Hey folks! I acquired an ‘87 4Runner a few months ago, and I’ve been googling how to repair some of the issues it has. Here recently, it wouldn’t crank. The battery was at or juuuuust below 12VDC, and nothing would happen when I turned the key. I jumped it, and I was reading 12.5 at the battery and 13.8 at the post on the alternator. The alternator harness (from the alternator to the plug at the passenger fender) ohm’d out fine. No opens or shorts. I decided that while the alt wasn’t at prime operating condition, it “should” be giving me enough to charge the battery - so I ran the “big 3”. Overkilled it, really: 0/1 from the alternator to +batt, with a 150amp fuse in hopes of an alternator upgrade in the future. 0/1 from -batt to a bus bar, with the same gauge wire going from -batt to the block, frame and passenger fender. Added 0/1 from the driver’s side of the block to the frame and 6awg from the head to the firewall. Cranked her up and I was reading 13.8VDC at the battery, and at the alternator post. She drove like a champ for about 10 miles and started to die out on me. First indication of it was the rear window going up slowww after my kid grabbed his football gear out of the cargo space. I killed the A/C & headlights and got back to the house. Pulled out the meter and read just above 12VDC at the battery and alternator. Turned her off and tried to crank her up again. The starter got it to turn over about twice and then nothing.

As far as the new wiring I installed, I’d read up a lot about this modification. I watched many YouTube videos, and all of them advised to reinstall the factory charge wire onto the alternator as well as the new fat wire, as well as hooking the fusible link to +batt,


0/1 & stock charge wire

-batt connection. All sheet metal connections were made to bare metal

+batt to fusible link, alt charge & starter

150amp fuse between the alternator & +batt (about a foot away from the battery)

6awg from the head to the firewall (bare metal)

Charred 80amp fuse
so naturally I heeded that advice.

I pulled the fuse box cover and noticed that the 80amp fuse element is now black. I distinctly remember that it wasn’t in this shape earlier this morning, before I ran this mod.

With this being the stock alternator, I can’t imagine this mod would be the reason that fuse is blown - but I’ve been wrong more than right.

On a related note, I was thinking of installing the jaguar alternator. With it being a “120amp” alternator, wouldn’t that blow the 80amp fuse?

 

First off, the wire from the alternator to the fuse block is a fusible link. At least the OEM one is. If you read 13.8 VDC at the alternator post, and 12 VDC on the battery, either the fusible link from the alternator, or the one from the battery to the fuse block, was bad. Most likely burned through. They're made to allow the wire core to burn through without the insulation burning apart. Protects the places the wire runs. If you're going to replace either of the two, make them OEM fusible links.
The cable from the battery + to the starter is a direct run, large wire. I think I used 1/0 when I replaced mine. Same with the two negative leads, one to the body metal, and one down to the AC Compressor mount. That's the engine block ground, that the starter uses. Obviously, it's pretty important.
That 6 AWG wire from the head to the firewall is just about perfect.
One of the wires from the plug in the back of the alternator is a ground wire. Usually run directly to the Power Steering mount. It's important for proper alternator operation.

The jaguar alternator being 120 A won't be a problem. It won't produce more current than the draw on it. Chances are, you won't ever draw more than the stock alternator can provide. Not unless you have a heavy duty amp for a sound system installed, a dozen KC Highlighters, or something like that. Even when the starter is cranking, it will probably not draw more than the stock alt can provide. Actually, it draws off the battery almost entirely, that's why the two cables direct to the starter from the battery, and once the engine starts, the alt then charges the battery back up. Once again, no draw more than the stock alt can provide.
If the 80A fuse failed, there's a reason. There's a draw of more than 80 A on it somewhere. You need to find that draw. Someplace, there's a short to ground. Probably, a wire worn through, and touching bare metal someplace. Probably.

Since your battery was low at the start of this whole adventure, you need to take it to a parts store or something, and have them test it out. Including a load test. That's vital. Does the battery charge drop slowly if it sits unused for a time? Like say a day or two, maybe? If it does, you've got a bad battery. Connected to the truck's systems or not, shouldn't matter, but, if it does drain down connected to the truck's systems only, you've got a parasitic draw somewhere. Could that be what failed the 80A fuse? Maybe?

Ok, OK, shutting up now.
Good luck!
Pat☺

 

Thanks, Pat! I do think that the fusible link is still good, or was at the start of all this. I had thought of pulling the fuse box to ohm it out before buttoning everything up, but at this point there’s no reason not to since I’ll already have it apart to swap out the 80amp fuse.

