Charging system problem, help!

I just replaced my alternator with a Denso (i think) remanufactured alternator about 2500 miles ago or 5 months ago. The other day the battery light started coming on intermittently and flashing. Then it went solid, so I drove home and tried to figure out what was causing it.

I tested the voltage at the battery with the truck off and on.

Off: Battery at 12.45 V
On: Battery at 11.4 V

So clearly the charge from the alternator is not getting to the battery. I checked the obvious, belt is still on there, pulley is spinning, and battery terminals are clean. Then I disconnected both wires from the alternator and both wires from the battery. The B+ terminal on the alternator was very corroded so I cleaned it off and put it all back together. Battery light was now of. I fussed with it some more and put all the wires back where they were. Then I started it again and the battery light was back on. So I did the same thing, took it all apart. Carefully put it all back together and the battery light was off again.

Fast forward two days (It stayed off for about 50 miles). I was on my way to work and was just about to pull into the parking lot when the light came back on. I'm gonna test it some more at lunch but I am thinking there is a bad connection somewhere and not necessarily that the alternator is bad.

Any advice?

 

Stop at Pepboys and have them do a load test on the alternator

 

First check your grounds, Clean the mount locations and test again

 

Absolutely check those two things. That’s a good start.

 

Where are the ground points? (Aside from the mount)

 

So I went home during my lunch break and messed with it some. First I tested continuity between the frame of the alternator and the negative post of the battery. It was perfect. Second I started the car and tested the voltage at the alternator hooking the jumpers to the B+ post and a ground point. It was reading about 11.5V. Next I disconnected the pigtail on the alternator, cleaned with some mineral spirits and reconnected. Started it up and battery light was still on, I went and jiggled the pigtail and the wires themselves with no luck. This was the point where I started to get frustrated and made more than one change at once because I needed to get back to work. I disconnected the positive battery "accessory" terminal (the one that the alternator, fuse box, and add ons are connected to). Once disconnected, I cleaned off all the terminal rings and put it back together. I also snugged the battery terminals for good measure when putting it back together. Lastly I tightened the nut on the B+ terminal (I was able to tighten it another half turn very easily). Ta da battery light off and it stayed off the entire drive to work.

So here is my logic/thinking. This does not seem to be a load related issue, turning on accessories does not make the battery light come on. I'll test this theory some more by driving with the AC and lights on on my way home. This seems to be an issue of the alternator not delivering charge to the battery. This would mean the voltage regulator is not properly "turning on" the alternator. The "accessory" terminal on the battery feeds an ALT-S fuse which then feeds the voltage regulator (I think of the voltage regulator as a very very large two position relay). If there is too much resistance along this path then the voltage regulator might be "reading" the wrong voltage and not turning on. The corroded battery "accessory" terminal might have been the source of this. If that was not the source then I believe the voltage regulator itself is faulty.

Any thoughts on this logic?

I am going to warranty the alternator and replace it if the light comes back on cause I am tired of dealing with this.

 

I think it was a combination of a few loose and corroded connections keeping the alternator from charging .

The quality of re manufactured parts has gone down hill

Mom and Pop shop or do it myself on electrical

 

Would it be better to get a "new" alternator instead of a warrantied reman replacement for the one I bought. Like this one https://www.rockauto.com/en/moreinfo.php?pk=1962914&cc=1418442&jsn=376

OEM is obviously too expensive, and I can't seem to justify it...

 

Could it be the battery? Have you tried another good known battery.

 

Don't think it would be the battery. When the truck is running alternator is not outputting the correct voltage and when the truck is off the battery voltage is normal.

 

If you have a bad battery, it won't necessarily accept the voltage the alternator is putting out.

How old is your battery? If it's more than ~5 years old, it's likely due for a replacement anyway.

 

I think I replaced it 3 years ago. I'm gonna head to autozone after work and get the battery and alternator tested. But doesn't the alternator provide all of the power while the vehicle is running?

 

I've definitely been in both situations.

I had what I thought was a bad alternator, since it wasn't putting out enough voltage, but a new alternator didn't solve the issue. New battery and everything was fine.

Then, I had a battery go dead on another vehicle, so I had the alt tested, they said it was fine so I swapped in a new battery, but it still went dead. After a new alternator it was fine again.

If there's an internal short or a bad cell in the battery, you won't be able to get full voltage out of it, or into it.

 

Well, battery tested bad at autozone alternator tested good. Battery was 4 days out of warranty lol but they were good sports and gave me a new one for free. I broke off one of the battery hold down bolts in the parking lot so now it’s time to try and get that out.

We’ll see how this battery holds up and it the battery light stays off.

 

It's like they put self destruct timers in these things!

Glad they still honored it.

 

Does the voltage increase if you rev the engine? I'm guessing no.

You were on to something with the wires. One powers the regulator and one is used by the regulator to sense the voltage. The regulator isn't an on/off switch. It increases the magnetic field in the alternator which makes the alternator output more power when needed.

You can check those wires. One should have constant 12v. One should have 12v only with key ON. The 3rd wire goes to the charge warning light (maybe a yellow wire if I remember cirrectly).

 

Thank you. If it comes back on that’s what I’ll be checking

 

Often a bad alternator will shorten battery life since lifespan is tied to proper charging.

 

The battery was 3 and a half years old or so by the way.

 

Well battery light came back on this morning after about 40 miles. New battery didn’t help. Must be something up with the wiring or the alternator.