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Discussion Starter #1
Hi my 96 amp metor gauge is about at 12.5 volts until I get up to 55mph. What I would like to no is there a voltage regulator, or is it built into the alternator. thanks
 

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The regulator is built into the PCM (computer) and costs $500 plus to replace.

Once you are certain the problem isn't the alternator or batteries you can disconnect the PCM regulator and use an external one. It's easy, cheap and won't effect anything else. Any 12 volt regulator will work. Remove and tape the small wires on the alt replace as in the picture below. Make certain the switched positive is at the top of triangular connector, the other two wires are interchangeable. The regulator doesn't have to be like the one below, in fact the number on it is no good.
 

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Discussion Starter #3
How can I test the alternator? Or should I bring it to an AutoZone or somewhere like that.
 

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Start your truck, then disconnect both batteries, if all your lights and everything still work and the truck still runs, alternator is doing its job, if not, the alternator isn't doing anything or you have a bad connection..

Another way gauranteeing its the alternator is with a voltometer. Hook it to the batteries when its not running and it should be around 12V, when you start it up you should be up to around 14Volts. If not, then its the alternator, but I would run through all the wires and make sure they aren't corroded or anything and that everything is getting a connection...
 

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Discussion Starter #5
Great tip ISX will get on that first thing in the morning
 

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Discussion Starter #6
wll i disconneted the batteries and the truck died . looks like i need a alternator.
 

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Don't be so sure. The alternator is just the slave, without the PCM to make it charge it sits cold. Unhook the two small field wires located on the back of the alternator attached with 8mm nuts. Keep them away from grounding on the block etc. Attach two jumper wires, polarity doesn't matter...one to each stud, then the other ends to positive and the other to negative. This will full field the alternator and bypass the regulator. I wouldn't hook up the jumpers until the truck is running. It will display full output so don't leave it that way but for a couple of seconds. Voltage can go sky high by full fielding. Once you have done this, leave the truck running and using a voltmeter check for voltage acrosss the terminals you took off. One wire is pos and the other neg, you should see between 7-12 volts. This will tell you the PCM is supplying field voltage and current to the alternator. In which case keep looking. If the alternator didn't labor the engine, or show high voltage on a meter, it is most likely short on brushes. If it did, well keep looking again.

I wouldn't go throwing an alternator at it until you've diagnosed it down a little further. Taking off the batt cables is not a good thing to do to modern charging systems, and it still doesn't tell you the problem with the charging system.

GL
Chris
 
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Discussion Starter #8
ok i will try this thanks gl
 

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Discussion Starter #9
i got 12.24 volts across the 2 wires i guess it is the alternator
 

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Don't be so sure. The alternator is just the slave, without the PCM to make it charge it sits cold. Unhook the two small field wires located on the back of the alternator attached with 8mm nuts. Keep them away from grounding on the block etc. Attach two jumper wires, polarity doesn't matter...one to each stud, then the other ends to positive and the other to negative. This will full field the alternator and bypass the regulator. I wouldn't hook up the jumpers until the truck is running. It will display full output so don't leave it that way but for a couple of seconds. Voltage can go sky high by full fielding. Once you have done this, leave the truck running and using a voltmeter check for voltage acrosss the terminals you took off. One wire is pos and the other neg, you should see between 7-12 volts. This will tell you the PCM is supplying field voltage and current to the alternator. In which case keep looking. If the alternator didn't labor the engine, or show high voltage on a meter, it is most likely short on brushes. If it did, well keep looking again.

I wouldn't go throwing an alternator at it until you've diagnosed it down a little further. Taking off the batt cables is not a good thing to do to modern charging systems, and it still doesn't tell you the problem with the charging system.

GL
Chris
Thanks for the informative article.http://www.cumminsforum.com/forum/images/icons/icon7.gif

I tried this with my '95 that won't charge and was getting field voltage from the PCM wires and also got a jump in volts when I briefly full fielded the alternator.

Any other clues or tests would be greatly appreciated.

Thanks, Pile
 

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Thanks for the informative article.http://www.cumminsforum.com/forum/images/icons/icon7.gif

I tried this with my '95 that won't charge and was getting field voltage from the PCM wires and also got a jump in volts when I briefly full fielded the alternator.

