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Ok, recent thread on here drove me past the point of looking at wiring diagrams and giving direction. Somethings are just easier explained with pictures and tested against a known good working system.

So in an effort to help others, I tore my own truck apart in the dark outside in Alaskan November temperatures with a few simple test tools, a head lamp and an IPhone camera so bare with me if the pictures aren't the greatest.

I do prefer that testing be done with test light when possible. A meter is useful tool but if you don't 100% completely know how to use/test with it you can easily be lead in the wrong direction. Just throwing that out there from 11 years now of turning wrenches for a living. See it happen all the time. No current flow no voltage drop. Testing circuits when they are loaded is best. Also when touching pins, TOUCH them only, do not force whatever probe you are using it in. This will spread the terminal and cause poor connections.

First of all you are going to have to look at a diagram, going to make you.

On this diagram we can see
- Powertrain Control Module or PCM - Controls Alternator Fields (Turns the alternator on/off)
- Power Distribution Center or PDC - Fuse box under the hood
- The Generator or Alternator - If you need this explained....

1. There is a Black/Grey Wire between the Gen/Alt and the PDC which contains the 140amp fuse and then a red wire to the battery. This is the large charge wire that electricity flows from the Gen/Alt to the 140amp fuse in the PDC and onto the battery. This is the large wire on the large stud on your Gen/Alt and you should have battery voltage or alternator output voltage on this stud. Check the fuse if you don't, or wires associated with that circuit.

2. Now into the meat of things, there are two wires from the PCM to the Alternator.
- Here is the two pin plug that plugs into the back of the Alternator, notice the Green and Dark Blue Wires.

- The dark blue wire is the voltage supply to the Generator fields. Meaning this is a positive wire and should have 12V on a meter or light a test light like this when the test light is hooked to battery negative as in the picture. (A meter would be the same with the negative lead) Engine MUST be running, key on engine off does not work for this test. *For demonstration purposes pictures are taken with engine off so the test light is not lit in these photo's.*
- The green wire is the control wire, meaning the PCM grounds this wire to turn on or off the alternator fields (DO NOT THINK YOU CAN JUST JUMP THIS TO GROUND FOREVER TO BYPASS THE PCM IT WILL FRY LOTS OF STUFF BECAUSE THE ALTERNATOR WILL BE FULL FIELDED TO 16+ VOLTS) This is a ground circuit so with the truck running a test light or meter hooked to battery positive should show 12V or light like this. If you have power on the blue wire while the engine is running and can hook 12v to ground on the green wire while the engine is running and voltage on the large stud your alternator is bad, but if you don't have one of the other.........

This is where the PCM lives behind your air filter box, notice I removed mine for better access, TURN THE KEY OFF before disconnecting.


The connector closest to the passenger side is the Grey C3 connector. The Connector in the middle is the White C2 connector. Reference the wiring diagram at the PCM. We are concerned with pin 25 in the Grey C3 connector as it is the opposite end of the dark blue wire at the alternator. We are also concerned with pin 10 of the White C3 connector as it is the opposite end of the green wire at the alternator.

NOTE - The connectors have a locking tab on the top and bottom, do not force, use shop air if need to blow out dirt so the locks release and you can unhook the connectors.
Noticed the pins are numbered, double and triple check your are on the correct pin when testing.
Grey C3 shown here

White C2 shown here


Now using your meter set it to continuity or resistance test and check between pin 25 in the Grey C3 connector and the BLUE wire pin at the alternator plug. There should be a connection between these two pins.

Next test the Pin 10 of the White C2 connector to the GREEN wire pin at the alternator plug. There should be a connection between these two pins.

Meters vary from make to make as to what they will read (some beep) when showing continuity vs an open circuit. Simple test is to simply look at what the meter says when the leads are not touching each other..... like this.


VS

With the leads touching each other. (no resistance, good connection)

This is what you want to see when testing both of your wires between the PCM and Alt plug

Now if these wires both test good, more than likely your PCM is bad. To be 100% positive it is the PCM, I would pull the plastic cover off of the C2 and C3 connectors and back probe the blue and green wires and see if you have 12Vs that way, this would eliminate the entire harness and test only the PCM. I personally would back probe the pins at the PCM and test just like I showed at the alternator plug with the test light.

