Last night I started the car before driving 7 miles back to my house with headlights on. The car('78 240D) started up on the first bump. The battery light comes on rather dimly and fades in and out for the first three miles of the trip. The light goes out by the 4th mile. When I get home I plug my volt meter into the cigarette lighter and it reads 12V with the car off. I start the car and it reads 12V with no battery light on.
This morning I check the voltmeter and it reads 12V with car off. I turn the key to the glow cycle and voltage is 10.5 until she starts on the first bump and then jumps up to 12V. I figure the alternator must be doing something since the glow plugs usually cycle for another 15 seconds after starting. This is not the updated relay with afterglow. I went ahead and tightened the alternator belt a little to see if I'd get a higher reading in voltage, and there was no difference. The belt wasn't all that loose to begin with. I ran some errands, and started the car three times with no drop in voltage. The battery is only about a year old. Any ideas? thanks

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'81 300SD