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620 alternator ?'s

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Ok gonna try & explain this to you guys & see if anybody can help. Back when i got my 74 on the road i bought & installed a new [reman.] alternator from Kragen. I believe it's an internally regulated unit. Since then ive been through 2 batteries & tracked it down to a bad alt...[i believe]. When i disconnect the + battery terminal the truck dies & from experience that tells me the alt. is bad. Ive read up on the Saturn & also the 77-81 510 alt. swap/upgrade & really want to do one of them. Im leaning toward the 77-81 510 unit because it's an easier swap. I now know that the newer 510 unit is internally regulated so heres my question. Do i have to unhook/remove from the truck the voltage regulator? & if so what should if anything happens to the wires/connector that connected to it. I read a tech article on the dime quarterly site explaing how to connect & rewire them but it's explained for a 510. Im not a big fan of electrical stuff but i gotta learn so i can get this swap done & start driving the 620 again.

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Yep toss the regulator. You will need 3 of the wires.


The one that goes to the battery (I believe it's yellow, it will be the heavy guage wire)


The field wire (it exites your alt. telling it to charge) not sure which one it is, but it will have power with the key on.


And the wire back to your dash light...I have no idea which one that is (check your trucks wiring diagram)


The yellow (battery wire) goes to the main lug on the alternator.

The field wire goes to the #2 (I think) blade (plug) on the alt. plug.

The light wire goes to the other blade (plug)


This is for the the Saturn alt, but most internal regulated alt's are set up about the same.


Again check the wiring diagram for the truck and the car the alt came from :D

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External regulator




Internal regulator



I looked up your '74 regulator wires and they are (or should be):




E................... Black (ground)

N................... Yellow

F................... White/Black

IGnition........... White/Blue

L.................... White/Red


Your 620 alternator and regulator wire colors should be the same color as the 510 regulator/alt. in the top picture. Check yours... do you agree???


If so, looks like all you do is install the new IR alternator and plug in your old wires to it. Disconnect the voltage regulator on fender, (and remove it) and on the harness plug, use two jumpers wires to connect:

ONE: the Yellow wire to the White wire, and...

TWO: the White/Black wire to the White/Red wire.



The second picture shows which wires to connect, BUT, I don't know why they changed the colors around like that. I ignored the colors and went by the terminals.

All 620 alt wire colors are the same, weather it's internal OR external regulated.



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i think most wire colors are the same in most Datsuns


heres a 510



just make 2 wires with male blade connectors and install it on the connector with those wire colors. keep the reg if you want to go back to the reg and no cutting of wires, nothing ,very simple


I have NOT done it myself as I still run the ext reg alternators.

I forgot when was the cut off year when they became IR type alternators.

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Could it be that because i left the VR wired up thats what was killing the battery?



what does this mean?


you left the ext volt reg and put the IR alternator on thinking it was a ext type? Then yes I think it causes a overvoltage proplem and COOK the batterys. If it kills the batterys buy dying then I dont know. I would think you would ck for voltage and make sure the volt meter reads correctly and see how it functions when you load down the sysytem(wiper heater,loghts are ON.


PS a 50 amper is harder to install on a truck(at least my 521 the ider arm get in the was to stretch the fan belt out) I grinded a part of it off

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ah well Y dont u go get a brand new 620 alt..... i sure did...... i paid 50 bucks for a brand spankin new one....... if u want to know where i got it i got it @ ADVANCE its a auto store if u have one....

The stock units put out roughly 35 amps. The 77-81 unit puts out bout 60. The Saturn unit puts out more than that. Id like to have brighter lights & maybe 1 day install a stereo :D

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OK guys a few more things before i tackle the swap. Will the 77-81 510 alt. bolt right to the stock L lower alt. mount? & does anybody have a part # for the alt. belt i'll need? I understand it's a bit bigger than the stock 620 alt. belt. Thanks...........

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OK checked the wiring harness colors. The T connector at the alt. colors are YELLOW & WHITE/w BLACK stripe. Found the 2 wires on the regulator that need to be cut/connected to the T connector @ the alt, they match the diagram. Did some digging & found out the alt. i installed last was deffinately a exturnally regulated unit. I may have gotten a bad unit from the start or the voltage reg. may have went bad too. New VR's cost bout $70 ish, way too much, the "stock" 35 amp alt's are $35 ish. The 77-81 reman 510 alt's are bout $54 & are easy to find new [reman] so thats the route im going to go. I can also see where it's going to make contact w/ the idler arm, cant i just run a smaller belt to avoid this? Hainz, do you have a pic of your modified idler arm?

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If you run a smaller belt, get the lower rad inlet hose to the block that sits just above the alternator. It's swept back more so there's more room between the hose and the alternator.


I also got the alternator block mount from a 720. It positions the alt. up slightly. Even so I had to put the belt on the pully first and then mount the alt. Still close. Grease the fitting then remove and install a plug. This will give additional room.



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Thanks D-mike for the info. Ill be going to the yard & picking up the 720 alt. mount & i already have the 280 z lower rad. inlet. I removed the ER alt. today to find a hole on it where it was rubbing against the idler arm..


Im sure it didnt help that it was rubbing all this time but o well :rolleyes:

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Ok i got the new alt. installed & wiring modified. I was able to use the stock 620 alt. mount & went with a 36 inch belt. Test drove it last night & after about 35 miles i noticed the headlights were going dim! This tells me that the alt. is'nt charging the battery. Time for me to tear into the wiring diagrams & see why it's doing this. Any ideas you guys may have as to whats causing this???...

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]2edeye may have nailed it... the sense wire if you refer back to the diagrams datzenmike posted is the S(N) wire, in that pic it is yellow and is shown at the top of the "T" plug... in the diagram the S(N) wire is spliced into the white wire which powers the fuse block, ignition switch, and charges the battery... if the sense wire is mixed up or disconnected oir anything but correctly hooked up you draw straight battery power.


Good luck bro. :thumbup:

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TY guys..today ill remove the loom & double check how the wires look & the colors. Im hoping it's as simple as me switching the connectors on the back of the alt. Are there any other wires coming off the + terminal on the battery? Now theres the thick one going to the starter & the fusible link. Is that all thats connected there & if not does anybody have pics of a + terminal 620 battery so i can compare?

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Ok i feel better now. I had the 2 extra wires going to the T plug on the alt. backwards...doh!! Is there a way to test & see if the alt. is charging the battery with it running in the driveway? Dont wanna drive it & have it kill the battery again if theres still issues.

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Is there a way to test & see if the alt. is charging the battery with it running in the driveway? Dont wanna drive it & have it kill the battery again if theres still issues.


Dude your making yourself look bad!!!!!!!!!

a volt meter!


I have done this but I will not say if this is recommended but With all accesories OFF i pull the pos or neg batt cable off. and if still runs I say the alternator is good. But I will leave this up to you. Could risk blowing something if its half assed already

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I do have a volt meter but the battery in it went bad..:rolleyes: I did know about pulling the + cable w/ the engine running method...did that & it kept running :thumbup: Remember..the tweekers got ahold of this truck before i picked it up & i had to undo alot of "stuff".

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