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*hiccup*


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I have mentioned this issue in some other forums/threads, haven't found any real clear solutions yet so here goes.

 

Okay, well there is no better way for me to describe this problem than as a hiccup; basically at random times (I will list examples below) the motor (L20B, it's in the ol' 1980 720) will either drop RPMs to 0 and then come back to life right after, OR develope this steady skip while cruising.

 

The sudden RPM drop happens when:

Taking off from a stop sign - particularly this one that is on a decent incline about 20ft from my house.

After coasting and starting off from a low gear (manual trans.)

-This bothers me because it happens over and over again, imagine a fat kid running up the side of a half-pipe, getting to the top and then rolling back down just to get up and try again. The engine will get up to about 1500 RPM then just shut off, but once it hits bottom it bounces right back up to idle.

That's what is going on, and that's what will happen as many times as you are patient to try, UNLESS you rev. the engine to about 3000RPM and then let the clutch out.

 

 

The skip happens when:

cruising at about 2500 RPM in 3rd gear or so, just a light *bap..bap..bap..bap* following rpm decrease or increase.

 

Lately I have gone over the electrical systems (the battery charging light STILL refuses to turn off). New alternator, new battery terminals, fuses are all good.

 

Would the bubbles in the fuel line be causing this? I put in a new fuel filter AFTER this issue started happening. I haven't noticed any leaking places in the fuel lines either. Plus the truck has been pulling those bubbles through ever since I can remember.

 

If anyone else has had this problem or anything like it, I would be glad to hear how you solved it. Or any insightful information is as always welcome.

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OK, I have mentioned this before, and the more you talk about it, the more I think it is what I said it was.

Do do you have a tach?

Does it drop to 0 when this happens?

If you don't have a tach, get a cheap one at Baxters or something, hook it up and see if the needle drops to 0 or if it jumps around when it happens, if it does, then you are having an interruption of power to the distributor. 

You see when you start out the engine shifts as you let the clutch out, when starting out up hill, it will even shift more, this shifting will bend/move wires in the same place over and over till things get loose or even start to break, even on the inside of the wire.

Your 1980 Datsun 720 has a matchbox distributor, it has a "T" connection on the distributor coming from the coil, while the engine is running, lightly move this set of wires around, don't yank on them, just push and pull on them sideways like when the engine shifts, you are trying to reproduce the situation that happens to the wires going to the "T" plug that goes to the matchbox, but it is a lot easier to see it happen with a tach.

Mine always lost power when I let off the gas, never when I gave it the gas, sometimes it would not get power till I either pushed in the clutch, or it died, then it would start again, because the engine returned to the center, as it wasn't under compression.

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But the issue does not occur at idle, it idles perfectly. It only happens when the RPM goes up at a particular rate. And yes it goes to 0, the whole truck shuts completely off for about half a second.

 

I am beginning to think the issue discribed here and the battery charging light issue are related. I checked to see if the truck was back to running off battery power for everything and sure enough when I disconnected one of the terminals the truck shut off.

 

But anyway here is a pic of the coil, I believe the "T" connector you're referring to is the white one to the right, under the vac. line

 

1229131123_zps04a2b4f3.jpg

 

The connections here are pretty tight, the "T", the wire to the dist., and the two bolted to the flange on the coil.

 

Here is a picture of the buzzing relay that the parts store said was a "blower relay."  

 

 

 

1229131139_zpsb679921b.jpg

 

It buzzes for a couple seconds on and off, then you hear a light "click," stops for a second then starts again.

 

But as it sits the truck is still running on battery power, holding a sucky 12V on the guage. All the fuses are good this time.

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The buzzing relay is common to the 720. It is most likely the choke heater relay. For the choke heater to be on it needs a fully functioning alternator output. If the alternator is not charging, the relay will turn on off on off and produce a buzzing sound. You might check your choke as well as without the choke heater relay working the choke probably does not warm up and shut off properly.

 

I have to agree with wayno, if your tach drops to zero then this indicates a loss of ignition signal. Something loose is likely aggravated by engine movement when letting the clutch out.

 

 

 


Lately I have gone over the electrical systems (the battery charging light STILL refuses to turn off). New alternator, new battery terminals, fuses are all good.

 

.

