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Charging issues--'77 620d

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well, I am new around here, and have been browsing as a guest for a while, and now I have a problem that it seems like the knowledgable folks around here may be able to help with, so here goes....


I had to let my 1977 620 (king cab, california model) sit for around a year due to a carberatuer issue that I was finally able to resolve, and then I drove it for a week with no issues, until yesterday...


As I pulled off the highway on the way home from work, I noticed the CHG light come on on the dash board. I ran it home as fast as legally allowed, and popped the hood. While driving home, I noticed that the light appeared to flicker at random moments, so my first assumption was a loose or corroded wire, so I had my wife sit in the truck watching the light while I wiggled every wire I could possibly move, and could not replicate any flickering of the light.


So I jumped in it, with the intention of taking it down to the parts store to test the alt. and bat. and I noticed that I could get the light to shut off when the engine was up over 4k rpm or so, (no tach, just a "educated guess" based on the sound of the engine/travel speed).


Got to the auto parts store, and put it on the diagnostic equip. and got the results, battery is good, and nearly full charge, and "no voltage detected" coming back into the battery. Tried the test again while maintaining enough engine speed to have the CHG light go out, with no change in the test results. The guy at the parts store did some part look-ups to see what was available, and since the voltage regulator was closer then the alt. we went ahead and ordered one, with the intention of "trying it out" before I paid for it to make sure that that was the issue.


Fast foreward a day and a half, during which time I operated the truck a few times, even using it to take the backroads in to work, being careful to maintain as high an engine speed as possible by undershifting a little bit, trying to keep the charge light off as much as possible.

I got it back to the parts store to check on the VR, but the guy who had ordered it originally was not in, and the other employees would not let me try the new one on before purchase for fear that I would just burn it out, so instead I opted to have a more experinced guy re-run the diagnostic, only to return nearly identical results, (the only difference is that the battery was now reccomended for charging...)


So, I pulled the battery out, and tossed it on the charger at the store, and in the mean time, pulled the alt. out to get it tested on the dyno in the store. While I had it out, I noticed it was in less then perfect condition, but did not seem to be too damaged, and the dyno confirmed that it was operating well within the proper range. I put it back on the truck, and when the battery was all charged up, I put that back in too, fired right up, but the light was still on, so I tried pulling the + cable, and the truck died. If I am not mistaken, that means to me, that the truck was indeed running solely on battery power, (at least at idle), and indicated to me that the VR was indeed the issue,(I had also visually inspected all of the wiring coming from the bat. and connecting in any way to the alternator while I had a little extra room in the engine compartment so I went back in and bought the new one. I took it out the the truck, mounted it up, and turned it on. Now the light was still lit, but would not turn off at 4k, instead, I would have to push it up to (guessing...) around 7k to get it to go out.


I took it down the street to a friends shop, and we got out the voltometer and test light, and got to poking around. We found that the bat. voltage was good, there was power going in and coming out from the alternator, the output at the VR harness seemed to be correct, and the CHG light would still not go out.


I'm stumped. So my question in a nutshell, would be...


Why is the CHG light on at low rpm, if the alt., VR, and battery cables have been eliminated as possible causes??????


Thanks for any suggestions or help you may be able to offer....




oh, and does anyone know where to find a free wiring diagram for the 620 engine with external regulator? My mechanic friend was pretty sure he could find the problem with access to one, although neither of us can find one....



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Check that the belt is tight

Clean the battery connections.

Check the voltage regulator is tightly bolted and grounded to the inner fender.

There should be a ground strap from the body sheet metal to the negative battery terminal OR from the body to the engine block, whichever is easier.

Unplug and plug in the regulator a few times to clean the contacts.


With engine running, battery voltage should be just over 14 volts if charging. If not charging it will be 12.3 or less.


If you tried a second VR and it ran the same then likely the alternator is bad or a bad wire. Connect the voltmeter to the battery and run motor while moving wires and connections. When it jumps to 14 volts you've found it. If nothing try an alternator. Better yet get an internally regulated alternator and eliminate the VR from now on. Hainz has a diagram of the two wires you have to jumper on the old VR plug to make it work.

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Check that the belt is tight

Clean the battery connections.

Check the voltage regulator is tightly bolted and grounded to the inner fender.

There should be a ground strap from the body sheet metal to the negative battery terminal OR from the body to the engine block, whichever is easier.

