battery draining electrical hodge podge - Mercedes-Benz Forum
 
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post #1 of 4 (permalink) Old 07-31-2018, 10:14 AM Thread Starter
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battery draining electrical hodge podge

I have a '84 300sd with a hard to find parasitic drain on the battery. I've tried finding it with a ammeter by removing the fuses one by one without any luck. The PO did lots of odd electrical "work" on it .There's lots of electr ical stuff on this model so lots of places for problems. So far I've found that tha diodes in the alternator are burned. If I unplug the wires from the back of the alternator, there should be no drain from the alternator but there still is one. With a milliamp meter across the negative terminal and the cable there is a strong pulsing drain on the battery.I'm wondering what would cause a drain and generate a pulse like a ticking clock. I've tried all the fuses I can find. My main questions are: could the alt be causing the problem even with the wires disconnected? What electrical devices on this car would generate an electrical pulse? Thanks for any advice.
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post #2 of 4 (permalink) Old 07-31-2018, 11:06 AM
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If the alternator is disconnected then it cant be drawing anything.
try looking at the warning flasher/turn signals.

https://mercedessource.com/node/4924

also under the passenger carpet is the alarm system, unplug it.
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post #3 of 4 (permalink) Old 07-31-2018, 11:48 AM
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Remove the flasher unit(s) to see if that eliminates the drain. Since the 4-way hazard flasher is wired to be always "hot," many tend to use it as a source of power when adding aftermarket accessories, If you have removed all the wires from the alternator you have eliminated it as the problem.
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post #4 of 4 (permalink) Old 07-31-2018, 10:03 PM
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When pulling the fuses out, you may want to keep pulling until they're all out - instead of putting each one back before pulling the next.
The PO may have wired something to two power sources (one switched, one always hot). I've seen this most often on alarm, audio, and auxiliary lighting.

The pulsing you're registering could be a capacitor charging and discharging (like when an amplifier is turned on and off).

What about flood damage?

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