Battery and brake light came on & the voltage gauge dropped the other day. Checked all the fuses & wires, as the battery was new & I was only getting 12 volts with the truck both running and off, all signs seemed to point to alternator.

Changed the alternator, and no difference. The truck dives fine, drove it around for a couple days, even though it's only running on 12 volts the battery doesn't die so the alt must be working, but obviously something is wrong.

Cleaned the connector for the alternator wiring harness, changed the alt 80a fuse, new fusible link from the battery to the fuse box, new battery terminals... Trucks starts just fine, no struggle & runs great.

I've been pulling my hair out trying to figure out what's going on. Any help or direction would be appreciated.

1992 4runner, 3vze

 

With the truck running, use a multimeter to check the voltage at the battery terminals and then check it at the alternator wire stud. You should have 14v+ at the stud. If you don't, then probably the alternator you just put in is bad. If over 14v on the stud but still only 12v at the battery, then I would look at replacing the main alternator wire. There should not be more then a .5v difference from stud to battery.

 

Yeah I forgot to mention that I checked that, I'm getting 12 volts at both the battery and the alt stud.

You gave me an idea though and I went one step further, I disconnected the wire from the stud with the truck running to isolate the alt & see what it was putting out... Only 2v at idle and around 4v revving.

I'll have to wait until Monday to get another one but it seems the new alt I put in was defective, I can't really see any other options.

 

That could be the cause, but getting bad components "out of the box" is rarer than some people think. If it doesn't pass the basic voltage check suggested by Robert m, I would check the rest of the components next: http://web.archive.org/web/201102051.../2onvehicl.pdf Especially the "ENGINE" fuse. http://web.archive.org/web/201204170.../2powersou.pdf

 

On these newer computer controlled engines you never want to disconnect parts of the charging systems. While the engine is running.

I suppose getting bad parts out of the box might just be my bad luck after all I married a closet Lesbian

 

I wouldn't consider it rare. I've replaced lots of alternators and have gotten a few bad alts out of the box or had the "new" ones fail within a week or 2 of being installed. The cheap/crappy remans that part houses sell are a crap shoot and I now make sure to have them tested in house before I walk out with it.

 

Rare or common, what are you going to do if you have a blown Engine fuse or a broken IG wire (color R)? When you take that alternator back and it tests "good," will they still give you a refund? Even if they do, will they sell you another one? No store wants you skating through their stock of alternators just because you don't know how to diagnose a charging issue.

You can often "get away" with just buying replacement parts at first because it is a little more likely that an alternator will fail than one of the wires will break. You still risk an unnecessary (or useless) trip to the store.

Of course, if you've had Wyoming9's or Robert m's experience, having the store test the "new" alternator before you leave is probably a pretty good idea.

But hishameen doesn't have that option right now, so he can pull the alternator and haul it down to the store, hoping that it will test "bad," or he can check the inputs to what is supposed to be a good alternator.

 

Trouble shooting is the easy part pulling the parts off is a pain in the butt if it comes off I know it is bad .

 

O.P.,
Scope103 brings up a good point...
You may have checked fuses, but you may have missed the ENGINE fuse because it is not named properly. The "engine" fuse is more critical to the charging system than the "charge" fuse. It provides excitation to alternator via the IG wire. If engine fuse orIG wire is bad, alt will not generate anything.

You can quickly check if you're getting +12V at IG by probing the back of the alternator connector with a paper clip connected to positive probe of multi-meter. If no voltage there, it could be bad engine fuse or the IG wire like Scope mentions.

Many things can cause the engine fuse to blow. For example, a back-up switch or its wiring shorting to ground like this

See schematic and suggested test points below. Not sure if same as your model truck but concept remains the same.

 

I had already checked the alternator wires, they were all good.

It turned out to be a bad alternator, I had the my original one rebuilt.

I probably would have just chalked it up to that in the beginning if I hadn't had wiring issues in the past, it is after all 24 year old wiring. At least it's all new now.

Thanx for all the input.

 

BAHAHAHAAHAHAHAHAH! You are hysterical!

 

FWIW, I have gotten a bad alternator out of the box before. It happens. And you get better at swapping them as a result.