Any other clues or tests would be greatly appreciated.

Thanks, Pile
This indicates your alternator is fine.
Problem is the PCM ($$$) or wiring.
Just install an external voltage regulator.
http://www.cumminsforum.com/forum/94-98-tech-articles/37468-alternator-external-regulator-how-89-02-a.html

External Regulator
 

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Don't be so sure. The alternator is just the slave, without the PCM to make it charge it sits cold. Unhook the two small field wires located on the back of the alternator attached with 8mm nuts. Keep them away from grounding on the block etc. Attach two jumper wires, polarity doesn't matter...one to each stud, then the other ends to positive and the other to negative. This will full field the alternator and bypass the regulator. I wouldn't hook up the jumpers until the truck is running. It will display full output so don't leave it that way but for a couple of seconds. Voltage can go sky high by full fielding. Once you have done this, leave the truck running and using a voltmeter check for voltage acrosss the terminals you took off. One wire is pos and the other neg, you should see between 7-12 volts. This will tell you the PCM is supplying field voltage and current to the alternator. In which case keep looking. If the alternator didn't labor the engine, or show high voltage on a meter, it is most likely short on brushes. If it did, well keep looking again.

I wouldn't go throwing an alternator at it until you've diagnosed it down a little further. Taking off the batt cables is not a good thing to do to modern charging systems, and it still doesn't tell you the problem with the charging system.

GL
Chris
Thanks for the reply, but just to clarify-

I did get 7-12 volt reading from the regulator wires when truck was running. Doesn't this mean the regulator is working (according to the above field test). Not sure if I understand the test correctly.

p.s. Also got volt jump when full fielding alternator

Thanks again, Pile
 

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Take the alternator off and take it to a shop that does alternator repairs and rebuilds (not a Autozone or NAPA, etc.). They will check it out using a scope and under load. If it's bad, have it rebuilt. If it's good, you may have a PCM or wiring problem (have you checked your grounds?); set up a external regulator for it.
 

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I found the culprit, only I don't know quite what it is called. It is some sort of 3 pronged sensor that sits right at the front and center of the engine and goes to the crank or something. This had vibrated loose and just needed to be pushed back together. I just happened to see it had a bit of a gap from being totally closed.

Anyway thanks to all who help out clueless guys like me save some money and learn a little about the truck.:party018:
 

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Sounds like you found a messed up engine speed sensor (ESS)
If it was the problem you should of had many more symptoms.
If the ESS doesn't pick up a tach signal it tells the PCM the engine isn't running, nothing electronic will work.
 

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The ESS was only partly open (not separate) and the tach has been a joke for a long time. I'm guessing it has been slowly creeping apart with maybe some sort of connection, don't know if this makes sense but now tach works and batteries are getting charged. :confused013:

Once again, thanks to this forum now I have replaced my oil pressure sensor, fuel shutoff solenoid and hopefully solved my charging problem.:headbang:
 

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I have a 1999 3500 Dodge Cummins Diesel with a Denso alternator. The instructions I have read for replacing the PCM with an external regulator all mention two posts on the back of the alternator. Mine doesn't have two posts but it does have a connector that connects the computer to the alternator. Can I just clip the wires on this plug and wire the external regulator to this plug? I have already mounted my Mopar regulator- I just need to wire it....

Rusty
 

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Guys
When adding the external regulator to the trucks, do any lights appear? Battery or otherwise? I am down to the PCM being bad on my truck and considering the external regulator.
 

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Start your truck, then disconnect both batteries, if all your lights and everything still work and the truck still runs, alternator is doing its job, if not, the alternator isn't doing anything or you have a bad connection..

Another way gauranteeing its the alternator is with a voltometer. Hook it to the batteries when its not running and it should be around 12V, when you start it up you should be up to around 14Volts. If not, then its the alternator, but I would run through all the wires and make sure they aren't corroded or anything and that everything is getting a connection...
I have read many Many times that running the engine without the batterie(s) hooked up will fry the alternator as sure as taxes on April 15th. I repeat.... Do not unhook the batteries while the engine and alternator are running.:badidea::badidea::badidea
 
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