

Then go and see if your LBS has Cateyes on display, with batteries. You could check whether one of those register anything when mounted on your bike. If they have bikes with Cateye wiring, you could try your comp there to see if it registers anything. That should help narrowing down the search.

--J