I started having this problem right before I was about to be out of town two weeks in a row, so I haven't had a lot of time to do more than basic testing.

While driving in to work one day with most of my electronics off, the voltmeter suddenly pegged at 18 volts and the "check gauges" indicator came on. I recently installed a Lincoln e-fan and knew its a power hog, so I turned my A/C on to activate it, and turned my lights on to add some heavy loads to the system. That took the voltage back down to about 14.

As it sits now, as long as I keep my A/C on while driving, the voltage gauge sits at about 14-15 volts. I have not had a chance to rig up my multimeter so that I can see exact voltage while driving, but at idle with no loads the system is putting out 15.8-16 Volts. So far, there have been no trouble codes from the computer.

Sure sounds like a bad voltage regulator to me, and from the service manual that is actually a part of the PCM? Is it possible the alternator is to blame somehow? I'm going to pull it and have it tested over at AutoZone as soon as I get the time.

Also, I currently have the 117 amp alt, and was thinking I might want to add a larger alternator to handle the added load of the Lincoln e-fan. From reading on here, my understanding is that the 136 amp alt for a 5.9 is a direct bolt on replacement, is that correct.

 

If it is your voltage regulator which it sure sounds like unless you want to get another PCM there's a DIY on here for a guy who added an external voltage regulator it's a cheaper option than a new pcm

I was bored so I found the link haha

https://dodgeforum.com/forum/2nd-gen...onversion.html

 

Yes indeed, the voltage regulator is in the PCM. If it's truly failed, you can rig up a two wire regulator to take its place. There are instructions for that in many places on the web.

It is, though, entirely possible that you've just got a wire shorted to ground, or an internal alternator fault doing the same thing. Hoping it's not overstatement of something obvious to you: The alternator knows only two states: fully on, and fully off. What determines this state is a ground applied to the field winding -- the PCM regulates system voltage by maintaining a window in which the low threshold determines when to turn on the alternator, and the high threshold determines when to switch it off.

Once you've got your regulation problem solved, the 136A alternator is a bolt-in. Nuthin' to it but to do it.

 

Ugh, Thanks. In theory, If I unhooked the PCM wires from the alt, would that be a way to test to see if the PCM is bad? My thinking being that by disconnecting it, the alt would be open and I would see low (>13) voltage if the PCM was the culprit.


I hate an always on check engine light...but I also hate Spending $300 for a new ECU, that decision is gonna suck if i have to make it!

 

A little piece of black electrical tape will solve the always on cel unless its a mental thing where it will bother you knowing its under there on. I completely understand though I hate when the cel is on

 

Its mental for me; In my mind, if its "fixed" that light should go off. And if its always on I might not find out until its too late that there is a bigger problem.

 

I completely understand. I wonder if there's a way you could hook a tuner up to it and see if you could delete that code from the PCM? Might be something to look into on my car I had a couple codes deleted because they were something like the gas cap and something else anyways they're not needed so I had them deleted from the PCM I'm not sure if you know anyone with a tuner but its a possibility I would think

 

That would tell you that something external to the alternator is grounding the field wire, NOT that the PCM is bad. The PCM's regulator might be bad, but that test would not prove it. If the dark green wire between the PCM and field coil is grounded anywhere along its run, it'll cause the overcharging condition AND disconnecting it would make the problem go away.

If the alternator is truly good, and there are no faults in the conductors (two) that run from the PCM to the field coil, then you'll want to inspect hell out of the conductors of the PCM plugs and the grounds before pulling the trigger on a new PCM. I can't say that I am familiar with where the regulator circuit gets its feedback, but I can say that it would suck to plunk down the cash for a PCM that's not needed so I'd look things over very carefully.

Oh, about that second conductor that runs to the field coil: It's just the high side, and comes by way of the ASD relay. It's pretty much a there/ain't there thing and is only implicated if you get no charge as opposed to overcharge.

 

Well, this morning my truck started for about two minutes, and died right as I pulled out of the driveway and now won't start. Getting "No Bus" on dash. going to do some of the things I've seen in other threads (checking grounds, swapping relays, etc) but I'm guessing the PCM just crapped itself completely