warrenm

Hi, can anybody help please? I've just acquired an Escort diesel van. Runs fine, starts ok, but the alternator doesn't charge the battery. Battery is less than a month old. Just tried swapping alternator (with one I know works ok), but still the battery shows 12.15v across the terminals even when revved (the same as did with the old alternator).
I'm now guessing it's a break in the wires - but I don't have a wiring diagram so don't know if there is anything else between alternator and battery that needs checking. I can see a thin wire coming out of the alternator that goes to some sort of electrical device on the radiator - can anybody tell me what this is and whether it's relevant to my problem please? Or any other suggestions as to what might be the problem. I have a 'donor van' that I can take bits from if necessary.
Thanks in advance,
Warren