Alternator not charging battery?

Factual Questions
1

My truck has dumped me twice this weekend.

Just gently drifting to a stop along the side of the road. Of course, it had to happen while my kids and girlfriend were in the car. 10pm a night between the off ramp and on ramps on a four-lane. It’s clear that I was running on the battery and not on the alternator. The lights were dim, the voltmeter on the dash was very low.

I replaced the alternator (and integraged voltage regulator) this Monday and it dumped me Monday evening after a day of around-town driving.

I was crawling all over the car with a flashlight last night, the only “charging circuit” appears to be a wire from the big lug on the alternator through a fuse to the battery. Simple.

The battery takes a charge from an external charger, it seems.

It failed after a day of hard driving up mountains, for what it’s worth. It gets as hot as hell under that hood.

Is there more to the charging circuit than a wire and a fuse? (the fuse is intact. It’s marked “80” but I’m assuming it’s “8.0” really. 80 amps is asking a lot of a car alternator.)

Can the battery fail in a way that will allow eternal charging but refuse an alternator charge? I guess it’s possible I put a DOA alternator in, too. I’ll have to find a way to check that. Can I buy a simple wire-with-a-fuse replacement or am I fabricating something if necessary?

2

No it is for sure an 80 amp fuse. You probably have a 50-60A alternator, as the fuse has to be bigger than the output of the alternator. For comparison, my car has a 160A alternator.
You say that the battery takes a charge from an external battery charger, did you fully charge the battery between when the alternator first died, and when you went driving around town when it died again? Alternators are not designed as battery chargers, they are designed to keep a fully charged battery fully charged, or to charge an almost full battery back to fully charged.

Not that I have seen, but I would suspect a dead alternator or bad connection first.

Do you have a voltmeter?

:confused: To replace what? I’m lost.

3

[QUOTE=Rick]
You say that the battery takes a charge from an external battery charger, did you fully charge the battery between when the alternator first died, and when you went driving around town when it died again? Alternators are not designed as battery chargers, they are designed to keep a fully charged battery fully charged, or to charge an almost full battery back to fully charged.
Not that I have seen, but I would suspect a dead alternator or bad connection first. Do you have a voltmeter? :confused: To replace what? I’m lost.
[/QUOTE]

Yeah, after the alternator died, I charged the battery fully and drove around all day, it dumped me that night. The battery was topped off that morning, so to speak.

I’ve just returned from the auto-parts store, their tester says the new in-place alternator is bad. (I charged the battery last night again and I’m riding around on that.)

They want me to pull the alternator, they’ll bench test it, and if it tests good, then it’s something else. Of course, they won’t let me take the new one home so I have to do this in their parking lot.

If it’s a short, draining the charge out before it gets to the battery, can I just get some heavy guage wire, add lugs, and run it from the alternator to the positive terminal of the battery? (That’s the “fabricate & replace” thing above.)

Might be a good thing in general to replace that cable. I think I found a splice in that wire that makes me nervous. There’s three wires on the battery’s positive terminal. One ultra-heavy duty that is no doubt going to the starter, a red one that leads back through the firewall, I assume it’s the dash guage, and a brown one. Trace the brown one and it turns white, there’s a splice about 4 inches back from the battery. The white half disappears into the fuse box, I assume the other side is attached to that 80A fuse.

The wire’s not all that heavy, though. Maybe 12 guage? Certainly not a thickness that’ll carry 80 amps for anything more than a split second.

I have a voltmeter if I can find the bugger - looking high and low hasn’t turned it up. It’s probably in my storage unit. I don’t have a clamp ammeter, tho.

4

OK if I am reading this correctly the fuse is on the wire connected to the alternator, and you have three wires to the positive battery cable. I also believe if I am reading this correctly that you cannot see just where the wire from the alternator joins the rest of the vehicle harness. If this is correct so far, I suspect that the output of the alternator goes to the starter, and then back through the big-assed red cable to the battery. The smaller red and brown wires feed the balance of the electrical system.

Find or borrow a voltmeter, and I can walk you though several tests that will tell you if you have a short to ground, or the splice is bad. I would not replace the splice unless it tests bad, as there is an excellent chance that the two splices you replace the single splice with be inferior electrically, and cause a problem where none existed before.

