PCM/Alternator Protecting Fuse Blown

[AKGriz]

Here's my tale of woe.  '99 3500 5.9 Cummins A/T--Started out with the the torque converter lock/unlock issue, found the cure purely by accident (pulled the alternator charge line which was lying directly on the alternator away from the alternator, and problem went away for a couple weeks until the line returned to its original position--ah ha!!!).  Traced the charge line, and ended up basically doing the W-T Ground Mod, things ran perfectly for about 4 years.  I could not believe that crazy ground splice on the port side of the motor--engineer must have been hitting the sauce on that one.  Wish I'd found this site back then!!!

 

Forgot to mention, I bought the truck used in 2015 with only 46,000 original miles on it--when I had the tranny rebuilt at 90,000, the tranny guy told me there were some non-OEM parts in the tranny--in other words, someone had been in there trying to fix the torque converter issue thinking it was a transmission problem.  This has been a source of great amusement to me, as the original owner sold this great truck because it had an "unfixable" transmission problem (and of course didn't tell me)--so thanks, MM1973, for letting people know about this easy fix even though I had to find it by accident!

 

While on a trip out of Alaska, got a no-charge condition and ended up doing an external regulator fix to get home.  Never liked the external regulator, as frequently the voltage would run high (15+).  After reading MM1973's article on the fuse, decided to implement it and restore wiring to original condition (with the addition of the 5A fuse)--purchased a used PCM, sent it off to be programmed to my VIN, installed it when it returned.  Lo and behold, the voltage was in the low 14s and the vehicle ran great with the exception of Brake and ABS lights (turned out the VIN programming did not take; the ABS module did not like an incorrect VIN!) and an occasional "surge" in RPM.  Meanwhile, I sent the original PCM in for repair.  A friend rode with me and monitored system voltage on his scanner--there was a very slight "spike" of 0.1V every time the "surge" occurred (could only see it on the graph). 

 

Strangely enough, the PCM rebuilder reported there were no problems with the original PCM (I asked them to replace that circuit regardless, which they did, and then tested for heat and vibration).  When I received it back and installed it, all was well with the exception of the tranny surge--it seemed to be bad one day, and disappear the next.  It was not throwing any codes, at least that my Edge reported.  About 2 weeks later, while driving to work (temp was about minus 30), the surge got pretty nasty so I locked out the overdrive to smooth things out.  Just before I pulled into work, I pushed the overdrive switch--it didn't shift, and my voltage dropped to 11.9.

 

As I suspected, the 5A fuse had blown.  I replaced it with a 7.5A, but still had no charging.  I'm guessing the original PCM has acted up again.  So I put my backup PCM in and went for a test drive.  Voltage was good, and she shifted into overdrive.  Problem solved, or so I thought.  I turned around to head back to the shop, and got on it a little to ensure things were working properly.  Instantly the voltage dropped to 11, and no overdrive.  Edge pulled down a P1765, P1682 and P0753.  Put the ohmmeter to the dark blue line coming off pin C3 25 to the alternator, have good continuity and no continuity to ground (same with green wire from C2 10).

 

Going to have transmission solenoids and wiring harness replaced this weekend based on transmission guy's previous experience with similar issues.  Based on the wiring diagram, it seems it has to be a transmission circuit issue.  But I'm still totally baffled as to the origin of the no charge situation--either the PCM rebuilder doesn't know what he's talking about or I've got some real gremlins running around in my truck (he said it tested good, and it worked well for a couple of weeks, but now this--and after I put in the protective fuse).  And full disclosure--I'm totally dumb when it comes to understanding automatic transmissions and 12V systems, but I did sleep in a Holiday Inn Express once.

 

Sorry to be so long winded!  I'd like to hear other members' ideas on this.  I'll report back after the solenoid/harness replacement this weekend.  Thanks in advance!

[dripley]

Well welcome to the forum. I cannot help with problem due to my electrical dumbness. But I have had many moments that a stay in a Holiday Inn express has helped my education. My only overdrive hunting issues involve me looking for 7th in the 6 speed.

Edited January 22, 2020 by dripley

[IBMobile]

Have you redone the two ground wires for the PCM?  They are spliced together lke the ones on the port side.

 

[AKGriz]

No, I have not.  I will definitely check it out this weekend when I get the truck inside (it's possibly headed down to minus 50....).  That's crazy!  Some solder might work wonders.

[Mopar1973Man]

If the protection fuse is blowing out its the blue wire to the field lead is either shorted to ground internally in the alternator or in the wiring. This is NOT a PCM ground issue. It a +12V feed issue from the PCM to the alternator and is drawing excessively on the field wiring.

 

So if it was me iif you replaced the fuse and unplugged the alternator field lead and the fuse does not blow then the wiring is fine. The alternator is shorting out and need to be replaced. 

[Haggar]

You don't mention which alternator you have (denso or bosch).  The Denso rectifier bridge is all soldered together.  Most people won't de-solder it to check each individual diode.  (the bosch is different and easier to check.)  My alternator guy and I had a good argument over this.  (all my dodges have had the denso)    I wanted my rectifier bridge replaced.  I had a bit of AC coming out.  His normal checks showed it just fine, but agreed he saw the AC output too.   I said it causes problems, especially on an automatics.  lol  next time I was in for some other stuff he mentioned he talked with his transmission buddy and they thought about it and his buddy said yes it is possible and may be a source of the trouble.  So alternator guy de-solder each diode  to check them out.  Sure enough he found one failed...  We have been good buddies since.  

