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As McDuck says, the alternator is not charging. Not meaning that it is a bad unit, but that the system is not working.
Using a voltmeter do this test: Measure voltage across the battery terminals with the engine stopped. Should be 12.5 or more. Less means either a low charge or a bad battery. In your case, probably the former.
Then get the engine started and measure the voltage again. Should be 13.5, better yet is 14.5. If it is below the engine stopped measurement the alternator is not charging.
There is a voltage regulator on the instrument cluster that provides a controlled voltage for the fuel gauge and temp gauge, since the car system voltage varies and that would make variations in the gauge reading.
I think the control is set at 10 volts. If the system voltage drops below that, the gauges would be effected. Thus your declining fuel level linked to the engine quitting. Ran out of juice.
Check the wires connected to the alternator. Should be three.
1) Fat red wire on an insulated stud. Carries the charge current to the battery via the starter. Not a problem, usually. It is hot all the time, so keep metal things away from it unless the battery is disconnected.
2) Relatively heavy wire connected to the alternator frame to the block. This one is easy to miss and is vital. Color is that of grease, or blue.
3) Thin red wire connected to an insulated spade lug. This one is critical: If it is not securely connected, the alternator will not charge. Often it is the weak link. Start checking by watching the warning lights on the dash when the key gets into Bulb Check position. That is Position II before starting the engine. No lit bulb = no charge.
Damage to that thin red wire comes about from the crumbling insulation problem, (1983 to 1988 240s), or damage inside the under-front-of-engine harness. That location is really hostile. Heat, road grit, and - worst of all - chafing from the metal holder clips when the rubberized coating has worn off. I found mine to be almost cut in two when I hit the problem.
That harness has three wires: The thin red one, the fat red one, and a black on that connects to the oil pressure sensor.
There are several fixes. Check the system and post back, OK?
Good Luck,
Bob
:>)
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