I know this is an old thread - but I was able to fix my tach pretty easily. It turns out the grease in the gear box turns into glue, and everything binds up. When that happens, the gear attached to the input breaks loose from the shaft, which spins inside it. I fixed it by pulling the gear off the shaft, Then I roughed up the areas where the two meet, both the shaft and the gear (I used a vice-grip to crudely knurl the shaft). Then I used some super-glue to re-attach the gear to the shaft and let it dry. Finally, I put it back together with some silicone grease (the clear stuff also called dialectric grease - not the nasty stuff used to coat the backs of brake pads). It's been working great for several months now. BTW - this is a technique used by owners of old MB and SAAB cars to fix their stuck VDO odometers.