mis-reading speedo

[thebilby]

driving along in my 1982 2.0 Aircooled the speedo suddenly started to over read. one minute its showing 30 mph next minute, with no increase in road speed its showing 70!!

any clues would be helpful.

[ghost123uk]

That can only be one thing, the actual speedo head has developed a fault. It will need removing, stripping and examining. (or replacing). One tricky bit is taking GREAT care of the rather delicate blue coloured plastic printed circuit that runs all over the guts of the instrument cluster.

Speedos work by having a rotating magnet on the end of the cable that generates a small voltage in a coil of copper wire, just like a small weak electric motor. The needle is attached to the shaft of this motor and has a very fine spring that limits movement and acts as a return spring. As the van goes faster the spinning magnet generates more current and this moves the "motor" more and more against the needle return spring.

So your fault is almost certainly summat to do with that little "hair" spring. It's all it can be really ;}

[Footprint]

A similar thing happened to mine, and in my van the pin on which the needle and cup turn had worn down, meaning the part of the assembly that is turned by the cable could just graze along the cup and physically turn it, rather than the field it produces pushing it against return spring pressure.
I carefully dismantled the speedo and very very carefully tapped the pin further through the cup a tiny bit, problem solved, speedo as accurate as it ever will be.