keep blowing alternator fuse

[marshull]

2002 4runner, SR5, 2 WD
Was having an issue where I was slowly losing power. The clock was diming, I began to lose acceleration, tach dropped to 0, then I heard a click like a circuit breaker popping then everything was fine. Took it to the parts shop and they tested battery and alternator. They said the battery was fine but the capacitor on the alternator was faulty. I put in a new alternator and as soon as I connected the positive cable to the battery, the alternator fuse popped. Went to the junkyard and cut out an alternator plug just in case there was an issue with the old plug (it didn't help that I snapped the connector off of the old plug). Used a wire brush to make sure all of the connectors were good and put in a new fuse. As soon as I connected the positive cable to the battery, there was a slight spark and the fuse blew again. I don't even get a chance to start the car or turn the key.
I am thinking that with the issue with the old alternator, it fried out a wire somewhere. I have no idea even where to begin testing the wire for breaks. Should I just buy a whole new wiring harness?

 

[photoleif]

2002 4runner, SR5, 2 WD
Was having an issue where I was slowly losing power. The clock was diming, I began to lose acceleration, tach dropped to 0, then I heard a click like a circuit breaker popping then everything was fine. Took it to the parts shop and they tested battery and alternator. They said the battery was fine but the capacitor on the alternator was faulty. I put in a new alternator and as soon as I connected the positive cable to the battery, the alternator fuse popped. Went to the junkyard and cut out an alternator plug just in case there was an issue with the old plug (it didn't help that I snapped the connector off of the old plug). Used a wire brush to make sure all of the connectors were good and put in a new fuse. As soon as I connected the positive cable to the battery, there was a slight spark and the fuse blew again. I don't even get a chance to start the car or turn the key.
I am thinking that with the issue with the old alternator, it fried out a wire somewhere. I have no idea even where to begin testing the wire for breaks. Should I just buy a whole new wiring harness?

this might be a long game of cat and mouse to troubleshoot. i had a bad alt once, and some of the same symptoms... loss of power, tach low, impending sense of doom. is your replacement alternator brand new? you hooked up the battery and immediately the fuse blew... with the engine off? or was that upon starting?

i doubt there's an issue with the heavy-gauge wiring that goes from the battery to the fusebox, but you might want to just check that out -- make sure it's tight. the alternator fuse... you're meaning the big one in the fuse box? 120A? that is a crapton of current, so something on the far side of that is drawing over that 120A, which is insane. is there anything new, recently installed? nearly any cable (2 ga or smaller) that's being asked to carry 120A is going to be fried. you might just get a bright light and shine it around near the alternator to see if there's any visible burned cable.

i'm suspecting a short (gee really) to ground, and that could well point to an issue with the alternator cable. with the battery positive cable disconnected, i'd check continuity and resistance between the battery positive cable and various points.

i realize this is a lot of shots in the dark, but it at least might help you isolate something.

 

[marshull]

this might be a long game of cat and mouse to troubleshoot. i had a bad alt once, and some of the same symptoms... loss of power, tach low, impending sense of doom. is your replacement alternator brand new? you hooked up the battery and immediately the fuse blew... with the engine off? or was that upon starting?

i doubt there's an issue with the heavy-gauge wiring that goes from the battery to the fusebox, but you might want to just check that out -- make sure it's tight. the alternator fuse... you're meaning the big one in the fuse box? 120A? that is a crapton of current, so something on the far side of that is drawing over that 120A, which is insane. is there anything new, recently installed? nearly any cable (2 ga or smaller) that's being asked to carry 120A is going to be fried. you might just get a bright light and shine it around near the alternator to see if there's any visible burned cable.

i'm suspecting a short (gee really) to ground, and that could well point to an issue with the alternator cable. with the battery positive cable disconnected, i'd check continuity and resistance between the battery positive cable and various points.

i realize this is a lot of shots in the dark, but it at least might help you isolate something.

Only thing new is the alternator. Well, new to me at least. Yes, the fuse is the pita bolted in one. The 120a. I connected the neg cable, then as soon as I touch the positive terminal to the positive pole on the battery, it sparks and I blow the fuse. Never get a chance to turn the key or start the car.

When I pulled out the old alternator, I noticed the bolt where you screw the wire to it was kind of fried looking and the insulation around the wire feels brittle. Like it cooked for a bit.

The neg is grounded to the frame right next to the battery, I unscrewed it and ran a wire brush on the spot of the frame and the bolt to make sure it had good contact. Guess I can give it the once over and make sure it really is grounding.

 

[marshull]

Well, I am at a loss. Everything is grounded fine. I don't see any wires that are showing any other issues.

Admittedly, I am not a serious car guy, much less an electrical guy. I know enough to get myself in trouble and thats about it. If it is mechanical, I just pull out the broken thing and put in a non-broken thing and that's about it. But I am confused. As I see the wiring, the positive connection on the battery has 2 wires. One is the thicker main cable and then a black smaller wire. The smaller heads into the fuse box. The white wire that comes out of the fuse box heads to the alternator. The 120amp fuse is in between the black wire coming from the battery and the white wire running to the alternator. So for the 120amp fuse to blow, more than 120amps has to be moving from the battery to the alternator. What in the world could be causing that without the alternator even running or the key even turned on? I am just really confused.

