First, 13.5V is not unusual for a EURO built car. The other thing, the voltage will be low until the battery is fully charged. Also, a battery can hold down the voltage if it is bad. You really need to have the amperage checked with a fully charged battery and without a load (a/c, lights, etc) then checked when under a load (a/c on high, lights). I don't know exactly what the amperage should be for a 79 300D but it should be able to supply that much under a load and engine running at 1500 rpm or so. Very few people have a AMP meter that can read up to 100A so I suggest you drive the car for a while and see how it does. The voltage readings you are seeing could be normal.
By the way, I would much rather see a voltage of 13.5 with no problems than see 14.5 with no problems. The battery charged to 13.5 will last a lot longer. A battery charged at 15V will have a life span of maybe 1000 miles or less depending on the outside temp. It could also blow up but will at least have that green messy stuff on the terminals.

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84 500 SEL (307,xxx miles)