After I'd had the sail drive gaiter changed on my B36 in 2011 by which time it was nine years old and still looked like new I found, as Nigel suggested, that the Tachometer was not working. Fortunately before getting the yard in to do the job, I'd made a careful note of where each of the alternator wires were connected. So when I noticed that the tachometer was not working I could see that they had replaced the alternator connections, but not to the correct terminals. I called the yard back so that they could see the problem and got them to do the job properly just incase having made the wrong connections had caused any electrical damage. Luckily for all it had not, but it shows how making a few notes before before the job was done paid dividends. Now whenever anything electrical has to be disconnected, it gets photographed and recorded before the job starts.