12.4 is undercharged. So that's not good.
11.9 under load from the glow plugs is OK, the voltage will conduct down. But you should verify that your battery is in good shape, and connections are good.
Ive never heard of an OM617 afterglow relay. It seems that some biofuels operators have used them, and its a potential that your vehicle has one... Im not sure that the 617 glow plugs are designed for that, but others may have other info, and/or the shorter length of the 617 plugs versus, say, the OM606 glow plugs may make this less of an issue.
But all the same, again I dont have experience with afterglow and its ramifications on a 616/617, and none of my cars have ever had that. IIRC, the older 603 engines have longer glow plugs, the later (3.5L 603) engines have shorter ones with a 60s max afterglow, and the OM 606 engines have up to 180s of afterglow. But these vehicles with the afterglow relays also have a temperature input so there is less to no afterglow, with the threshold for none being 40C. The grid heaters on my cummins trucks work similarly.
What these older 616/617 vehicles with small alternators do require is some additional speed and field on the alternator... Ive seen simple alternator systems in these and other diesel vehicles (e.g., my Dodge Ram Cummins trucks), and there needs to be some off-idle time. When the alternators are highly loaded, their vltage will drop, the battery wont charge, and the voltage wont rise... Alternators make far from full rated capability at low speeds. The 55A alternator might only make 15 or 20A at idle.
Example:
Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 5
Are you waiting three minutes of idling, or actual driving and use?
If youre sitting idling, no doubt it will keep way low if your alternator cant sufficiently excite, and your RPMs are low because of cold, set speed, etc.
I could also imagine that perhaps the relay is stuck and takes some vibration to shake it loose... three minutes is just an artifact of that...
Can you start by snapping some photos and giving some details regarding the relay that you have? I have no idea if a 6 cyl more modern relay could be plugged into a 5 cyl engine, and how it would work with no coolant temperature input... I think that would be more of a fluke thing....
When you did the test, did the car sit and idle when it took three minutes or so to get the voltage up? Did you watch it closely? Did it slowly creep up, or did it kind of step up where one second it was 11.9, then one second it was 14.3?