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Discussion Starter · #1 ·
I've found a few posts that have been helpful, but nothing matching this circumstance exactly, so I thought I would create a new post. This problem is with my 2015 Ram 1500 Bighorn EcoDiesel.

I loaned my truck to my son in-law last weekend and after he was done with it, he drove it back and parked it in my driveway. He said everything worked fine- no issues with the truck. The next morning I went to start it and the battery was dead- the voltage was around 7v. This is a pretty good AGM battery only about a year and a half old. I recharged it and had it tested. It tested good. I put it back in and started the truck. The EVIC said it had 11v, so I checked at the battery while running, and sure enough, 11.4v.

I thought maybe the alternator was bad, so I pulled it out. Apparently, these can't be tested at your local Kragen due to the digital control setup, so I took it to an alternator shop. They said it was producing 14+ volts, but could not check the controls.

I re-installed it in the truck, and upon restarting it, got 13v on the EVIC and at the battery. I took it on a drive, and the EVIC stayed at 13v, so I thought I was home free, but last night it dropped back down to 11v. I registered a U0120 code ("Lost communication with starter/generator control module") and got a "Battery Low" warning.

This time I checked the voltage at the alternator, and it was 14.3v, but still only 11.4v at the battery. I turned it off and disconnected all the cables from the positive terminal of the battery. The only cable that had continuity to the alternator was the one on the 250 amp fuse. Does anyone know if anything else is directly connected between the alternator and the battery? There is one other cable that is not fused, but I think this goes to the starter. The schematic shows a connection between the alternator and the PCM, but not where the splice is (could be at the terminal).

As I understand it, there are only 2 inputs to the PCM as it figures out how much voltage to send to the battery: alternator voltage (via the B+ terminal on the alternator), and battery voltage (via the battery sensor). The PCM sends a signal to the regulator in the alternator which then charges the battery through the main cable.

Here are the things I'm going to check for today:

1) Bad ground​
2) Faulty regulator in the alternator​
3) Short or loose connection in the cable from the alternator to the battery​
4) Test the battery sensor (I still need to figure out how I'm going to do this)​

I'm sorely tempted to bypass the alternator cable and just run a known good cable directly from the alternator to the battery to see what happens, but I'm concerned that might fry something. I do have another (used) alternator on the way, so if I can't figure it out before then, I'll just drop that in and see what happens.

Any other ideas what to look for? I'm hoping to track this down today.
 

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Discussion Starter · #2 ·
Update- started it first thing this morning: 11v indicated
Pulled all connectors off, checked grounds, verified continuity. Restarted: 11v initially, then after a couple seconds 12v, then after a few more seconds 13v. At the terminals, 13.85v. At the alternator, 14.7v

Going to check a few more things and then let it sit for a while before starting again
 

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Discussion Starter · #3 ·
Restarted again when completely cooled off: 11v

I ran out of time for troubleshooting today, but I want to see if it starts charging again after disconnecting the battery. If so, then it could indicate that something is "resetting" when power is cut in which case I will start looking at the PDC and battery sensor.
 

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Discussion Starter · #4 ·
Back at it this morning.

1st start(everything left where it was yesterday): 11v
2nd start (disconnected the voltage regulator wire at the alternator): 11v
3rd start (reconnected the regulator and bypassed the main charge cable from the alternator to the battery): 14v

Now I need to find a better schematic for the charging system to confirm that the cable I bypassed does indeed go directly from the alternator to the battery. I've never seen 14v on the EVIC, and I don't want to fry anything. On the other hand, it could be that the battery got so low that the PCM is "supercharging" it. If that is the case, then the culprit is a bad cable.
 

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Discussion Starter · #5 ·
As an aside, when I jumped the truck the other day, I was able to start it, but it was giving me all kinds of weird messages (Service Anti Lock brake system, Radio locked, Traction control disabled, etc.) and the truck would throttle up and down between about 500-1100 RPM all by itself).

Because the charging system was faulty, jumping the battery gave it just enough voltage to start, but as soon as I unhooked the jumper cables, everything went haywire because the battery was somewhere between 8v-10v. It was a great reminder as to why the battery should be the first thing checked for any electrical problems!
 

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Discussion Starter · #9 ·
Thanks to the schematic above, I was able to determine that the alternator is directly connected to the battery (through the 250A fuse), so yesterday I did some more troubleshooting.

With everything hooked up I was still getting 11.4v at the battery, but 14.5v at the alternator B+ terminal. It became strange when I decided to measure voltages at the 250A fuse and found that I had 14.5v on the alternator side and 11.4v on the battery side of the fuse (ie ~3v drop across the fuse). I then bypassed the fuse, and the alternator quit working (only generating about 2v). I have another alternator on the way which should be here next week, so I will pick up the trail again next week and start with a new fuse and the new alternator. It is possible that either the alternator was intermittent and finally gave out, or the fuse was somehow creating a load. More to come...
 

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Discussion Starter · #12 ·
quick update. Since I can't really continue troubleshooting until I get the new alternator and megafuse, I decided to take a closer look at the old alternator. After removing the plastic cover, I saw that the brushes were good (held in with 2 screws), and the regulator looked OK, although i did not test it.

However, when it came to the fields, I found that 2 of the 3 had good continuity (<1 ohm), but one was open. The alternator is definitely bad, but whether it is the root cause of the problem or not remains to be seen.
 

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2016 Big Horn 4X4 Engine and Transmission tuned by no longer available (but truck runs good)
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Hoping the "new" alternator fixes the issue (y) and no issues with other electrical components:oops::unsure:
 

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Discussion Starter · #14 ·
The new (used) alternator showed up today. I installed it and it had the same problem still (only outputting 2.8v). After tracing the sense lead up to the PCM, and checking a couple other things, I decided to jumper the alternator directly to the battery, and it worked. So it looks like the culprit was the main cable from the battery to the alternator. This is one I had repaired when I bought the truck (due to the engine fire), but the symptoms were not in any way pointing to the problem :(

Anyway, all's well that ends well! I have a temporary cable routed from the alternator to the battery, but I will put in a more permanent solution next time I have the engine out (planning to put the experimental motor back in at some point).
 
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