Bad PCM now?

[AllenG]

After a year of slight surging and seeing this from my OBDIIscanner I swapped out my apps.  I also have completely re done my fuel lines, draw straw, and swapped to a raptor pump.  While I was at it I swapped out my alternator since it looked like the original was on the truck.  The truck seems to be running great now with no surging.........but now I have another issue!  No signal from pcm to the alternator plug so it’s not charging.  The ac clutch is also only engaging now at startup, once it kicks off it will not come back on until the next start up.  The old alternator was putting out 14v and the ac was working perfect, tried swapping back to the old alternator and no luck.  Put the new one back in again and all is the same.  Then tested the plug and it looks like nothing is coming from the pcm to the back of the alternator.  Yes I have checked all fuses and relays.......is my pcm bad????

Tested the blue wire on the plug with a test light and now power.  But the green wire does light up.......reading the write up from mopar1973man in alternator and pcm protection it sounds like I am going to be replacing the pcm and the old alternator already damaged it.  I did find a spot on the wire at the back of the old alternator where it had rubbed through and shorted out on the case.  I fixed that before I hooked up the new alternator though.  Can the faulty pcm also cause the ac clutch to not cycle except right at start up?  If you put your foot on the throttle the ac clutch kicks out sooner and will not come back in.

[JAG1]

I'm hoping Mopar1973Man would jump in soon to offer some help. I am guessing if I say anything pertaining to your trouble.

 

A new alternator can have a bad diode sending excess AC volts ripple throughout the system. If I remember correctly AC ripple can confuse the computer, but I'm not certain the PCM even controls when the Air Conditioning compressor clutch kicks back on. Again I am guessing when I say the low pressure switch to the ac might be faulty. It's around a 35 dollar part and sits on top of the accumulator tank. I think that's right. IBMobile has educated me on this some, but being involved in many unrelated projects lately....

Edited September 16, 2019 by JAG1

[IBMobile]

 

 

 

In this article is a section on testing the AC clutch

 

 

[AllenG]

Thanks for the replies, IBMobile I did that exact test.  No power to the blue wire.......

JAG1, I swapped back to the old alternator which was putting out 14v and have the same issue.  New alternator or old one makes no difference neither will charge.  But neither are getting signal from pcm to charge now.

[JAG1]

Check the alternator for bad grounding. The bracket and mounting bolt(s) could be corroded enough to be losing ground.My OHM meter showed a very weak ground thru the bracket. A lot of guys clean up the bolt and bracket real good, but I ran a ground wire from Alt housing to battery ground.

[Mopar1973Man]

Blue wire - If there is no +12V after tach signal (engine running) then the PCM is fried. 

 

Green wire - is the variable ground.