Alternator Wires on a Leyland Daf 45 130 1992?

Hi, i have a 1992 Leyland Daf 45 130. The speedo has stopped working, i check the wires to the speedo generator at the gearbox and no power at all.
Whilst under the truck i noticed a broken wire just below the starter motor. I traced the wire back to the alternator and have no idea where its supposed to connect to or what is for, there are 3 connectors on the alternator this wire is number 3 if looking to rear of alternator....anyone any ideas.
Oh and the wire had a voltage reading of 2.