I’ve been looking at the pics I uploaded, and thinking…. It looks an awful lot like my new 0/1 terminal lug at the alternator is possibly touching the case. That could be my short. I’ll verify in about an hour and a half when I get home from work.

in your opinion, would it matter if I just unhooked the stock power wire from the alternator? Of course, I’d like to remove the whole run going to the fuse box. I ask because I’m wondering if the fuse box will still be operable with the power to it is coming from the battery through the fusible link, as opposed to the power to it is coming from the alternator.

thanks,
Joe

 

Well, the system is designed to have the alternator feed the fuse block. The alternator feeds the far side of the 150A from the battery, and the FL from the battery feeds the "near" side, IIRC. That's where the fusible from the alternator goes, I believe. I've been wrong before. Often. Check your local FSM Schematic to be certain. YMMV, and so forth.
Anywho, The alternator feeds into the fuse block through it's fusible link. That's how the system is designed. If you want to replace that FL with a heavy cable, it's on you. You might go years, and thousands of miles with no trouble at all, but if, IF I say, there comes a time that the FL from the alternator will save the fuse block from a fire, you'll regret replacing the FL with a heavy wire. It's similar to replacing a fuse with a chunk of wire.

As long as the fuse block is getting a feed from the alternator, the big post on it's side, the system will be happy. If you remove all feeds to the fuse block from the alternator, what's going to charge the battery? Yes, the system will run on the battery alone, but not for long. Again, check the schematics. The fuse block needs a viable alternator feed to function correctly. It spreads from the fuse block to the rest of the system. That is what keeps the battery charged up, as well as running all the trucks systems. Yes, you could replace the FL with a big wire, as long as it goes where the FL went into the fuse block.

Does that help at all?
Pat☺

 

Got to the house and started probing around. The battery has dropped 3VDC in 24 hours, and I believe it’s because I fried my alternator.

I removed the 80amp fuse, and it read zero ohms. Same for the fusible link. Since it’s already apart and AutoZone has them in stock, I’m going to go ahead and swap out the fuse. As far as the fusible link, I cannot find them anywhere.

None of the other fuses in the fuse box are even slightly damaged. Clear as a bell & totally in tact. The fuse box did have a burnt smell to it, but I couldn’t find any damage. Nothing charred or melted underneath.

Batt + to head

Battery voltage

Batt + to charging post on alternator. I suspect that I shorted the case with the terminal lug and this is the result.

 

I need to get one of those FSM’s you speak of!

Also, am I reading your post right in that there are fusible links on both ends of the fuse box? One in from the alternator and one in from the battery?

My thoughts were to run the alternator cable to the battery, at the positive post, also therefore to the fuse box through the fusible link attached to the same post.

My brain likely looks like my 80amp fuse right now, trying to process this.

 

Burnt up alternator usually has a distinct oder...

 

OK, here's a section of the schematic in the FSM. It's the section involving what we're talking about. FL 2.0L is the fusible in from the battery to the fuse block. The red arrow is where the wire from the alternator comes in. No, they don't show the FL off the alt in this schematic, but it's there. One blue arrow points to the 80A Fuse which they call a fusible link, for whatever reason. The other points to the 140A (in this schematic it's 140A, in your truck it's 150A). The electrics in Toyota trucks change year to year, sometimes month to month, but overall, this schematic is accurate. You can see where the wire from the alternator meets the fuse block. Right between the 80A and 150A fuses. The wire that specifies that it goes TO the alternator is th always hot that provides the field coils with the voltage that makes the alternator work.

The 140A and 80A fuses connections are on the bottom. You have to take the part of the fuse block out and turn it upside down, essentially, to get to the connections. Be careful, some of the connections are a little close together, so a large terminal could connect something you don't want connected.

Does this help, at all?
Pat☺

 

Ok I’m picking up what you’re putting down. However, my fuse box doesn’t have and fuse over 80A. The other two that remain in the box are 30 & 40A. Also with that said, I cannot tell which wire the runs to the fuse box that could be a fusible link. I trust what you’re telling me and the schematic clearly shows two fusible links, one going to the box and one coming out - but I can’t discern which one is the FL. The parts store has an 80A fuse on hand, so I’m going to have my alt tested and put everything back together. If I can get her cranked up, I’m going to check voltage at each fuse, as well as what I’m getting out of my alternator.


Top of the fuse box. No 150 or 140A fuse. I’m guessing that if it were, it would’ve men installed below the 80A?

Wiring going to the fuse box from the harness. Other than the wiring going to the 80A fuse, and the two plugs going to the top of the box, These are all the wires I have. I can’t see where anything was clipped or pig tailed back into the harness in any kind of “delete”.

 

With the alternator out, I’ve taken some voltage reading with the key switched “on”, and with 12.61VDC, I had 12.49 @ B, 12.55 @ S, 12.34 @ IG & 11.83 @ L.

11.92 @ both sides of the 75amp fuse
.004 @ both sides of the 10amp fuses
12.57 @ both sides of the 15amp fuse

All these were with the switch on, and not running. I’m having a local shop rebuild the alt and they’re hopping it up to 180 amps all for less than a parts store reman. They also did say that it was really fried. I’m just trying to figure out if the alternator was the problem all along.



