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1972 620 electrical problems


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Hi Ratsun, need some help. I've done all the basic stuff like check and changed all fuses, cleaned their contacts, even disconnected almost ALL harnesses, horn, aftermarket relays one at a time during trouble shooting.


Upon checking voltage at the fuses, it shows 12.6, but when I put the ignition to on or turn on the parking lights, the 12 volts goes away and I get 1.0 - 0.651 (something like that), nowhere near 12v.


I tried pulling all fuses one at a time until they are all gone, but cant seem to find the culprit. Am I going at it this wrong way?


I'm guessing it's a shorted or broken wire somewhere, since I've unplugged most everything, but not exactly sure how to find it. The only devices I didn't disconnect was the alternator and starter.


Any advise would be greatly appreciated. TIA!

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So any time you put power on and put a load to it the voltage drops on all circuts????


I would ck the battery cable and ground esp the battery ends.Guam has alot of salt corrossion


then work up from the Round connector from the starter post up to the fuse box.Is that tight?



a guy at work his Forerunning did not start.

I told him put the key to ON. then I measured a 2 volt drop from the battery post to the cable. so the post and or cable inside was dirty.So when he hit START it just KILLED it.


cleaned it then it was 12volts from post to cable lug

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Yeah, 12.6 at the 4 fuses (3 on the right and top left), turn the ignition to on, no dash lights (charge, oil, brake) and voltage is 0.165. 0.00v on the 2 bottom left, 0.105 with the ignition on. Weird thing is, when I turn the ignition off, I get 10.65v at the 4 fuses.


Went an tested voltage from the battery. I get 12.6 at the posts, the cable lugs, and both ends of the positive cable. Tested Ohms; on the negative cable I get 0.2, on the positive cable I get 0.2. Of course, I'm typing these as I do them so I don't forget. I cleaned them up and try it out, IT'S ALIVE! I couldn't believe it, since there wasn't much gunk on the battery cables. Never the less, she's alive again! Thanks Hainz, your my second best Datsun Hero. Of course DatzenMike is still #1 in my book, no offense.


One more thing, I test the battery with all lights on including high beam and I get 12.1 at 550 rpm idle, turn all the lights off and it's at 13.34 are these o.k.?

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550 rpm idle is way low. The alternator charges more as it revs up. Turn the idle up to 650-750. 13.3 is barely a charge but .... ok.


Engine off and battery charged you should have about 12.6 volts


Engine on and charging about 14+ volts but, the 620 has a very small alternator and only provides 28 amps of current at 2,500 rpms and a lot less at idle. Headlights on high beam will draw at least 16 amps not counting all the tail and running lights. Heater fan and wipers? It adds up.


Is your fan belt properly tensioned? Slipping will squeal.

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35 amps are about right you just learn to drive with it. since you don't need a heater in Guam Im sure your fine.


sometime with different volt regs installed and can get higher or lower readings.


start car and run it first at a higher rate before truing stuff on. I notice on my 521 it ai put on the lights it will charge at a lower level unless I rev it up to get the volts up then it seems to lock.


get a cig lighter volt meter so you can monitor this on a daily babsis and then youll see before you get the idiot light that something is going bad

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The '73 has an LT135 and nominally rated at 35 amps. Under real world conditions the output is 28 amps at 14 volts and 2,500 RPM or 1,000 crankshaft RPMs.



550 rpm idle is way low. The alternator charges more as it revs up. Turn the idle up to 650-750.

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