I have an old Datamarine Wind Instruments - LX-360, I think. It has worked
fine up until now, but this year, the wind speed just reads 0.00, while
direction works fine.

I plugged the masthead unit directly into the back of the display, and there
is no change, so presumably cable is OK.

I see from http://www.dmimarine.com/ that there is a masthead speed rebuild
kit. Has anyone done this? Are there any checks that I could make to see
whether problem is in head or in masthead unit?

Thanks