alternator "Gen out" no voltage

[Sugmadique]

Im have a charging issue along with the transmission not having od. I checked the blue and green wires at the back of the alternator and the blue wire has no voltage. where does the voltage come from? does the pcm send 12v to the alternator or does the alternator send 12 volts to the pcm? or do both get their power from the pdc where the trans relay is?

[IBMobile]

[Sugmadique]

Thanks, I already have the wiring diagram and have checked continuity in the wires and they are fine. I’m just curious what is the origin point of the 12v signal on the “gen out” wire. Does the voltage come from the alternator to the pcm to let the pcm know the batteries need charged and it controls the rate of charge via the “gen field” ground signal, or if the pcm sends the 12v signal to the alternator. When I patched in a external voltage regulator the alternator was charging but that didn’t get voltage to the blue wire controlling the trans relay. I already replaced the pcm but I’m concerned that the alternator may have some issue and fried both pcm’s. 

[IBMobile]

From the alternator to the PCM.  You might find this interesting.

 

[JAG1]

Mopar1973Man can't access the site for awhile. He will be able to make some points about what to check. I hope you find out where it is and report back.

[Sugmadique]

14 hours ago, IBMobile said:

From the alternator to the PCM.  You might find this interesting.

 

Thank you so much, that article was extremely helpful. So when this first happened I removed the alternator and sent it to Napa to be tested and they sent it back saying it was fine. I’m not sure how they test it but I took it to an alternator repair shop and they said it needed rebuilt so o spent the $90 and I’ll have it back Monday. I have pcm on order and I’ll put a fuse inline in the meantime. Is there anything I can do to get moving until it arrives? I have a voltage regulator and a pigtail for it already. Will supplying the blue wire with battery voltage (and fuse)get my trans working? After I add the fuse holder to the blue wire I can leave the fuse out to protect the pcm if necessary. I just need to be able to get to work and back until the pcm arrives. 

[IBMobile]

If you power up the trans relay do it with a power source that is switched on with the key in the run position only.

[Great work!]

Blue wire gets 12v from PCM. The green wire 'TOGGLES'  (PWM) to ground through PCM. So blue to 12v and green  to solid ground equals max alternator output. Basically the blue and green each go to a insulated plastic brush holder. The brushes where out and the springs behind them can make contact with other parts of alternator internals or excessive field current overheats the regulator inside the PCM. If you use an external old school mopar regulator the case of it must be grounded. The trans relay can be bypassed too but you will hear humming under the truck with it powered up and engine off. That sound is the governor  solenoid doing its job. My truck has lots of bypass and an external regulator. I also have ECM split wide open dangling by hay wire totally  hacked and I rigged the vp44 with an external transistor. I also have the lift pump and grid heaters bypassed. I just refuse to buy anymore black boxes for my truck. LoL.

[Mopar1973Man]

Yup. The green and blue are control by tach signal so the engine must be running to measure. the blue lead is typically +12V hence why the protection fuse for it. Prevent PCM damage from a shorted alternator. The green wire is a variable ground to control the charge rate vs battery temperature which the PCM checks and adjust accordingly.

 

I do not suggest external regulators I've seen too many hooked up wrong and placement of the external regulator will  cause under charging if too cold and over charging if the regulator is too cold. I still is a common problem with the older 89 to 91 trucks. Cold mornings here in Idaho (minus) they could over charge quite a bit. 

Edited August 16, 2020 by Mopar1973Man