If the alternator is putting out about 13.5 volts but 13.5 volts is not present at the battery terminals (engine running of course) then there is likely a fault in the circuit between the battery and alternator.
Check the underhood fuse/relay box for a popped fusible link. If that’s the case it’s a simple cheap fix.
As for my prior comment about Milliamps, milliamperes, MAs, mas, or whatever I’m not that big a stickler on lingo which varies by locale.
Being in the auto world I’ve dealt with amps and milliamps; never mega-amps. I checked several of my multi-meters and nope; not a Mega-amp setting anywhere.
If this is the voltage was from a test of the stud on the back of the alternator, then check if you have that voltage at the battery too. If not, then as I mentioned earlier…the large wire running from that stud on the alternator to the battery is bad and must be replaced.
This means that the alternator is putting out the charge, but the wire is broken that carries that current to the battery.
FYI, a typical diode used for alternators has a spec of reverse current at 150º and 18 volts of 100µA. Multiply by 3 to get 300µA, well below the max parasitic drain of about 50 mA.
So the diodes would have to be really bad. And why doesn’t the alternator manufacturer test them? cost, probably. But the manufacturer tests them, so there should be no problem.
Perhaps a sleazy alternator manufacturer buys rejects from the diode manufacturer. Or untested units.
Somebody told me one time that if you charged your battery (with a battery charger) without first disconnecting it from the car, doing that could cause this alternator diode problem, where the diodes start to flow in both directions instead of what they should do, one direction only. It seems dubious to me, but if the OP has charged his truck’s battery with a battery charger and didn’t disconnect the battery from the car first, something to consider anyway.
I should add that in the winter I charge my Corolla’s battery with a battery charger several times a month without first disconnecting it from the car and never had this problem.
Just my 2 cents worth but go back and read Testers comment on the 120 amp fuse, then it might help to look at the actual wiring diagram to see what else is there.
I’ve never disconnected a battery to charge it and never had a problem.
Diodes that are used in alternators are pretty tough, but like all solid state devices they can get damaged. Electrical spikes are a big thing to try to prevent. Spikes can occur when you connect a charger to a battery and you see sparking occur. I usually don’t turn on the charger until after the battery connections are already connected in order to keep the sparking down.
I have seen a number of alternators fail the day they were installed because the tech was too lazy to recharge the battery. The strong draw of a rapidly discharged battery can overheat the alternator diodes in less than 20 minutes and fail.
OTOH all we know about the OP’s car is that the engine stalls while idling in traffic and a volunteer removed the alternator numerous times for testing.
Here is the update. After 4 days at a mechanic that specializes in electrical along with a friend who works at subaru. They replaced the “bad altenator” with a bad altenator. Then the car seemed to be good after second replacement. She drove it for about 2 days and then went out of town for 3 days leaving her car. 2 days in to driving again and it does the same thing. They determine the battery might be bad so we take this one back again and go with the 35 group instead of 26. Both seem to be recommended but 35 was the original one she had. She drives car the next day but leaves it unplugged when off so she can make it to 2 doc appointments. After doc appointment it was dead again. Mechanic will be back on Monday. It is verified by several people that it is not Parasidic drain. Not to mention that driving it seems to be what kills the battery. She gave subaru the .14 amp info and they agreed it is not parasidic. Also 2 mechanics agree as well who checked for it. She still has all nonessential fuses unplugged anyway. We are guessing either a 3rd bad altenator or wiring. We don’t think it could be anything else.
The battery sense lead and the field lead tying to the back side of the alternator need to be checked out for a problem. Both of those leads should be very close to the battery voltage normally while the engine is running. If the battery sense lead has a resistance problem that will cause the alternator to over work trying to charge the battery more than it should and possibly damaging the alternator. Proper testing will quickly locate where the problem is at along with the parasitic drain issue.
I think in Koldblue’s defense he/she was merely indicating the possibility of multiple alternator fails in the box depending on if all the alternators are coming from the same source. Even if from different sources, possibly the same diode supplier is involved. Me, I only buy Delco rebuilds anymore after suffering repeated fails with NAPA, although I haven’t needed one for quite a few years now.
“Me, I only buy Delco rebuilds anymore after suffering repeated fails with NAPA, although I haven’t needed one for quite a few years now.”
That’s a good plan. I’m fortunate with help nearby. I only have my alternators and starters rebuilt locally, while I wait. I had an Auto Electric Shop an hour’s drive from me, but I now have one 10 miles (only one stop sign, some deer, and 55mph highway) from here!
I haven’t needed one for a long time, not until the 17 year-old Impala hit over 300,000 miles and started to have intermittent starting (cranking) problems that got progressively worse.
I removed the starter and I easily diagnosed a bad solenoid, took it to Kermit at the shop 10 miles away and he said it would be exactly $25 (total including tax) for a brand new one, over-the counter or installed, my choice. He put it on, tested it, and I was headed for home in 5 minutes. Two bolts off, two bolts on, starter problem fixed!
I for one would still like to have @Dispatcher or @Thatgirl29 to answer the questions that I and others have asked.
No one here is trying to say that either of you are dumb, we just need answers to our questions so we can make an informed opinion.
What was the voltage (engine running) at the stud on the alternator and at the battery.
And please give us numbers…not “it was within spec’s!!!”
on a side note you will get a poor reading on a analog meter. It just cannot display 10ths of a volt.
A digital Volt/ohm meter is what you need.
You can have a good battery and a good alternator but if that wire that feeds power to the battery is bad…the current has no way to keep the battery charged as you drive. So when you charge the battery then drive the car, the battery is drained after awhile. This wire can look ok, but it is broken inside…not allowing current to flow.
Did you peal back some insulation on the battery cables and check for corrosion, and clean the terminals and battery posts. Even a slight coating of corrosion can impeded the flow of electricity.
When you jump the battery the jumper leads dig through the corrosion and make a good enough connection to start the car. But once the jumpers are removed the engine is running on only the alternator and the battery gets little charge.
Battery cables also can be bad to the point that they can carry the small load to run the engine, but cannot carry the heavy load to run the starter.
What year is this mystery vehicle???
Keep us informed. We really do want to help you fix this problem.