Unplug and plug in the regulator a few times to clean the contacts.


With engine running, battery voltage should be just over 14 volts if charging. If not charging it will be 12.3 or less.



pulled, tested, and re-installed alternator this afternoon, belt tension is good...

pulled and charged the battery this afternoon, cleaned all connections before re-installing...

Removed, and re-mounted VR, twice...

visually and physically inspected both terminal wires for signs of corrosion and voltage, both tested good...

during testing, plugged VR in/off around two dozen times today, all connections bright...


with engine running, battery tests at 12.7 volts...so, not charging...




I thought about swapping the alt. for an internal, but that is a last resort for me, as I am not really a "car guy", and being able to have any parts store look up "by the book" for me is a big plus, although if that fix eliminates the wire that may be the ultimate culprit it may be the way to go.


The haynes book for 72-79 620 has been discontinued according to all my local parts stores, I did find one for more then I would like to spend on amazon, although all I really need is the wiring diagram, which seems to not be available in a free format online, although I can find plenty for the 510 model...


Does anyone have a diagram for the 620 they would be willing to share???


thanks again....



Edited by deadhead
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Scroll down to 77 diagram. Also a lot of other helpful info, just browse the site. :thumbsup:


My original post was getting long winded, so I didn't get the chance to mention that My only computer at the moment is my work issued laptop, which has some quirky security settings, intended to keep me off the internet I think.... Most of the time, it doesn't cause many issues, however in a few instances, (old datsun.com for example), the AV program will not let me view some websites. I have tried to go there in search of good info, but since I can't see the site, haven't been able to search it. Maybe I will try to check out the library tomorrow to see what I can pull off of that site.


In the mean time, what I feel makes this case unique is the fact that the CHG light will go out once a certain rpm level is reached. I have not been able to tell why or how, but I feel like that is central to the cause of the light being lit up in the first place...


has anyone heard of a similar issue ever???




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The CHG light is controlled by the Voltage Regulator.

If the alternator is putting out 13.5-15.0 volts @1100 RPM or higher, the alternator is OK. If the alternator is OK, and light won't go out, it's probably the Regulator. You could remove the cover and lightly, carefully sand the points. Easy to mess it up...


But if the alternator is not putting out enough voltage, then that is the problem.


Also if the battery is really low the light won't go out for a while. My battery was at 11v, it took a while for the alternator voltage to go up. Maybe after five minutes it finally went out.

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yeah, gzilla, that is what I assumed, and after changing all the parts except the alt. I am still having the issue.


A friend of mine who owned a 510 some years ago suggested that there may be something in the coil circuit that may be contributing to the issue, but that doesn't seem right to me... what do ya'all think?


I am thinking that if I install an internally regulated alt. and bypass the wiring and VR loop in the circuit that may help alleviate the issue, but I am not quite sure exactly how to proceed with that, even after reading about so many others who have done just that, I am not entirely clear about which model of alternator would be the best fit. I am not the handiest when it comes to messing around with changing pulleys and fabricating brackets or drilling mounting holes in the block, so what would be a direct bolt on replacement alt. for this?


thanks for the input....




oh, and I did manage to get on olddatsuns.com and print off some wiring diagrams, so if I can't figure out what a direct fit after market alt. will work, I am going to start rewiring the entire charge circuit.....(a last resort, but one I am confidant will work....)

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with engine running, battery tests at 12.7 volts...so, not charging...

You can bypass the Regulator to test the alternator by itself.

1. Disconnect T-shaped connector at alternator

2. Start the engine

3. Put Voltmeter across battery

4. On alternator, jump the F terminal to Battery +


- If Volts jump up (rev engine to 1100 RPM or more), alternator is OK

- If Volts stay same, alternator is bad. Probably just the brushes, a $5 fix


Read up more here: Alternator testing. The 620 uses the same alternator type as my Datsun 1200.


(Nissan Factory Service Manual diagram)



Of if you just replace both the alternator and regulator with one internally-regulated alternator, that will probably fix it too, unless it's a wiring harness or fuse problem. Best to test it.

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I like that diagram. that should be on olddatsuns.com


I had this proplem also once.

I have to rev the motor up to get the light out.

Schucks tested it and said it was good. Took it to NAPA they said it was bad.


Took alternator apart find one brush was short. So assumed when the stator or whatever is spinning there was enough expansion where the brush made contack.


swap another alternator in there!



