I have 1996 Disco 300tdi. Apparently the mechanic disconnected the battery when replacing a broken bolt on the alternator. When I got it back, the tacho does not work (alternator is still charging the battery ok). The fuse for the instruments is fine, the connection on the alternator is fine. Does anyone know if there is a module at the rear of the alternator that might have blown which sends to the tacho ?

Just tried swapping the spade plugs on the back of the alternator. Works fine now. The mechanic had plugged to the tacho wire on to the wrong pin.