Jump Starting Blew A Fuse?

[Sandrailstudent]

I killed the battery by leaving the lights on, and connected jumpers to the + terminal in the front of the truck and the - to the engine lifting lug in my 85 diesel MM. It charged for about 20 seconds, started to smoke under the hood. I disconnected the jumper cables and looked for the source of smoke, but couldn't find it. I used thick jumper cables with more than 1/4" diameter copper and jumped plugged it in to a running car battery.

After the smoke all cleared and the places that I clamped to on the truck cooled down, I noticed that none of the cabin lights turned on. I tried connecting the jumper cables again, but it didn't seem to push any more charge through, or light up the cabin. All of the fuses in the fuse box are still good, but nothing works.

The other perplexing thing is that when I removed the truck's battery and measured resistance to the + charging terminal in the front of the truck, it had no resistance. The same was true with the - battery terminal to the engine. So the positive and negative cables in the truck have continuity. I also confirmed this by getting the battery charged and putting it back in the truck. The + terminal in the front has 12.6V, which is the same as direct to the battery. I would expect everything in the electrical system to work, but nothing does. I also questioned whether the battery was somehow not giving enough current to actually run something as small as a dome light, so I removed the truck battery and replaced it with jumpers to a running car, but nothing came on.

My question is, are there any fuses in the charging line? If so, where are they and what do they look like? I am super confused at this point because I haven't been able to isolate the problem. I am not sure if I need to start looking for shorts. I would expect that a big short in a hot line would cause a bunch more smoke, and make itself obvious.

Thanks in advance,

Chris

[4cylinders]

These trucks use a fusible link. It's a section of wire that melts with an overload. It's located around the battery main harness area.
Any time.

[SubGothius]

I think 1st-gens use the same fusible-link box as my early 2nd-gen, should be a box that looks like this on the battery-side inner fender:



...where a blown/failing link may look like the one on the right here (this one was obviously a sketchy replacement to begin with):



Reckon these Datsun 280Z green-wire links should be a suitable replacement:

https://www.ebay.com/itm/314615621571

One thing puzzles me about this bit:

connected jumpers to the + terminal in the front of the truck

I wasn't aware any of these trucks had a dedicated (+) terminal for charging, apart from the actual battery's (+) terminal? Maybe a 1st-gen or diesel-only thing?

[Sandrailstudent]

Thank you both for the responses.

SubGotius, yes, this seems to be a diesel only thing for the California models that have smog stuff located where the battery normally would be. Threre is a long hex head that looks like a 3/8" allen key that the + jumper cable can hook to.

I will go looking for that fuseable link.

Thanks again,

Chris

[Sandrailstudent]

I did not find any fuses in the positive side of the system. I did find one broken and partially melted wire connecting the main positive cable to the rest of the wiring harness. The insulation was gummy and seemed like it was dissolving. It was a short wire that disappears into a buddle, and I didn't trace it or test it. I couldn't get the "quick connector" apart, but will eventually figure out a way to get the two plastic pieces apart.



I will post a reply when I figure out if this was what was keeping everything from working. If this was powering the ignition switch, then it would make sense that nothing worked with it broken.

Thanks again,

Chris

[xboxrox]

My 1986 truck has 2.6L gas engine BUT perhaps your truck has similar wiring harness fuses; fuse thread here:
http://www.mightyram50.net/vbulletin...-Fusible-links

[Sandrailstudent]

Thank you all. I got some 16ga fusable link from Autozone. I couldn't read what the burned up wire said, but I counted every strand and dial calliper measured one strand from the old and one from the new. Everything matched exactly, so I'm sure that it will work just like factory.

This was WAY more effort than just buying a regular knife blade auto fuse.

I will not be jumping an entirely dead battery again on this truck. That suck'd.

Chris

[xboxrox]

Good job Chris & thanks for the update..!