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Alternator to battery voltage drop


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Hello - following up on my other thread - https://ratsun.net/topic/79647-idle-stuck-at-3000-lost-anti-dieseling-low-voltage/



Now home with time and tools and turns out the ground is totally fine. Battery ground wire is 100% solid and there are zero volts and zero ohms between the battery and the block (and other places too).


BUT I have 2 volts and 124 ohms between my alternator and the battery positive post. Looks like that wire goes through quite a loom to get to the positive post. Can I just put a new heavy duty wire in there direct from the alternator to the battery and bypass the whole mess? Do I need a fusible link in-line?




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No it should go through the fusible link. In fact the fusible link may be the problem. I only have the '84 FSM but likely the '85 is similar...


There are 3 fusible links. A single Green one and a Black and a Green paired together. The alternator connects to the positive battery through the Green link that is beside the Black link. All links have plastic connectors so they can be unplugged to be replaced. The spade connections may be corroded or the fusible link may have overheated in the past. Find the cause of the resistance, there are 3 possibilities. 


1/ the wire from the alternator to the Green link

2/ the Green link itself

3/ the wire from the Green link to the positive battery post.

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I separated the connector and it looks nasty.  I cleaned it up as best I could.  There is no resistance between the alternator and the input side of the connector, nor between the battery terminal and the output side. But there is still resistance and a voltage drop across the connector, which I assume is at the end of its life cycle.


I used a jumper cable to run from the alternator B post direct to the battery and immediately got 14 volts and all of the lights got very bright.  So I am just going to get some 6AWG wire and a fuse block and bypass the whole thing.  I am thinking that my recently fried alternator may have been that it was seeing only intermittent connection to the battery and pushing through high resistance.


I also fixed my parking/dome/dash/tail light short by contact cleaning the $%#$@ out of the combination switch, which now has a more affirmative "click" to it and everything seems all good.

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I just installed a 4AWG bypass wire with 80 amp inline fuse direct from the alternator B post to the positive battery terminal.  I also cleaned the heck out of all of the connections at the battery, including the fusible link connector.  It's quite incredible, the headlights were like dim candles before, now they are blindingly bright.  Pulling 14+ volts at idle, and about 13 volts at idle with A/C on high, turn signals, wipers, and high beams all on.  Wipers are now screaming fast and my turn signal blinks almost too fast.  I highly recommend this mod/clean up as well as the hot start bypass I did a few months ago.  Getting all of those electrical gremlins out.

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On 11/9/2021 at 12:10 AM, MaddieCycle said:

Running out of moving and electrical parts on this truck to replace.  Soon it will just be a new truck. 🤣

Mine has so many parts from hardbodies and frontiers it's no longer a datsun. Anyways make sure that fuse is as close to the battery as possible for the best protection. Good job on tracking this down so many folks are shy about electrical. Just convert volts, amps and resistance to gallons, pressure and flow of water. Switches become valves and it seems so simple.

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