Alright guys and gals, coming home from work the other morning my volts gauge was pegged to the left and check gauges light on.
I have a 1999 Xj 4.0. First I checked the terminals and cleaned them. Got the colt meter out and with the engin off I had 12.4 volts at the battery post. The same volts at the post on the fuse block, and the same at the post on back of the alternator.
I then started the jeep and had the same readings hence telling me the system was not charging. Took the alternator(original 185k) and took it up to O'reillys, failed 2 test. So I bought a store brand remy one and still have not charging.
I proceeded to check fuses and wiring/battery cables and replaced all battery/ground cables with 4gauge and new ends.

 

The fusible link wire going from the alternator stud to the stud on fuse box was also replaced with 4gauge wire with a 120amp inline fuse. Checked all the connections had Walmart check the battery again and charge it back up, still not charging....

Tommorow I'm gonna take the alternator back off an exchange it for a new one to see if I just got a crappy remy one. Any other thing that I'm missing??!

Searched all the forums and read lots of threads with same problem, just wanna check all my bases before getting a new pcm.

Any help would be greatly appreciated. Thanks!

 

Ecm has internal voltage regulator.

 

Yes I'm aware of that, just looking at all other possibilities before replacing the pcm or doing a external voltage regulator

 

Just took the alternator back up to O'reillys and it passed with a 14.6 output, I'm stumped now

 

Alternator is obviously fine, so it isn't getting a signal to field (AKA Activate). So you need to make sure all the electrical connections are working.

From AllData:
"GENERATOR OUTPUT-PCM INPUT
Provides a charging system voltage input to the Powertrain Control Module (PCM). It is sensed at the battery input to the PCM.

GENERATOR FIELD SOURCE (+)-PCM OUTPUT
This output from the Powertrain Control Module (PCM) regulates charging system voltage to the generator field source (+) circuit. The voltage range is 12.9 to 15.0 volts . Models of previous years had used the ASD relay (directly) to apply the 12 volt + power supply to the generator field source (+) circuit.

GENERATOR FIELD DRIVER (-)-PCM OUTPUT
This output from the Powertrain Control Module (PCM) regulates charging system ground control to the generator field driver (-) circuit."

So, simplified: Make sure the alternator has voltage at pin 1, and ground at pin 2. Engine might have to be running to test it. Back-Probing (Small paperclip or "T-Pins" in the back of the connector where the wires go in) would be the best way IMO, to make sure nothing is shorting out while it's plugged in.

 

did you ever get this figured out?? I have the exact jeep with the exact problem.

 

Having had this exact problem, and trying to diagnose it to no avail, I determined the generator field driver (-) wire had somehow shorted or broken in the harness. There was no continuity between terminal 2 on the connector and it's corresponding pin on the ECU connector. I cut, soldered and spliced a wire between those points, and problem solved. charging properly. it took me about a month of testing everything else, though!