Morris Minor Owners Club

any advice welcome: after a very gentle bump (in the road and from the front of the car) the speedo ceased to work. Inspection showed the cable to be too long, coiled once under the sump and the outer sheath had been worn through. I obtained and fitted a new cable. No difference. I connected an electric drill to the gearbox end of the cable and the speedo worked as normal at all speeds. I inspected the drive from the gearbox and replaced both the gear/pin and the oil seal, which the cable screws onto. No difference. I borrowed two spare speedos, both of which ran normally on an electric drill. Both of these spare speedos behaved exactly the same as my original speedo, which by now is afflicted by some form of extremely irregular St. Vitus dance, when connected to the gearbox. There is no consistency or pattern to the speedo movements. Any suggestions please for a cause/ solution.

Thanks (Stoneporch).