Charging voltage should be between 14.0V and 14.4V measured at the battery teminals.

If anything, the charging voltage should rise after startup. The alternator control box will attempt to maintain the voltages above, no matter what load. When the engine is first started the battery voltage will be the off load voltage of about 12.6V for a good battery. (This might be somewhat lower, say about 12.2V, because the battery has been discharged by the glow plugs and the starter.)

For the first few minutes, the charging current will be the order of 20 Amps, slowly tapering off as the battery voltage rises. (This is why loose fan belts squeal just after startup.)

You say that the alternator is OK. That only leaves the control box at fault. Funnily enough, the internals of the control box aren't electronic, but rely on relay coils and adjustable springs switching in and out field resistances like the old Joe Lucas B90 CVC boxes. Eventually the contacts burnt out, or the resistances went open circuit, but this took one hell of a long time. Generally speaking, the agricultural technology had been refined over 50 years or so, to the state that it was at least as reliable as solid state stuff and totally bomb-proof in comparison with solid-state, so far as unintentional overloads were concerned.

When I say a very long time, I'm talking of at least 150000 miles.

The other odd thing about the 4trak, is that the control box is external to the alternator and is mounted on the left hand wing inside the engine compartment. Most alternator control circuits since the late 70s have been electronic and internal to the alternator.

My control box hasn't given me any trouble, so I haven't had the need to poke around inside it, so I'm sorry I can't suggest any likely faults to investigate.