Something (Consumer) is not getting enough Voltage and the Computer thinks
It is (the Under-Voltage Consumer) Defective.
'Don't know whether the computer thinks it's defective due to Low Voltage
Consumption...Or there is not enough Volts being delivered to the UVC.

Or the computer may be trying to tell you It's not getting enough Voltage.
And you are to find the problem and fix it or be hounded into lunacy.

One of those $100,000.00 Mercedes Diagnostic Machines could probably
tell you what's going on

This is an official "Arthur Dalton" type question.

Hey,a Cheapo Multi-Meter from HF (Or Sears on Sale) is only about 12 bucks.

Autozone,Advance,Etc.,Etc.,O'Reillys will all check your charging system
for free.

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'84 300SD sold
124.128