The gauge has probably died.
That was my solution until i found a fully working set of instruments on fleabay. Note that Yazaki and VDO instruments are
NOT interchangeable. There is an obvious difference on the odometer, i think it's that on the Yazaki there are bars separating the digits, on the VDO (mine in the picture) there isn't
BUT i might have that the wrong way round - it tells you somewhere in the FAQ which is which.
It's about 6 years since i did that so my memory is a little hazy on it now.