 

Several things here.

 

Most importantly, look on or near the battery positive cable. There should be a short length of  RED and a GREEN wire with plastic connectors joined to the battery positive on one end and going into the harness on the other. These are the fusible links that protect the wires going to the fuse box, the ignition and headlights. They are designed to melt like a fuse and may not appear burnt. Wiggle these wires with the engine running and see if it disrupts anything. The alternator is also connected to the RED of these fusible links, so have someone watch the charge light goes out.

 

Check also the ground cable where it bolts to the head. Clean and tighten. There should also be a ground connection to the body sheet metal near the battery.

 

 

If this was a $40 replacement alternator from Rock Auto (or where ever) it may have been bad in the box. This is very common these days with a 50% failure rate. Have it tested.

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I have one tan wire coming from the terminal, and one red wire with the yellow end on it coming from the pos. terminal aswell. The long white fuse container at the bottom left, goes to a wire that runs allllllll the way under the truck to the back passenger wheel, and just stops, it doesn't tie into anything. Which wires would have melted? There are a couple small, like 16-20ga wires that are there, one running to the alt. (has the blue splice peeking out from the wiper fluid resivoir). And then that tan one, but the tan one is good because I can disconnect it and the engine will die. And the alternator is a $55 remanufactured one from O'Rielly's. Also should the alternator pulley be able to spin without any noticable resistance?

1229131329_zps254e1c35.jpg

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The red wire on the right may be for a stereo power lead.

 

The pale yellow one has been modified for sure. The fusible link has been replaced or jury rigged. The red wire (with white paint) has an inline fuse. See if it's blown. Remember one of these has to connect to the alternator.

 

Red and Green fusible links are rated at...

 

Although they do the same job, I think the fusible link is slower to melt than a fuse. A sudden current spike could blow a fuse but a fusible link takes a few seconds.

 

Here are the Nissan fusible link color codes and ratings.

 

Pink..... 30amp

Green... 40amp.... 0.50 sq mm.

Red...... 50amp.... 0.69 sq mm.

Yellow.. 60amp

Black.... 80amp... 1.25 sq mm.

Blue..... 100amp

These fusible links (or lack of) are at least part of your problem....

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The pale yellow one has been modified for sure. The fusible link has been replaced or jury rigged. The red wire (with white paint) has an inline fuse. See if it's blown. Remember one of these has to connect to the alternator.

 

 

These fusible links (or lack of) are at least part of your problem....

You check this?

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Yes I just pulled the section of wire that would serve as a "fuse," put the meter on it and it would read continuity...sometimes. So for today I will drive it with a piece of 10ga. wire in its place and see if there is any difference in the charging light/skip/firing problems. And I replaced that fuse yesterday, 15A32V.

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So I just tried something...

 

I had opened the throttle, brought up the rpm to about 2500, then disconnected the battery....ran FINE. Until I let off with the gas. It got down to about 1000 rpm, then died.

 

I have done this several times with the same results.

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Two things...

 

The buzzing relay is common to the 720. It is most likely the choke heater relay. For the choke heater to be on it needs a fully functioning alternator output. If the alternator is not charging, the relay will turn on off on off and produce a buzzing sound. You might check your choke as well as without the choke heater relay working the choke probably does not warm up and shut off properly.

 

 

If this was a $40 replacement alternator from Rock Auto (or where ever) it may have been bad in the box. This is very common these days with a 50% failure rate. Have it tested.

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Voltage on the terminals is a steady 12.02V across the board. unchanging, unless you turn on lights or radio, then it drops to 10V and some change.

 

I had O'Reilly's test the alternator, pulled it out of the truck and took it over there. The boy behind the counter's expression was "herpderp, ain't nothin' wrong with it." Which according to their machine was true. It tested 3 points, input, output, and magnetism.

 

So on my way back, there is an Advance Auto Parts. I decided what the heck, let's have a second opinion. According to the machine at Advance, which tested for all the same points as the O'Reilly's testing station, the alternator FAILS the "DC Charging" continuity test (not even an option at the O'Reilly's testing station) and one other I cant remember. Basically the Advance machine was a 5 point test, and the O'Reilly's machine was a 3 point.

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See the orange on the top of the "T" plug in this photo of the distributor.

