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1978 620 King Cab electrical mystery


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Hello Datsun fans. 

 I have a total mystery on my hands. My '78 620 King Cab has an electrical glitch that causes it to instantly quit when sitting at idle or in park. Ongoing problem. Worked on it for two weeks the first time and a month the second time without coming to a solution. Had it towed to the shop both times. Last time it was $800 with a fist full of new parts. Two months later, it's doing it again. Only this time, I'm able to restart it after 15 minutes. Runs perfectly, maybe for 10 minutes or maybe for several hours, then does it again. Very frustrating. Every component of the electrical system has been either traced,  checked multiple times or the parts replaced. If it were a component, it would have showed up and been replaced by now. I honestly don't think the shop figured it out, they may have just gotten lucky. This particular issue has accumulated almost $1500 in parts and labor over the last year.  All the obvious things have been done. Looking for something that isn't obvious. Perhaps you have had a similar problem. I'd love to hear about how you solved the mystery. 

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To run an engine basically needs two things. Gas and spark. I find it odd that you can't tell me which is missing as this is the place to start. Next time it quits check the following.



Pull a plug wire off and put an old plug in the end and hold it against a grounded surface like the valve cover. Have someone turn the engine over with the starter. Do you see any spark??? If yes go to Fuel.



Look on the front of the carburetor for the small round window that shows where the fuel level is, It should be at the dot on the glass. Is there gas? Won't run without it. Find out why.



Do these two things now while it's running so you know what to look for. Mechanics are crooks so find another one who's more honest as you've paid $1,500 to not fix this problem. If he was your doctor..... you'd be dead by now. In fact he may have put in other 'new' parts that do have problems or are inferior quality. There is a tendency to believe a new part is working properly. Go to e-bay or alibaba (what ever) and get a Nissan FSM (factory service manual) for $20- $35 for your year 620 truck and leave it in the bathroom to read. Don't pay a mechanic to learn how to fix your Datsun (or not fix) If you can't work on it, at least know what's wrong with it.




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what mIke said,

This would have saved 1200 of the 1500 right off the batt!!!!!



no spark would more or less be the coil or more like the EI distributor or the remote module you have, Dirty connection?


If you have spark then check for gas.  cycle the linkage and if no squirting of gas the bowl is empty

Edited by banzai510(hainz)
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King Rat and L motor God,


Thank you both for your input. Obviously, my question needs to be re-framed. First, just a tad bit of history. This truck has been in the family since new off the lot. I have had it personally for the last 10 years. There isn't much about this rig that I haven't had apart, replaced, repaired or my hands on.  New radiator, new carb, new muffler, new this, new that.I think the only original part of the engine is the block.  I do most of the service myself, except when I DON'T have the tools or diagnostic equipment to work on it. I don't replace tires on wheels and I prefer to have the brakes done at the shop. That's why it was so frustrating to have to take it to the shop for this problem. After a month of playing with it, it had me stumped. 


Yes, I am aware that it is either spark or fuel. Fuel does not seem to be the problem. Mechanical fuel pump works, clean fuel filter, gas showing up in the carb. So yes, spark. Starts right up, when it wants too. That eliminates the starter, battery, alternator, coil, spark plug wires, fuse-able link, distributor electronic ignition , the electronic ignition module, ignition switch, ignition relay and fuse box. Now, could one of these go intermittently bad? Sure!!!  Symptom: running just fine and instantly dies. Will restart after 5 to 15 minutes.  When dead, it has no spark. Minutes later, it has spark. This happens after it's just started, it happens when it completely warmed up. There is no pattern. There is no sputter, shudder or notice when it will die.  New coil doesn't make any difference. Coil tests fine. Everything that can be bench tested has been. 


One of the parts replaced was an OEM electronic ignition module. Found an old stock, unused unit. I bought it for the shop (at less than half the price they were going to charge me) and had them install it along with some other work being done. They also replaced the coil while in the process. I asked for the old parts. When I got home, just for fun, I switched out the new units for the old coil and the old EIM and sure enough, it started right up with the "bad" units. I put the new units back in and kept the old ones as back-up parts. This is one of the best shops in the area. The shop may be okay but the mechanic may have issues. Probably has a big boat, two ex-wives, hot girlfriend, horses and a drug habit. 


Dirty connection? Loose wire? Temperature sensitive connection? Poltergeist?  I'm hoping someone else has had a similar problem and found a solution. This problem probably isn't exclusive to this brave little truck. Seems like every other time I'm out with it, someone asks me if I want to sell it. Holds it's value for being 42. 


Thanks for the help. Daddio.    

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Bought a new pick up coil for the distributor. Thought that might the problem. Nope. 

Installed a new ignition unit (under the glove box), both less than six months old. Old one worked when I tested it after it came home from the shop. 

Yep, been there, done that. 

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We know all parts have been changed so it would appear that there is an intermittent  bad connection between some of these parts.



Get a volt meter. Next time it quits unplug the box and check......


Power supply on plug by connecting + meter lead to the Black/White stripe wire and - lead to the Black. Ignition on should read 12v (or battery)  From ignition switch. Could be this.


Primary circuit on plug  + lead to the Blue wire and - lead to the Black. Ignition on read 12 v This reads ignition switch through the coil.


Pick up coil continuity on plug. + lead to Green and - to Red. On resistance setting should read 750 ohms. Much less or much more is a fail.


Check pick up coil's power pulse signal. Disconnect coil wire from distributor cap so engine can't start. On plug connect + lead to Green, - to Red, same as above. Meter on 2.5v range. Use starter to turn engine over. Meter should deflect. No movement... no good.



On coil....

+lead to the coil - terminal. - lead to ground. Turn ignition on should immediately read 12v through the coil.



That's enough for now to check. Hopefully it's one of these checks. I'm hoping it's the ignition switch. With all these pieces being changes all connections have been unplugged and plugged back in.




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King Rat, appreciate the checklist. There are several things on that list I have not tried. Wasn't looking for a short then. 


I'll print it off and run though them next time it acts up and report back on what I find. At this time it starts right up. 😒 Can't trust it. 


I bench tested the ignition switch last time it wouldn't run. It seemed fine at the time. However, it is a stock switch so I couldn't blame it if it did have issues. 


Much appreciated.  

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If not spark when this happen then yes its electrical and rule out the fuel system.

Im not familiar with the Nissan EI systems myself except they work or don't.


just make sure they didn't put a point coil in a truck that requires a EI coil.  a point coil will get HOT  HOT HOT so check that out. Cause they not running a ballast resisitor. EI cars don't need this and are made to run on 12/14 volt w/o a ballast



My father had a ford truck that would do this exact same thing everything was changed except the coil. By the time I told my dad maybe the coil is failing he gave the truck away. I didn't want it either as he had to be towed 3 times!!!!! on side of freeway.

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