Due to the tacho being electronic and fitted in the timing cover, sometimes metal filings get attracted to the sender unit. If you unbolt it and pull it out, wipe the end with a rag and refit it , it may well fix your problem. If not you'll need a new sender.
K1W1TankaYanka
October 2007
Which cab & serial number is the tractor?? I think you'll find you need a new sender on the front of the engine, tho' looking at the parts catalogue there are some newer versions depending on tractor / engine serial number.. See your local dealer or look at www.johndeere.co.uk & navigate your way to the parts catalogues from there