As said make sure you have battery light, btw on Cosworths this is common for wire to break, will still charge if revved hard enough as field windings get excited. If you have battery light and it goes out once running it will be cable from alt to batt. Probably there was nothing wrong with old Alt.

Relised that it's Diesel as well make sure you haven't mixed cables as Diesels normally use one terminal to feed tacho. (If applicable)

Martin