How you go about fixing the issue is up to you. With the manual transmission trucks that is all these two wires do. BUT with the Automatic trucks notice in the diagram that the BLUE wire also powers the transmission relay through a splice.

I'll try to update this with information as to PCM repair or external voltage regulator solutions. Hope it answers some questions.

Engine running for testing at the plug. Key off before unhooking PCM.
 

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This is a great write up man! I’m currently dealing with this problem. I did figure out the pcm is bad and used your write up to double check my testing.

I decided since my truck is a auto. And I still have 12v on blue.

I’m going to use the external regulator kit with a adjustable regulator. I used a non adjustable regulator and couldn’t get my truck below 15v.

So I ordered a adjustable one and hope it will be good now.

I also found out The green and blue wires do not need to be hooked up for the dash Gauge to read charge.
The computer picks up the voltage somewhere else to signal the dash the charging state.




Sent from my iPhone using Tapatalk
 
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Discussion Starter #3
Thanks white moose, glad it helped someone at least. Its really a simple test procedure if you just ignore everything other than the 3 wires hooked to alternator. %80 of most diagnostics is research, hope this shortcuts that for some of the guys who don't know where to start with finding that information.

If anyone has anything to add, chime in. The more we can keep these old trucks on the road the better in my opinion.

The newer diesel are not what the older ones are when it comes to reliability and cost to maintain. I know these can seem to nickel and dime us to death, I dare you to look at truck payments on a newer truck that is going to have problems just as much if not more with the emission systems and even if deleted of that, You still have the reliability and cost of HIGH PRESSURE COMMON RAIL. Which in my mind is an unreliable fuel system and expensive as heck to keep up with. Yes I said it, I don't care who makes the HPCR system, Cummins, Cat, John Deere, International, Duramaxx, everyone of them has fuel system problems that are expensive to fix way before they should be broken. Rant over.

If any Admin wants to add this as a sticky or to the tech articles, you are welcome to do so.
 

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Thanks akshooter! Well written and easy to understand. We are facing this problem. I believe one of the wires vibrated off at the alternator. We were getting intermittent charging and assumed the alternator had failed. We replaced and still have no charging but one of the field wires came off at the alternator. I would hazard a guess that if it was hanging on by a thread on and off again, it likely triggered a nasty response by the ECM with it being intermittent! As we examined the broken wire to see if we could somehow reattach, the other wire fell off and we lost the connector! I believe it fell into one of the holes in the frame, we spent hours including fishing expeditions into the frames but could never find! Murphy's Law. We have now ordered a replacement pigtail. We have also talked to our alternator man and he can bypass the ECM (he claims) if the ECM was fried by adding internal regulation? inside the alternator.

We are just in the process of troubleshooting and we will troubleshoot the wiring leading from the box. Would the blue and green wires be interchangeable with each other as they are connected to the coil inside the alternator? Thanks for posting that detailed picture as I can now get the orientation correct when we get the replacement plug for the green and blue wires!

This truck was purchased second hand. Someone who had it before us had soldered wires to the back of the plug and use some shrink tubing. Unfortunately his temporary fix only lasted so long and appeared to vibrate apart! The 2 wire connector seems to be associated only with the Nippon Denso alternator. We have many friends with similar trucks but none had the 2 wire connector. I was looking for the plug orientation so that I could reconnect the 2 wires like they were before. I couldn't find anything on the internet until I found your posting. Thanks so much. Keep up the good work!! I believe the other alternators used a 3 wire for some reason!
 

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So I have this issue on my 98.5 5m9 24v 2wd automatic. I recently installed group 31 stud post batteries and cable ends to match. Then my voltage gauge began to drop and I lost high gear. Replaced alternator(twice, 1st one busted the +stud) I read your post and tested to find a bad diode in my pcm (sent off to be repaired)(repair place stated to check and repair wiring for a short) got it back reinstalled and everything worked for 2 wks now I have the same issue.

