I have noticed this as well. As long as you are above 13, you are OK. Mercedes uses alternators that are at almost 90% duty when all accessories are on...This did present a problem with the early 124's. I have converted several over to the later Diesel alternator. It puts out about 35 more amps.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS