I have been having an issue with my truck not charging. When I first start the truck everything works as normal but after some driving the check gages light will come on and the voltage gauge in the dash will drop to zero. This will sometimes happen in 5 miles, sometimes 12, sometimes more. Every time if i turn the truck off and start it again it operates properly. So far I have cleaned the negative battery terminal connection as well as the 2 pin field source connector on the alternator. I pulled the alternator and had it bench tested at Oreilly where they told me it is good. Checking the battery post voltage while running I get around 13.8 volts and around 13.6 with all electrical items on in the cab. When the dash light comes on this drops to around 12.6. I visually checked all grounds and connections. I checked for voltage drop on the main power wire leading to the alternator as well as the ground by touching the voltmeter leads to the alternator body and the battery negative post. I have continuity between the C2 pin 10 Connector on the PCM and the Alternator as well the C3 Pin 25 and the alternator. When I unplug the connector on the alternator I get I get battery voltage by touching the lead to the WT/DB source wire and the battery negative post as well as 12 volts between the DG generator field wire and the battery positive post. I get these same results with the engine running regardless of the alternator having proper output or not. I still need to check the batt temp sensor.

Seems to be a possible PCM malfunction?

 

https://www.cumminsforum.com/threads...ntrol.2366185/

 

Since you are either getting a charge or just reading the battery voltage, I'd look at the source wire for the alternator. I've only had this happen once so it's not a common problem, but I had a truck that would sometimes charge and sometimes not. The truck was a real rust buggy and ended up being parted out. The main supply wire to the alternator was corroded at the relay end where it supplies the charge to make the initial power to create the internal field power in the alternator. It wasn't a constant problem but once I cleaned the crud off the terminals, the charging system worked fine. I'm not saying this IS your problem, but it will eliminate one possible problem.

 

Ok, are you referring to the larger gauge wire that runs from the alternator to the PDC (Power Distribution Center) fuse box? I can check this again. I am still suspecting a PCM because my problem is resolved each time I restart the truck. Thanks for the suggestion. I will look into it

 

Unfortunately, this is a somewhat common problem. The voltage regulator is part of the PCM - it is unlikely the alternator.
Likely some solder joints are starting to fail, it happens due to heat cycles and age.

Some options are replacing the PCM or using an external voltage regulator.

 

Last time I drove the truck I was having issues every few miles. I would pull over and restart the truck which would work for a while. My friend had an idea to try a junkyard PCM to troubleshoot and see if it charges properly. My local junkyard has a 2001 Durango Auto V8 PCM. Would the truck even start with this? Again this is just to plug in and see if it charges then I would source the correct PCM.

For sourcing a PCM that will actually work, how close do I need to be to my truck specs (does the donor pcm need to be same year trans engine?)?

 

If the part number is the same, it *should* work.

 

Swapping parts is not diagnosing. I believe there is a test using a rheostat in the service manual. The rheostat is wired in to the two wires going into the pcm and will go from nothing to full field. Full field is the maximum output. Using a meter you will see the output go up as you move the rheostat. If it does then you know it's the pcm. Here is a video doing the full field test without a rheostat.

 

If you need a PCM to replace a bad one, you get one programmed to your VIN. I'd try something besides throwing parts at it. Give the video Moparite linked to test it. A used PCM will need to be the EXACT same drivetrain to work. A 5.7 engine won't work in a 4.7 or 5.2 or especially a 3.9 or 3.7 V-6.

 

Thanks for all of the replies. I had already essentially done a lot of the diagnosis but I had not done a full field test because the Cummins forum link that I had posted advised against it. Also there is no reference of it in the FSM. Lastly this is frustrating because it is so intermittent (I can’t just walk outside to test the truck-I have to drive sometimes a few miles, sometimes more than 12 before it starts malfunctioning).

So I took the truck for a drive today until it began to malfunction and then tried to conduct a full field test. I was successful a few times and only ran it for a few seconds. I did so by running a jumper wire from battery negative post to a paper clip backprobed into the DG Generator Field wire. This produced about 18ish volts. I was only able to do it three times. I tried once more and was unable maybe just due to a poor connection (back probe). I’m still leaning towards PCM malfunction and still a bit confused. I seem to getting close to a full voltage drop on the Generator Field Wire as well as full battery voltage on the generator source wire. So that seems like the computer is sending the correct signal? I did at one point get more in the 7 volts range back probing the field wire.

Lasty I am getting a P1682 code