All wires look good visibly and with ohm meter. Could the problem be the aftermarket batteries possibly bouncing and arching on the hood?
 

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Mod please turn this write-up into a sticky for future reference. :thumbup:
 

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Hello AKshooter, can I assume that this wiring is the same for a 1994 2500 diesel?
The dodge wiring schematic showed a different color wire going to the PCM, one is for communication link red/white, and the other is for the transmission, pink wire.
Please let me know what I am missing because your explanation of alternator/generator wiring is excellent easy to follow and not a lot of mathematical lingo. My son purchased this, his first pickup, and the previous owner cut and removed all the wires from the alt/gen to install an external voltage regulator. This should have been fine if it was done correctly, so now I need to re wire the charging system. Thanks for the great article
 

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Chances ae the PO installed the external alt because the PCM was not regulating charging. So properly rewiring the external alternator correctly would be the best way to go short of a new pcm. Use a hot wire to key hot, a field wire and ground wire and on some a body ground on the regulator. A diagram is available in the regulator box or on google for most regulators. Read the pictures and search google images, Sue the style(3 or 4 wire) or brand and part number in your search. You can not use the old hot or ground out of the pcm. You have to make sure all proper grounds are in place. Pretty simple circuits rally.
 

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Yes, Assuming that is what is wrong. There is other components involved but is most common if the alt is good. so take the alt to a parts store and have it bench tested. Or stop buy an alternator rebuild shop and have them test the whole system. All unless you are into a lot of meter testing and skilled at it.

PS: Welcome. When posting, it is best except for special circumstances, to post in the year group forum that matches your truck or system you are working on. Actually there a lot of differences with anything engine or electrical between 94-98 12v and 98.5 to 02, even some between 98.5 to 00 and the 01/02's.
 

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Yes, Assuming that is what is wrong. There is other components involved but is most common if the alt is good. so take the alt to a parts store and have it bench tested. Or stop buy an alternator rebuild shop and have them test the whole system. All unless you are into a lot of meter testing and skilled at it.

PS: Welcome. When posting, it is best except for special circumstances, to post in the year group forum that matches your truck or system you are working on. Actually there a lot of differences with anything engine or electrical between 94-98 12v and 98.5 to 02, even some between 98.5 to 00 and the 01/02's.
 

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I saw that you were new. Just wanted to let you know so that you get the most accurate best responses from those with similar vehicles. Another tip. Usually it is best to start a new thread in correct forum or get into one that has several current posts on the same subject, rather than posts that are months or years old that are not being currently followed and contributed to. Not as many people see or react to old posts. Like When I saw this was 2 years old I almost passed by it. Every forum,especially on older trucks have a few knowledgeable and helpful members.
No such thing as a "stupid question" as long as it is honest and on point. Also there you do not have to search your butt off before asking a question as some may suggest.
The only bad posts are those that are rude and condecending to honest polite people. The worst posts are those from people that are too lazy to use decent grammer and punctuation an are not understandable and hard to read. Often using her excuse "Sorry I was on my phone." Otherwise this a great forum.
 

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I saw that you were new. Just wanted to let you know so that you get the most accurate best responses from those with similar vehicles. Another tip. Usually it is best to start a new thread in correct forum or get into one that has several current posts on the same subject, rather than posts that are months or years old that are not being currently followed and contributed to. Not as many people see or react to old posts. Like When I saw this was 2 years old I almost passed by it. Every forum,especially on older trucks have a few knowledgeable and helpful members.
No such thing as a "stupid question" as long as it is honest and on point. Also there you do not have to search your butt off before asking a question as some may suggest.
The only bad posts are those that are rude and condecending to honest polite people. The worst posts are those from people that are too lazy to use decent grammer and punctuation an are not understandable and hard to read. Often using her excuse "Sorry I was on my phone." Otherwise this a great forum.
 

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You are welcome. You may get more info. After you get 50 posts yo can also PM anybody. Feel welcome to. Classifieds open up the same time. maybe after thirty days too.
 
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