What follows below is a very a shortened version of the many and various faults that can happen to a speedometer. These speedometers are old and usually respond well to a clean and service.
In practice a lot of problems are caused by the flexible drive cable from the gear box, being incorrectly routed to the speedo head. Any sharp bends should be avoided. The inner flexible drive of the cable needs to be lubricated occasionally, see the section on cleaning.
1) Speedometer does not work, neither does the odometer.
Check that the flexible drive cable from the gearbox is not broken. If it is replace it. If the
speedometer still does not work, the speedometer needs at least a service and probable parts
2) Odometer works but the speedometer does not.
This might indicate a broken rear pivot on the drum spindle. Service and replacement of spindle
is usually required.
3) Speedometer operates but the odometer does not.
Usually suggests that the worm on the eccentric is broken or seized. A new fibre gear of the
correct tooth count will normally need to be fitted as the eccentric spindle is most likely to seized
in the cradle.
4) Speedometer is very jerky and jumps between readings.
This might indicate a very worn rear pivot on the Drum Spindle allowing the Magnet assembly
to touch the Drum Spindle assembly occasionally. This is serious and the speedo should be
disassembled and repaired, as further operation might cause a very serious and costly failure.
5) Speedometer needle goes round and round the speedo dial.
Usually indicates that the drum and the magnet are touching. This has resulted in the hair spring
being broken and needle stop missing, Speedometer needs at least a service, and some parts
6) Speedometer is very slow and perhaps reading very low.
This might indicate that the speedometer is in need of cleaning and lubrication. If the car has not
been used for a while, or the speedo has been in storage, then suspect spiders webs. Or it could
indicate that the magnets are very weak. In either case clean and service.
7) I have taken the speedometer to bits and I cannot get it back together.
Collect all the parts you can find and post it to me.
Please note I do not sell parts.