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New battery, new alternator, 12.37v with lights off. Please help.


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This is for a 74 620 using a hitachi manufactured 79 200sx alternator. Thursday morning my lights were really dim. They kept getting dimmer till they were basicly off. Stopped at a red light and the truck died. Had the battery tested, it had a dead cell. Replaced the battery and still had dim(ish) lights. They were a dull yellow instead of white but I had to go to work so I drove it like that.


Today we took the alternator to have it tested. Everything checked out ok, but the clerk was nice and replaced it under warranty anyway. We get back and install the alternator and the lights are slightly brighter but still not correct. We tried the old trick of removing the battery cable and it dies immediatly every time.


I cleaned the fuse block, looked over all of the connections we could find, and nothing looks wrong. We unwrapped most of the wiring harness and found what looks to be the stock connection of the white/red wire from the alt and the black wire (should be fusible link) from the positive side of the battery. We followed this wire (continues as a heavy gauge black wire) to the fuse block and everything looks good.


As for back ground, this setup has ran fine since February of this year with the old battery. The voltage regulator is bypassed and there isn't a slow drain through the bypass.


So I guess I need to know exactly which wire should betested to see if the alternator itself is outputting any volts? The BAT connection reads the same 12.37v as the battery. Also, my alternator has two locations marked with an "E" one is where the ground is connected and the other does not had a bolt or nut to connect to. Is the second location just an alternate ground point? Thanks for you help guys.





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Your '74 620 will have an external mounted voltage regulator somewhere on the inner fender to the rear of the battery.  All alternators after '78 were internally regulated.



Not sure what by-passed means. You can't run what you have described without cutting and crossing two pair of wires and getting rid of the external regulator.

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Does your alternator light come on?  Ever?


Even if it's been properly converted to use the internal-reg alternator, it still needs the alternator light to work to generate the field.  If it's not coming on, usually from being burned out, the alternator may not charge (some will self excite, but crappy Orielleys alternators often are too weak to do so, considering their near 50% DOA rate).  The light also won't come on if the regulator was simply unplugged rather  than properly converted cross connecting a couple terminals on the harness-side plug.   If the light comes on with the ign ON and engine not running, then turns off but doesn't charge, it's usually a case of a badly "rebuilt" alternator that simply isn't working. 



"E" means "Earth" and there were multiple locations on some Hitachis.  Doesn't matter which one is used, it simply provides a ground to the alternator body.

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Thanks guys. Datzenmike, I bybassed the external regulator by taking the old regulator plug, cross connecting 4 of the wires (can't remember the colors atm) and plugging it back into the harness. The other two wires where cut and wrapped.


Datsunaholic, that is very interesting. Now that I think about it I found a bulb in my floor board wednesday before work, I didn't think anything of it but the light hasn't come on since. I will replace the bulb to see if that is what I needed. Are there any other things I should troubleshoot just in case?

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Wow, case closed. Thank you so much datsunaholic.


I put the bulb back in and nothing changed, but then I looked at the bulb housing and saw that one of the tabs had broken off. I replaced the housing with a known good one and, while the truck was running, I put the bulb in. Immediately the truck bogged down a little and I thought oh hell... Then I remembered that the battery hadn;t been charging and told my step dad to remove the cable. The truck revved back up and stayed running. Its charging at 13.5v with the headlights on and without revving the engine.


Lesson here: Replace those MFin bulbs on your MFin Datsun.


I also forgot to add that now my fuel and temp gauges are working again. The miricles of routine maintenance.

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