For the not charging issue you should have battery voltage at both of the large slots electrical connector that plugs into the alternator.

You should have no voltage at the small slot till you turn the ignition switch on and it will be slightly less then battery voltage.

Then of course there is that never ending issue of having a good contacts at your engine to chassis ground strap and the battery to chassis ground.
It is even possible for where your alternator bracket bolts up to the engine to be so rusted it is no longer grounded well.

Once your engine has started pull off both the square and rectangular electrical connector at the Glow Plug Relay and see if the voltage at the battery changes.
If after that the voltage goes up your Glow Plug relay is stuck on or there is an issue with the ignition switch keeping the relay on..

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84 300D, 82 Volvo 244Gl Diesel