Series III speedo problems

My series 3 speedo has gone mad. I was sat at 55mph behind the trucks (so I assume it is reasonably accurate) and then it just dropped to 15mph. When I eased off the gas it seemed to pop back up. It waived up and down for a bit and then seemed to go back to normal.

Its done this ever since, sometimes its OK, sometimes is goes nuts. Any ideas anyone? It has had the clock/head changed for a 109 one which apparently made it more accurate (it has bigger than standard wheels).

Any suggestions would be gratefuly received!

Cheers,

Mike

Although my reply is pobably a bit late, I have the same problem. I have found this advice:
http://www.series123.com/repairs_speedometer.html

One possible cause of an erratic speedometer reading may be due to the speedometer drive gear slipping. The drive gear is not splined but is a frictional fit between the rear driveshaft bearing and output flange. If there is wear in the speedometer drive then slipping can occur. The solution is to remove the rear propshaft and re-torque the castellated nut on the rear output shaft to 85ft.lb, fit a new split pin and re-fit the propshaft.


Let me know how you solved it (if you ever did).

aye or yer cable is fooked

If the wee square end of the cable is worn at either end that might cause that.