 

Whether you have one or the other, check for a failed diode.  They can fail and still make good power and not kill the batteries when the vehicle is off.

 

HTH
 

Hag

[JAG1]

16 hours ago, dripley said:

Well welcome to the forum. I cannot help with problem due to my electrical dumbness. 

It is because he has been electrocuted one too many times.

 

Make certain the firewall ground to PCM connection is in good condition too. I would like to hear what you A/C ripple level is when idling.

Edited January 22, 2020 by JAG1

[AKGriz]

Excellent thoughts.  NAPA tested the bosch as good, for whatever that's worth.  I did emory cloth the firewall to PCM connection last week.  A/C is <0.1A.

 

We're going to pull the transmission control relay at the PDC (should have 3rd gear only) and go run it to see if we have charging without interference from any tranny issues.  If that works, we should have eliminated the alternator as a source of the problem.  If it doesn't work, it points directly to the alternator having a short as the wire itself as a problem has already been eliminated through ohm testing. 

 

Does this sound like a reasonable plan?  There is no Holiday Inn Express in town, so I'm kind of out there flapping....

[Mopar1973Man]

PCM does not ground through the case. Ground is the passenger side battery.

[dripley]

I have to agree with @Mopar1973Man on that. The ground for cicuit board is in the wiring harness and ties to the passanger battery. I can only assume that strap to the firewall is just for the case for a static charge maybe?? Not sure.

[JAG1]

5 hours ago, Mopar1973Man said:

PCM does not ground through the case. Ground is the passenger side battery.

On both my second gens there are factory grounds going from firewall to the PCM housing. It runs a short distance toward the drivers direction. Oh well, if I'm wrong its okay.

 

W-T does talk about the alternator factory ground, very important, going thru rusty corroded mount bolts affording a poor ground thru the mounting bracket. Make sure all that is clean making good ground.

Edited January 23, 2020 by JAG1

[AKGriz]

16 hours ago, JAG1 said:

On both my second gens there are factory grounds going from firewall to the PCM housing. It runs a short distance toward the drivers direction. Oh well, if I'm wrong its okay.

 

W-T does talk about the alternator factory ground, very important, going thru rusty corroded mount bolts affording a poor ground thru the mounting bracket. Make sure all that is clean making good ground.

 

Yes, I'm wondering if I have a ground issue.  I've had those bolts out twice in the past 2 weeks and cleaned them up. 

 

We're going to pull the transmission relay this evening and go for a ride, and she's going to the tranny guy for solenoids/harness Friday night.  I'll update after the tests.

 

Unfortunately our electrical rebuild shop closed down a few years ago (couldn't compete with the big companies)--otherwise I'd have him tear into the alternator.  Closest shop is in Anchorage, 367 road miles away.  I am prepared to hit NAPA up for a new one, but that's no guarantee of getting a good one either!!!

[kzimmer]

I recently put in a 160amp alternator from a 2001 Durango. It's a hairpin stator which is supposed to be a better design. Just had to change the pulley, and I used an aftermarket smaller pulley from a ferd super duty to spin it faster. Been great so far. Still need to do an AC voltage test though. 

[AKGriz]

On 1/22/2020 at 6:14 AM, Mopar1973Man said:

If the protection fuse is blowing out its the blue wire to the field lead is either shorted to ground internally in the alternator or in the wiring. This is NOT a PCM ground issue. It a +12V feed issue from the PCM to the alternator and is drawing excessively on the field wiring.

 

So if it was me iif you replaced the fuse and unplugged the alternator field lead and the fuse does not blow then the wiring is fine. The alternator is shorting out and need to be replaced. 

 

Disconnected the field leads on the alternator and took her for a drive.  Interestingly enough, the transmission shifted normally (including into overdrive and lockup) and the fuse did not blow.  Consequently, there is now a new (as in "new," not rebuilt) alternator sitting in the backup vehicle which I will install in the morning.  More to follow.  Thanks, Mopar1973Man!

[AKGriz]

Changed out the alternator, only got to run it a few minutes before work--but have charging voltage and overdrive works without blowing the fuse!  Still going to have tranny guy work his magic this weekend, but looks like the alternator was the issue.  Thanks to all for their help!!!

[Mopar1973Man]

Bad field lead or armature. You just saved yourself $750 dollar repair of the PCM. Auto Computer Specialist took care of my failure but it was so bad the printed circuit board burned through completely making the PCM total junk. 

 

That 5 Amp fuse saved your PCM from certain death. 

 

Thank you for confirming that... 

[AKGriz]

2 hours ago, Mopar1973Man said:

Bad field lead or armature. You just saved yourself $750 dollar repair of the PCM. Auto Computer Specialist took care of my failure but it was so bad the printed circuit board burned through completely making the PCM total junk. 

 

That 5 Amp fuse saved your PCM from certain death. 

 

Thank you for confirming that... 

 

I agree!  When I saw your article I knew I had to do it--sure glad I did.  Reading that one article paid for my subscription for numerous years!