Battery voltage

75 amp fuse

Top left 10amp fuse

15amp fuse

Bottom left 10amp fuse

 

I'm not one to quibble, but that first fuse you checked was a 7.5 A fuse, not a 75 A fuse. Seven point five amps, not seventy five amps.

Because those two fuses on the left have zero volts on them, BTW: did you check BOTH sides of the fuses? and did you check the fuses for continuity? anywho, because there's no voltage on them, check the schematic. Where do they normally get their voltage from? Back-track until the line(s) feeding those two fuses shows good voltage on them. They could be the headlamp, OR the high beam fuses. If so, without the right switchology, there won't be any voltage applied to them.
Never use the ohms portion of you multimeter with the battery connected. Remove the cables from BOTH sides of the battery, or you can blow up your meter.

Anywho, sure sounds like the alternator was the failed item, yes. Having said that, it's a lot cheaper to go through all the testing we recommended, than to just jump on parts right off. It may be frustrating, it may be long and involved, and so on, but what if it was just a fuse, or a wire rubbed and shorting, or something like that? You would have paid for a alternator rebuild, for no reason. For some of us, like me, on a low, fixed income, that stuff matters. Not to mention draining the radiator, and removing the bottom hose, just to get to the alternator. If your careful, and save the coolant, great, but oft times it's old, and needs replacing anyway. Stuff like that, that MAY not need to be done.
BTW, once you've drained the radiator, you have to burp the system after you fill it again. Takes a while, and is kinda a PITA, but gotta be done.

Good luck, and let us know how things go for ya!
Pat☺

 

good catch on the 7.5 amperage and I’m truly grateful for all the help and knowledge you’ve shared with me. It’s been an immense help. I honestly mean that!

Also, I did get those same voltage readings on both ends of the fuses - and after I turned my headlights on, I got 12VDC at the 10 amp fuses.

Thank you so much for all your help in the matter. After I get the alternator installed, I’ll post up the results so anyone else that may have this issue in the future could find some answers a little easier.

Respectfully,
Joe

 

There's an update on the matter. After installing the alternator last night, I read 14.02VDC coming from the B post of the alternator, as well as at the battery posts. All charging fuses checked very close to the same voltage. I took her out for a drive, and she died on me at a stop, about 1.5 miles from the house.

Pulling up to a stop sign, the engine shut off. All gauge lighting cut out. Turned the key back to "off". Turned it to "accessory" and my radio would power up, the cigarette lighter would get power. Turned the key to "run", and it all powered down. Turned the key to "start" and Nothing at all happened. No clicks. Starter did not engage.

Since I had some time (waiting on my kid to show up with the truck & tow straps), I checked voltage at the battery posts, and had 13VDC. I had no voltage at any charging system fuse. I believe that the fusible link between the battery post and the 80-amp fuse melted, because every fuse in the fuse box was in tact. No visual damage at all. All of the fuses above the driver's kick panel were good visually as well. I do understand that they should be tested with a meter, but it was late, my kid showed up to pull me home, and There was traffic to deal with. I have this unshakeable fear that someone will hit the vehicle while I'm working on it.

I plan on removing the link to test it this evening when I get home.

Also, a hokey thing was happening after the stop sign death: With the 4-way flashers on, they'd cut out when I'd open either of the doors, and the door lights would illuminate. On the tow home, when brakes were applied, the 4-ways would cease to work. On top of that, they wouldn't always work with the same switchology.

By the time I got it backed into the driveway, I was tired, fed up & needed a beer, so i decided to give it a look when I get home from work today. In thinking about it, I'm wondering if the ignition switch may be malfunctioning. Before I explore that possibility, I need to look at what I've been working on and see what all may have been affected by it.

 

Alright, I got to piddlin on the 4Runner as soon as I got home. Voltage readings at the fuses in the engine bay were non existent no matter the switch position, so I started checking for continuity via resistance.

Alt post to case: 89.6 ohm
Alt case to ground: 0.1 ohm
Charge (B) wire to ground: 3.2 ohm (short)
Fusible link to ground: 3.3 ohm (short)
Charge wire to FL: 0.3 ohm (short)
All fuses in box: 0.3 ohm (short)

In light of these readings, I decided to get a closer look at the loom coming from the driver’s fender to the fuse box. All of the wiring looked like it was in good shape.

I pulled the 30 & 40-amp fuses and narrowed the ground to the left pin (as seen while looking at the fuse box from the front of the vehicle) of the 40-amp fuse. This wire splits into two and is connected to a cannon plug at the driver’s side between the fender & air box, directly behind the headlamp. The other shorts were eliminated with the 30 & 40-amp fuses pulled.

I checked for ground at both terminals for these wires, and I read 7.0 ohms at the top right terminal, as seen while looking directly at the male plug terminals.

By then it was getting dark and I had to pack up for the evening.

Does anyone know where this particular section of wire goes to? It appears to be going to the marker & turn signal.

Left terminal for the 40-amp fuse is shorted to ground.

Top right terminal yielded 7Ω to ground…. Appears to be a short?