 

[Kanoe]

Sounds like something in the new alt. is shorted out. Where’d you get it? Check carefully the copper stud on the new alternator, it could be touching the case somehow.

 

[marshull]

Sounds like something in the new alt. is shorted out. Where’d you get it? Check carefully the copper stud on the new alternator, it could be touching the case somehow.

It is a rebuilt from ORielly. Is there a way to take it in and have them look at it and see if there is an issue? I guess I could pull it out and remove the cover to look it over for anything obvious.

 

[HiLife]

Ha, depends on the counter guys there. All locations should have the alternator/starter tester. Seems like you have to twist their arm to do it.
Don't know why most just "dont have" the Toyota plugins as there are lots of toyotas on the road.

 

[marshull]

Ha, depends on the counter guys there. All locations should have the alternator/starter tester. Seems like you have to twist their arm to do it.
Don't know why most just "dont have" the Toyota plugins as there are lots of toyotas on the road.

So I can just bring the alternator in? Thought they would have to test it on the car while it was running.

 

[ambush]

IT IS the ALTERNATOR

Hey electrical is funny on cars. I guarantee if you even decide to get a reman or another "new" one over the counter you will be changing it out again. I promise.
Go back to the junkyard and get an OEM one from a wrecked 4runner, Tacoma or Sienna! DENSO is the only one that will not give you problems. I PROMISE. Take that retail foolishness back.
Now if you want to really protect yourself after you reinstall, install your BIG 3 and fuse your ALTERNATOR to POWER as well..

this is what I did....

I added 150 AMP fuse just for my nerves. If if blows, it will hit that first before it hits the oem one in the fuse box.

PLEASE go OEM or if you can wait order one from Rockauto if you are going spend that type of $$

 

[HiLife]

So I can just bring the alternator in? Thought they would have to test it on the car while it was running.

They can do both. They have a hand held to test on a running car.
And they have a inside the store counter mounted machine that spins the alternator and tests the output.
If one location says they cant do it, just go to another location.
Hard to find a motivated enough counter guy to do the testing correctly.

 

[phattyduck]

It is a rebuilt from ORielly. Is there a way to take it in and have them look at it and see if there is an issue? I guess I could pull it out and remove the cover to look it over for anything obvious.

Gonna second the other poster here. Oreilly rebuild? Junk. Make sure it is an OEM or Denso reman. ONLY.

-Charlie

 

[thennen]

To echo others, it sounds like the "new" item isn't good - the alternator. If it's instantly popping the fuse as soon as you touch the positive cable to the battery, with nothing on and the vehicle turned off, it sounds like an internal short somewhere within the alternator. One way to quickly rule out the wiring that you suspect is to completely disconnect (remove, even) the alternator, then reconnect the battery. If the fuse still blows, it's obviously not the alternator. If it doesn't blow, maybe you can find a schematic online for the alternator and use an ohmmeter to check for shorts internally.

 

[Traveler74]

Only thing new is the alternator. Well, new to me at least. Yes, the fuse is the pita bolted in one. The 120a. I connected the neg cable, then as soon as I touch the positive terminal to the positive pole on the battery, it sparks and I blow the fuse. Never get a chance to turn the key or start the car.

When I pulled out the old alternator, I noticed the bolt where you screw the wire to it was kind of fried looking and the insulation around the wire feels brittle. Like it cooked for a bit.

The neg is grounded to the frame right next to the battery, I unscrewed it and ran a wire brush on the spot of the frame and the bolt to make sure it had good contact. Guess I can give it the once over and make sure it really is grounding.

I am pretty certain that you must re-install the POSITIVE cable 1st and then the negative. Doing it the wrong way can fry certain sensitive electronics.

 

[marshull]

Turned out to be the alternator. I pulled it out and took it back into Oreilly for testing. The guy put it on the machine and it counted down 3-2-1 and fail. He scratched his head then checked the cables and hit test again. 3-2-1 fail. Got a "new" one and dropped it in and no problems. Going to have to agree that the Oreilly rebuilds are crap. Considering that the one that failed was a replacement for another that failed after a month which was a replacement for another that failed after 3 years.

Seriously looking at doing the "big 3". The post is pretty well laid out and seems to really future proof the engine/alternator. Guess I have my next project lined up.

Thank you everyone for your advice and help.

 

[HiLife]

Glad you figured it out and didn't blow too many alt fuses.
Those fuses are expensive.

 

[thennen]

Glad you figured it out and didn't blow too many alt fuses.
Those fuses are expensive.

Yeah, O’Reilley should have reimbursed for those.

 

[IBallEngineer]

I am pretty certain that you must re-install the POSITIVE cable 1st and then the negative. Doing it the wrong way can fry certain sensitive electronics.

Sorry, this is untrue, although a good practice to bolt on the positive first to avoid grounding a tool, it is not necessary to any of the electronics.

OP, sorry I arrived so late to the party, but the first thing to check was the continuity between the bat terminal on the alternator, and the alternator housing. If you were blowing the 120A fuse, that thing was ABSOLUTELY shorted to ground. No question. DVOM classes should be mandatory at the high school level. Just sayin'