The symptoms:
I had the original battery from oct/2000 and had no problem starting in 0 degree weather over the winter.
My alternator just went but it was the bearings. I could easily hear the grinding and of course, the battery light came on shortly after.
So I got a rebuilt alternator an put it in. The battery had been on a trickle charger and showed a full 12.5 volts. Imediately upon starting the truck the battery light was still on so I put the tester on the battery and it only showed 5v. I turned on the charger again with the truck running and got 12.5+ at the battery. I put the tester directly on the alternator and that was showing 5V.
So I went and got a new battery. I figure at 10+ years I got my monies worth from that one. But same thing on a fully charged new battery. 5V at the battery and at the alternator.

The ebay guy I got the alternator from claims my 10 year old battery instantly killed the alternator but he will replace it if I pay shipping.
Before I do that here is my question(s).

Does that sound true? I've been working on cars for 35 years and I know repeatedly having the alternator charge a week old battery will take a toll on the alternator, but not instantly kill it from one start. Could it?

Is there another regulator on this truck somewhere that I should replace kind of like the old days when it was mounted to the fire wall and not built into the alternator?

 

Take him up on the replacement. Not sure I would trust a complete stranger to rebuild an alternator. Ive seen them come back right out of the box from suppliers. A brand new battery with a bad alternator will only last a few days, if that. Not sure if a weak battery would ruin an alternator though, I don't think it would. He might just be trying to put the blame on you. When the truth is, you probably got a defective alternator. So replace it.

 

He most definately is trying to place the blame on me. But he's willing to replace it so it's really just another week of down time and inconvenience. And shipping cost.
It's an Ebayer with very high feedback scores and rated a top seller.

If the next one is also 5V then I have a problem somewhere else and that's what I'm trying to determine before shipping this one back.

I used to rebuild my own alternators back in the day when it was just a couple of springs and brushes. Oh well....

Thanks Toyz

 

I would never buy an alternator on Ebay unless it comes with a lifetime warranty.

 

I'm not sure why, with a new battery you would be getting 5v at the battery when new. Regardless of any electrical issue, a "new" battery should read about 12.5v. And when running should be 14 or so. Maybe your not testing it right? The truck wouldnt even start if it only had 5v at the battery...The voltage regulator is part of the alternator assembly...and typically where you would test for voltage. I would check your ground cable on the battery, for any corrosion.

 

The battery itself does have 12.5 at rest but when I start it that same battery now reads 5V. If I throw a charger on the battery while running I get the 12.5 reading. The ground cable at the battery post is pretty clean. Do you mean follow it to the other end?
Maybe I'm not testing the alternator right? Positive off the post and grounding to the engine block. I did take it off again and now see that it's stamped with "ground here to test" but the little star bolt is on the back. you can't get at that when mounted.

 

Then something is wrong. I would inspect the cables from the battery to the solenoid and starter. Also inspect the wires from the battery ground. Make sure they all look good with no corrosion and are tightly attached. Sounds like a loose wire or bad ground to me or a bad alternator. Never heard of drops that low though. Most issues with voltage drops due to bad cables or poor grounds don't dip that low. And I'm not sure if you can get to the regulator on the back of the alternator or not while it's mounted. It is however, free at places like autozone to test the alternator for you.

What year and engine do you have? Some had a mega fuse on the firewall near the solenoid along with an alternator field fuse.

 

I have a 2001 4.6
Do I have a mega fuse or field fuse?

 

Pull the alternator and take it over to Autozone and get them to test it on their bench.

 

heading there with it tomorrow. But do I have one of those fuses?

 

failed the bench test.

Now the fun of the exchange and wait...

 

I don't want to **** you off, but why on earth are you buying alternators on Ebay? Spend a few more bucks and buy one from a local parts store where you can simply walk in when it dies and exchange it under the lifetime warranty. They may not be any better, but the exchange sure is easier and quicker.

 

Well I don't want to 'pee you off either' but after 35 years why don't you know how a charging system works?
First, 5 volts won't even allow the computer to function let alone turn over the motor.
.
Short course on charging systems:
1. the voltage difference between the battery and the alternator regulator internal 'reference' determines the charge current.
2. a battery down to even 12 volts will force a charge current. 14.6 - 12 = 2.6 volt difference.
The battery acts like a very low resistance such that many amperes of current flow to force the charge recovery, raising the voltage. As the voltage rises the charge current tapers off.
All you needed to do was monitor the voltage properly with the motor running to see the voltage should have raised to the 15 volt level +/- as a check of a good alternator and battery, then taper down as the battery recovered it's charge.
If it doesnot do this there is an issue with either or both the battery and or alternator.
Nothing is much different on any vehichle with this type charging system.

 

That's helpful Bluegrass. thanks.

I'm using a dummy light meter. Only the first red light (5v) illuminates.

I now have a new battery and a new reman alternator from advance auto parts.
I still have the battery light on.
Using the dummy tester the battery at rest is barely 12.5v. When I start it only the red 5v light is on.

 

Check the fuses? #30 in the central junction box in the cab and #11 in the battery junction box in the engine compartment. The light green with red striped wire should have 12v in Run at the alternator. It is the single wire at the alternator.