I doubt it is a short, as it would drain the battery while the car was parked.

When come back bring voltmeter and we can nail this down to exactly what is wrong.

5

I once had a car that developed a short which would kill a perfectly good alternator within seconds of starting the car. It’s entirely possible that your truck might have a similar problem. How you’d go about finding it and fixing it, I’ve no idea. I had to take my car to a professional mechanic to get it fixed.

6

I had a short on the battery cable once. It drained the battery in about 5 seconds. The starter cable had bubbling plastic on about two feet of it. The voltage regulator might be bad also. The alternator could be bad, and producing mostly AC current.

7

[QUOTE=Rick]
OK if I am reading this correctly the fuse is on the wire connected to the alternator, and you have three wires to the positive battery cable. I also believe if I am reading this correctly that you cannot see just where the wire from the alternator joins the rest of the vehicle harness. If this is correct so far, I suspect that the output of the alternator goes to the starter, and then back through the big-assed red cable to the battery. The smaller red and brown wires feed the balance of the electrical system.

Find or borrow a voltmeter, and I can walk you though several tests that will tell you if you have a short to ground, or the splice is bad. I would not replace the splice unless it tests bad, as there is an excellent chance that the two splices you replace the single splice with be inferior electrically, and cause a problem where none existed before.

I doubt it is a short, as it would drain the battery while the car was parked.

When come back bring voltmeter and we can nail this down to exactly what is wrong.
[/QUOTE]

Yeah, parked car doesn’t drain the battery.

There’s two connector sets on the alternator. One is three-pronged connector that locks into a receptical on the back of the alternator housing. All thin wires leaving from this.

The other is the attached to the big lug on the side of the alternator. The connector’s wires, and the lug’s wire all join into a single wrapped bundle and dive into some conduit. the conduit runs across the front of the car, just inside the grill, and then, right at the battery, dives down under it. From here I’m making a guess as to where it goes.

The wire on the lug seems really thin for 80 amps.

My Haynes manual is no help on this.

As soon as the rain here stops - I’ll search out that voltmeter and rejoin the thread.

The toyota tech I just talked to mentioned a fusible wire between the fusebox and the battery. That may be the “splice” I mentioned earlier.

8

The test stand at Autozone says the new alternator is DOA. I got a new one and replaced it in the parking lot (needs to be a mini-rant on that one).

I’m going to monitor for a day or two. I’m worried that I’ve got some sort of undetected fault that’s killing alternators. (Thanks Tuckerfan for that new thing to worry about. :wink: )

Key to “on” right now raises the volmeter to a particular position. It then rises above that when I turn over the engine - a good sign.

9

It’s not unheard of to get a faulty rebuilt alternator from a parts store. I’ve known of a it happening to a couple different friends over the years.

10

Having worked at an auto parts store, rebuilts are crap and routinely come back the next day. IMO the quality is so low that they shouldn’t even be selling them.

11

[QUOTE=Harmonious Discord]
It’s not unheard of to get a faulty rebuilt alternator from a parts store. I’ve known of a it happening to a couple different friends over the years.
[/QUOTE]

Not only is not unheard, of it is way more common that it ever should be.
I think my personal record is 5 bad ones in a row. :mad:
If you can lay your hands on a voltmeter it is very simple to see if the new unit is charging. and if all the diodes are OK.

12

You definitely need a voltmeter and an ammeter for this: you are shooting in the dark without one. Also, you need a wiring diagram. The best ones are the factory manuals. You can buy one from the dealership if your car is not too old.

Yes, it is possible that the rebuilt alternator is bad – and they’re too common. It is also possible that you have a bad battery.

In 1989 we had a customer who went through 3 “bad” alternators in a row before bringing it to the shop. What we found was that he had a partial short in his battery that would cause the alternator to deliver rated amps for an extended period of time. Normal car alternators can only deliver rated amps for a short period of time (enough to recharge the battery, defrost the windows, and turn on all the lights on a cold morning, for instance). The alternator will burn out if that kind load persists too long.

We replaced the battery and he never had a complaint again.

13

You may have good electrical connections betweent the alternator and battery, but a bad ground incompleting a circuit.

14

Two days on the new alternator and I think I’m still good.

Just a dud out of the box.

I’ve learned. I had them put this one on the bench tester before I installed it.