Revcounter and battry light

Hi I would appreciate some help I have a problem I started my Discovery this morning and the revcounter did not work and the battrey light did not go out so my first thought was alternator so set off to work thinking I will take it off and get it looked at when I reved the engine to 2500 rpm the revcounter came back on and the battry light went off if I droped below 2500 rpm the revcouter gos off and the battry light comes on so still thinking its an alternator fault I removed the alternator and have had it checked its ok on the test machine even at low revs as little as 500 rpm its still ok So befor I refit the alternator can any one shed any light on what the problem might be ???
By the way its a 300TDI Discvery 1996 manual

Ok so I have now refited the alternator and still have the same problem I have also relised that the alternator is a Britpart replacment having serched the forum I see lots of you have had problems with britpart so thinking I will see if I can find another alternator tomorrow is there any one make better than another don't think I want another britpart

It's always worth checking the condition of the W connection on the back of the alternator - often a cause of rev counter problems.

Had similar problem with my disco, The rev needle would bounce about then drop to nothing. It was the wire in the back not connecting properly.

Try liberally spraying the back of the alternator with WD40 or similar. Solved mine.