Steve's comments about the alternator low output are confirmed by my own experience checking output with a volt-ohm meter with various accessories on or off and a computer printed test of the alternator on my '77 300D. At an idle your lucky to get 20 amps, 10-15 amps is more realistic. At 1,200-1,400 rpm output is about 20 amps, at 1,800 rpm alternator output is 40 amps, at 2,000 rpm output is 45 amps, at 3,000 rpm (about 60 mph) output is 60 amps, and then output increases marginally to 65 amps perhaps a bit more with higher engine rpm's. When I have to idle for extended periods with accessories running I'll shift to neutral or park and increase engine speed a bit to say 900-1,000 rpm to speed up the alt. and get more output and decrease the drain on the battery. When I have a lot of accessories running and increase engine rpm's a bit from idle I can hear the aux. cooling fan pick-up speed as more amps are available to it.
If you're going to change sensors to have the aux fan come on at lower temps. and/or install other fans then you need to consider the alternator's capabilities and compensate accordingly (at extended idle time increase speed a bit, run the AC/heater blower fan at lower speeds, turn the stereo volume down esp if you have sub-woofers, charge the battery once a week to bring it back to full charge, etc.). If you replace or add fan(s), then try to find fan(s) with lower amperage requirements. Also, if you have a high power stereo system consider using capacitors to store power for those times when the music puts higher loads on the alternator (typically sub-woofer and low bass sound waves).
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1977 300D: 300,000+ miles
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