My alternator is over 20 years old with over 300,000 miles...but I want to make sure before replacing it. I was towing my trailer last week, and when going up mountain passes, the battery light started coming on.

I borrowed a friends Solar BA7 system tester. Now I don't know exactly what I'm doing, but following a Youtube video, I performed all the tests. My battery was at 12.6v, after sitting all night and most of the day before. No problems there. Cranking volts were fine. While running, battery is up to ~14.2v. Again, seems pretty good. With AC running on High and rpms up towards 2,000 (not totally sure since I was using the throttle from under the hood), voltage is ~14.1.

As far as I know, all of that is in spec. Do I need more of a load for the alternator test? Or any other thoughts on what to check?

 

If you suspect the battery, just checking the voltage isn't enough. You need to take it to an auto parts store, a chain type preferably, and ask them to do a load test. THAT will tell the tale.
Does the battery light come on when the engine is at idle? That might be an indication of the alternator. You might also ohm out the fusible link from the alternator to the battery, just to be sure.
You might want to check the battery terminals for corrosion. Those thin metal bands that seem to be all the rage now, corrode through very easily.
Not only those, but the cables attached to them can get corrosion from the battery's acid mist production, and it can get down inside the insulation. No visible indications, but the wire strands down inside the cable's start breaking, reducing the current carrying ability of the cable. Or, if it's bad enough, the cable can come apart when flexed a little, opening it, and preventing current flow entirely. Can be checked by ohming the cable end-to-end, and flexing it while doing so. If there's any doubt, replace the cable.

Check the grounds, too. Make sure both ends of the grounds are clean, and shiney, no corrosion, paint, grease/oil, etc.

Good luck!
Pat☺

 

That tester has the ability to test the battery not just the charging/starting systems.
Did you use that function?

 

It tested the resting voltage and cranking voltage (numbers in my first post). Anything else needed for the battery? Those were the only battery-specific functions it had.

 

I might be off base, but I'm not suspicious of the battery. I'm suspicious of the alternator. The times I've had battery problems, there were issues with starting and with holding a charge. This is an issue of the charge light coming on, under load. Which sure seems to imply alternator or a connection from the alternator.

 

It can also do a quick load test.

 

Turn all your accessories on and see what voltage you read. Rear Defrost, high beams or low beams and fogs, a/c on max and fan on max, all 4 windows in motion, brake lights, and the radio on plus any aux lights you might have. That will max the draw and you should be able to read whether the alternator can output enough to keep up.

 

Turn them on one at a time, and check the alternator output voltage once each is on. Watch the battery light, too. It may be something is causing an unusually high draw on the electrical system. Not enough to blow a fuse, but enough to load the system to the point the battery light comes on.
If you find one, try running the engine RPMs up a bit, and see if the battery light goes off. If so, it may be that the trailer's electrical draw, PLUS the normal systems draw, at a low RPM, is just enough to bring the light on. That would say alternator to me. Just my humble opinion, though, and we all know what opinions are worth. Really, 300,000 isn't bad for an alternator to last, really. I am not sure if the alternators in those year trucks still used replacable brushes, but if they do, I would say that's the trouble. Probably easiest just to buy a whole new unit, though.

Check the connection of the large cable bolted to the side of the alternator. Make sure it's clean and shiney, both the end of the wire, and the bolt it attaches to. Make sure the bold is on snugly. Make sure the plug in the back of the alternator is in and snug. Not loose or sloppy. A small dab of silicone dielectric grease for both the plug and the cable won't hurt a darn thing, either.

Finally, do you hear a high pitched squealing when the battery light comes on? If so, the belt is either worn, or too loose. If it's that worn, you should probably replace the belt right off the bat. If it's just loose, just tighten it a bit.

Keep us up to date on how things go
Pat☺

 

Absolutely, the next step would've been to load the electrical system one item at a time. I was just starting with the idea of a DIY load test.

 

Thanks. I'll do both tests today. Fwiw, this has so far only happened up mtn passes, towing the trailer. I only use AC as a very, very last resort, so it wasn't on. No rear defrost. Stock stereo system. No fog lights. All the same, all the tests you both mention make sense and I'll do both. One accessory at a time, then all at once.

 

Lowest I saw the charging voltage drop was 14.04.

 

Does this happen if you aren’t towing the trailer? Could it be a short in the trailer?

 

Yesterday I would have said that's a possibility. But today I did the same drive without the trailer, and for the first time not towing, the charge light came on again.

 

What were the engine RPMs when the light came on? Just an approximate guess, is sufficient.
What gear? Brakes on or off? Lights on or off? Any other electric loads you can think of? Engine temperature? Or, more importantly, engine compartment temperature, if you can guess at it? IE: Were you going slow with the engine at low RPM, working hard with low air flow through the compartment? Like going up a steep hill in a high gear, moving relatively slowly?
Or when you were going fast in a lower gear, so there was a high air-flow through the engine compartment?

I ask the above to see if perhaps the voltage regulator, or even the alternator itself, is failing under specific temperature conditions. Like could the alt be failing when it get's hot, or when it's under a high load from a specific source? Or a combination of the above. I'm not saying when the engine temp is slightly higher than normal, but when you can estimate the compartment is getting a bit hot, without sufficient air flow to cool the alternator adequately.

Just throwing things that are suggesting themselves to me out there. Maybe something will be a clue that will lead us in the right direction.
Pat☺

 

Yeah I hear you. After looking up alternators, and seeing I can get an OEM remanufactured unit from Toyota for under $200, I'm not opposed to just throwing money at it, but in the meantime:

70's and 80's (ambient temp), mountain passes, going highway speeds in 4th or 5th gear. ~3,000 rpm. So far hasn't come on driving around town.

 

IMHO, what I would do, is replace the alternator, after ohming the fusible link between the alternator and the fuse box to ensure it's OK. Same with the fusible link between the battery and the fuse box. If they both ohm good, the connections at the ends are nice and clean and shiney, etc, while being flexed along their lengths, I would say it's pretty much got to be the alternator.
Make sure the alternator plug, that goes in the back, is seating firmly and tightly, and the pins are clean, and free or dirt, oil and so forth. I've had a situation where the park brake and battery light would come on in my 87 4Runner if I went over a good bump. Turned out the plug just wasn't quite fully seated in the alternator. I pulled over and wiggled it a little, and click! Problem went away completely. My own fault. Next chance I had I popped it out, gave it a small shot of silicon dielectric grease, and never had another problem with it.

Sorry, I ramble again...
Pat☺

 

So far so good. Replaced the alternator yesterday and so far no charge light. Not that it tells me much, but I used a little cigarette lighter plug-in volt meter while I was driving the last couple days. Everything would be fine, up above 14V, then I'd notice the charge light come on and it would drop to around 12.7V, then back to 14, down to 12 etc. Couldn't find any pattern to rpm, moving or idling, whatever. But like I said, since the new alternator, no charge light.