DSCN1983.JPG

The "T" plug is orange again, and the distributor ground wire is right next to it.

DSCN1984.JPG

It has silicone on it because of engine shifting under load or compression, it would make the engine cut out, and the engine would run erratically at best. 

This was how they fixed the issue, and I inherited it from somewhere, as long as it runs, I just don't care at the moment.

Sometimes the wires start to pull out of the "T" plug and touch each other, this can also cause the same issues you have.

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Yeah it's definitely not the connector you're referring to wayno, I had someone look while I adjusted it and no change.

 

But I did finally get the new alternator from O'Reilly's (after two trips, and evidently a ghost alternator floating somewhere around their store) had it tested by Advance, all good.

 

During installation I disconnected the battery first, and went about bolting on the alternator. When I was plugging in wires (I have done this 3 times now), evidently something hit that shouldn't have (possibly the battery terminal bumped back against the post and connected the battery) and there was this spectacular crackling arc from my pliers to the bottom bolt of the alternator.

 

I fired t up, and sure enough when you disconnect the battery the truck STILL dies.

 

I am thinking this arc toasted an internal relay, or solder joint, or resistor? But I took it back to Advance and it gave the same reading as the FIRST one I got from O'Reilly's.

 

So I am thinking of taking it to the Alternator/Radiator/Starter rebuilders a couple towns over. I am done messing with the circus that is the O'Reilly's store.

 

Would this arc actually toast the alternator? It never actually jumped to any terminal, it just hit the outside shell near one of the bolting points. I didn't think these things were that delicate.

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as ratty as it may look nothing seems to be wrong with it, I am getting continuity on everything and there are only 4 places any of those wires are going: the battery, the starter, the alternator, and the wiring harness.

 

I did unplug the alt. except for the ground and started the truck, charging light went out.

 

I checked with the Hayne's book specifically for 1980-85 720s and it said that if you remove the leads to the "L" and "B" terminals on the alternator and check for voltage you should get NO MORE than 0.5V max. If less then the voltage regulator is functioning properly and the alternator is good.

 

If more than 0.5V the regulator is toast and the alternator needs to be replaced. Mine read a whopping 1.8V max on the vertical "L" prong and a 1.6V on the horizontal "L" prong (it's one of those "T" shaped connectors). Either one is past tolerances according to the book.

 

Would this be causing the charging light to stay on? Or the battery to drain? 

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I don't know what to say. The stock Nissan alternators are practically flawless and I would trust a used wrecking yard one over any re manufactured, NAPA, O' Rilies, Rock Auto, Pep Boys junk. Too many times I hear on here about having 2 or 3 alternator changes before you get a good one. These are $250-$300 new at the Nissan dealer... what are you getting for $60 these days? Stuck on the side of the road with a lifetime guarantee in the glove box.

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too god damn true. I am wishing I kept the old one and had it rebuilt.

 

I am not past the point of going full out mad scientist and robbing an alternator off a retired police Crown Vic interceptor and making that 200amp monster fit. Them bitches are bullet proof.

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Sometimes its worth the extra money, for a quality part.

 

If you spend that kind money, make sure your battery is disconnected during install.

 

Also, your battery needs to be fully charged, before you run the new alternator.

 

Cover all your bases, for a more satisfying outcome.

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Well, I guess the lesson of this thread should be: don't trust remanufactured alternators.

 

I just got my hands on one from Advance (O'Rielly's refunded the p.o.s. the sold me), put it in, hooked it up and behold, no charging light anymore! This one was a little more expensive and took a few days to get here but it did the trick. Voltage reading is well within tolerance from the alternator, the battery is receiving charge, lights and radio function properly, and the cab voltmeter is also reading normal (about 15-16, it wobbles).

 

I appreciate all the input and advice, without it I would have assumed that multiple faulty alternators in a row was just statistically impossible and tore the truck apart chasing a non-existent wire fault.

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The failure rate right in the box is unbelievable. If they sell it for $40 what the hell do they get them for? Cheap replacement parts, cheap labor, assembly line, crank them out the door. Never throw the original equipment away.

 

 

too god damn true. I am wishing I kept the old one and had it rebuilt.

.

Probably just needed the brushes replaced.

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