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First of all thanks for taking the time to read this. I'm a long time lurker who has found a ton of useful info here. This may be a bit of a long post as I have a lot of info to relay.


Truck: 98.5 24v with nv5600 manual trans. Replaced just about everything by now except the engine it self.


To start with, several weeks ago my truck started charging slower and never showing more than about 13v. I replaced the alternator with no improvement then discovered the voltage regulator was not working. So I installed an external voltage regulator and everything worked great again. I have not seen any voltages over 14.3V even at full throttle. I did not install a Field Replacement Module. I waited to see if the check engine light would come on because of this and it did not.


Shortly after I noticed it was surging slightly at idle. Only about 25-50 RPMs at idle and corresponding voltage surges with it between 13.9-14.2V. I am not sure if this is related. It may have started right after installing the voltage regulator and I just didn't notice right away. Im not sure. I put about 1000 miles on it with no further issues and hadn't got around to looking into this yet.


Today I drove about 250 miles to get home. About 50 miles in the check engine light came on. The motor was running fine and the only thing out of the ordinary on the gauges was very low EGTs. I pulled over and scanned for codes. Here is what I got:

P1682 Charging system voltage too low

P1594 Charging system voltage too high

P0622 Generator Field Control Circuit <- From installing my EVR without a FRM. Just didnt cause the check engine light to come on.

P0122 Accelerator Position Sensor (APPS) LowAPPS Voltage Input Too Low

P1693 Generic fault indicating a fault in the bus or PCM/ECM interface

P0253 Fuel Injection Pump Fuel Valve Open Circuit

P0253 Pending


That Injection Pump code scared me because I had 200 miles to go through the desert and the whole family in the truck with me. I replaced my VP44 about 50k miles ago with a Blue Chip unit and gave it a FASS lift pump. I watch the gauges religiously and fuel pressure never drops below 12psi read right before the VP44. I drove the rest of the way home without issue but kept it running the whole time just to be safe. I did not use cruise control again just to be safe but it was working fine 2 days ago. No gauges out of normal ranges except the EGTs. I could barely get it to read 400 degrees even when driving up a steep grade at 70mph with the pedal floored in 6th gear! Lightly loaded but thats still very odd. At idle it would read 100 degrees.


After searching I see posts suggesting that my PCM may be bad with all these codes. This makes sense with the voltage regulator having just recently failed. However that doesn't seem right because the truck runs fine besides the slight surge at idle. Lots of voltage codes in there though. I did a quick check for damaged wires and bad grounds but everything looked good. I'll check more thoroughly.


So I get home, unload and get the family settled then come out an hour later to look at the truck. Starts right up immediately and no check engine light. EGTs are back to reading normal. I go for a quick test drive and everything is normal. Still has the surging idle. Read the codes again and the P0253 is gone, as well as the P0253 Pending. So clearly that is the code that brought up the check engine light. Now I am wondering if the voltage codes were stored from when the voltage regulator went bad?


My next move will be to clear the codes and go for a longer test drive. Ill report back.




Ok. I noticed a few things on the test drive. At idle the volt meter fluctuates along with the idle surge, as I mentioned before. All incandescent lights darken along with it as well. When I am not at idle the volt meter does not register fluctuations but the incandescent lights do, they flash faster as the RPMs increase. When driving normally it's fast enough that it is hard to notice if you aren't looking for it.


After a while the pyro gauge started reading low again, however the other 2 gauges that are powered parallel with it kept reading accurately.


Clearly I have an electrical issue. Is the external voltage regulator just faulty? Should I wire in a field replacement module and see if that helps?


Only one code came back: P1594. The internet calls it "Charging system voltage too high". The scanner calls it "Manf. cntrl. veh. spd. Idle Speed Control Auxiliary Inputs"


Edited by Mikew
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Pull all that off. Send the PCM for repair at Auto Computer Specialist in Florida. External regulators are not a good permanent solution. (Only band aid at best). Here is what happened in your PCM.



Here is my full tale..




All your damage comes from a failed alternators. Blue lead of the field shorted out and burned up the PCM. I've got a fix for that. 

Then the other mod to protect the entire truck.




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Thanks for the help. I pulled the PCM and will send it to them on Monday. I just put a fuse on the blue wire to PCM C3 and will do the ground wire modification too.


I carefully opened the PCM and saw no visible damage. Then I reconnected the PCM, cleared codes, let it idle for a while and then rescanned. Now I have the following codes:


I found your diagnostics for P1388 and P1389. Followed the directions and both indicated a bad PCM. Also checked the alternator field wires from the PCM, 8.5v-9.5v when running.


You really have created a great resource on here for us cummins owners!

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14 hours ago, Mikew said:

I just put a fuse on the blue wire to PCM C3


Make sure its near the C3 connector and not out by the alternator. Since the PCM creates the 12V source then you should fuse near the source so if the wire shorts to ground it blows the fuse out. You should be fine with a 5A fuse which I'm running. 


14 hours ago, Mikew said:

PCM, 8.5v-9.5v when running.

PCM turns on the +12V on the blue wire and then turn on the variable ground on the green wire. This only happens after the PCM detects RPM on the engine. That Blue wire to actual ground should show 12V. Now blue wire (+) and the green wire (-) it should show like the 8 or 9V till the battery charges up then it will drop lower. Normal function of the voltage regulator.


14 hours ago, Mikew said:

You really have created a great resource on here for us cummins owners!

It was all a mistake. This web site was named after me and was just going to be a personal notebook. It grew and grew. From a simple static page web site in 2004. To this big beast of server and nearly 15G of data stored on the server. We are handling over 1,500 people per day. I never though it would blow up this big in mere years but now that it going I can stop. I even tried to shut down the site in 2012 and everyone push me to continue on. No one wanted the website to be shutdown. So here we are....


First url was http://frontiernet.net/~mopar1973man (still hold it and the server is empty)


Then the actual domain was bought 2007 for mopar1973man.com





Edited by Mopar1973Man
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