I really cant believe they couldnt put a volt meter on there right away to ck this out at the parts store???????????


usually a over volt proplem is a Volt reg.

Under volt is the alternator in most cases.


since you swapped to Volt regs I will assume its the alternator brushes.


if you never change this you can have a alternator shop fix this or get another alternator.


ps most alternators suck that come from the parts stores. Alway buy the most expensive one otherwise this will happen again in 3 yrs!!!!!!!!!!


or buy a nissan unit from the dealer or nissanparts.cc or do a upgraded

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holy mother of dats! Thanks a ton guys, By using the info from GGZILLA and then folowing the reccomendation of banzai, I was at last able to figure out what was going on.


After having the alt. tested at a number of shops and having it come up good on their machines, I just assumed it had to be a wiring or regulator issue, but after striking out on those as possible causes, I tried the short circuit test described by ggzilla, and then took banzais advice and just replaced the alt. with one from autozone with a lifetime warranty, and wouldn't you know it, the problem cleared right up, go figure.


When returning the core to the parts store, I had them test the old one just to see what it would say, and sure enough, the one store I had not previously tested it in returned a fail result. Sure would have saved me a lot of time and frustration if I had just gone with my gut and swapped that out first and foremost! live and learn I guess, that is what I get for being a broke guy, afraid to buy parts that may not be the right ones...


I still have the new VR I bought, which oddly enough was almost twice the cost of the alt., and what I am wondering is that I have heard such good things about the original, and now it is functioning properly with the oem VR in it, should I bother keeping the new one, and putting it on, or should the old one last? The guy at the parts store is my neighbor, so luckily for me, I can return it, and I am thinking this is what I should do, but since you guys have helped me this far, I just figured I would get your take on that too... So whatcha think?


thanks again,



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NAPAs tester put a LOAD on the alernator thats when it failed. otherwise it read 14volts.




Your volt reg?

Its alway good to have spare in the car/truck. But If possible find a Japanese made volt reg.


Most Jap volt regs are still mechanical type.


However I have seen some USA made mechanical type and they suck!!!!!!!!!One was sold thru Schucks under the Neihoff brand and the other at NAPA

I had 2 go bad in like 3 yrs.


I use the Wells brand solid state units. They are also sold thru Carquest I believe. They will be smaller in how tall they are. But I have had good luck with them. I pop one recently when we pilled a motor and I didnt have the batt cable tight so I assumed it broke while trying to start truck as it was fine before the motor pull. But now they are getting more expensive.


You can adjust the old Japan made mechanical units but never done one before.Mecanical mean ist uses relays with a coil on there to use contacks to regulate the voltage. really ancient tech now.



be honest maybe even a taiwan solid state one might be better!!!!!!!!!



As for Autozone?

Has you buddy told you the percentage of Bad alternators straight form the store?


The guy at Schucks told me once, the Cheap brand alternators was 3 out of 5 are bad in the box.



glade you got it worked out.

Dont be surprised it goes out in 2/3 years!!!!!!!!!!!!


If you ru a Volt meter or one of those diode ckers you plug in the cig lighter. you can sometimes see the proplem before the light comes on dash!!!!!!



left is solid state right is mechanical VR


Edited by banzai510(hainz)
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having worked at both autozone and advanced auto parts i can tell you the reason that they didn't just throw a voltmeter on the vehicle is because they're told not to. i was using a simple voltmeter with a load (like this one http://www.annco.com/products/pro_4.cfm?sku=tg-03037703). if you know how to read one like that it's quick and usually pretty accurate. after having very consistent results the rep from johnson controls came and made this huge deal about us using such old technology. told the store manager to not allow us to use that tester anymore. then they sent out a company wide memo saying that that tester would no longer be available and that we needed to use the bench tester as much as possible.


and that lifetime warranty on the alternator... that can be a good and a bad thing. it's good because it's true, autozone is supposed to replace that alternator everytime it goes bad for as long as you own that truck. however there are some managers and store managers that will only swap it a few times, or tell you to take the issue up with coporate that you must be installing it wrong. the reason they can afford to lifetime warranty them is what hainz said. they reman those things at such a poor degree that alot of them come back and have to be swapped multiple times to get a good one. not to mention the shipping and in store employees mishandle them alot.


fact is that the autozone you're going to might have awesome knowledgeable employees, but you still have to be cautious about their parts.

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