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My 2009 Jeep Compass alternator is not charging the battery. I was driving one day and out of the blue the battery dash light lit up. I was able to drive home, and had less than 12v while the car was running. Took it to a mechanic to get the problem diagnosed and they thought the alternator was bad as they weren't able to read any output, they offered to replace it but that alternator was less than 2 months old so I didn't want to for over the $600 for a new one. When I went to pick up the car from the mechanics I started it and was surprised to see the dash light was off, I went inside and asked what they did and they were as surprised as I was. At the battery I was measuring 14.4+ volts with A/C, radio, defrost etc. So I assumed the connector between the PCM and Alt. had wiggled loose and re tightened by the tech. The car drove perfectly for 2 weeks until the damn battery light came back on whist driving. At this point I knew something was wrong, after doing some googleing I tried some things to troubleshoot. I had the alternator and battery tested and they were both perfect, I checked all fuses and connections and everything was seated right, nothing was frayed or visibly damaged, I re-sat the PCM connections and made sure they were clean, checked continuity between pin 51 at the PCM and at the alternator. At this point I think the ECU in the PCM is bad, does anyone else have any other ideas of things to test/try?


TL;DR - My alternator wont charge the battery because, I think, the PCM is not sending signal to the Alt. and is therefore bad. Although the 2 weeks of drivability between the dash lights is puzzling.
I am experiencing an
My 2009 Jeep Compass alternator is not charging the battery. I was driving one day and out of the blue the battery dash light lit up. I was able to drive home, and had less than 12v while the car was running. Took it to a mechanic to get the problem diagnosed and they thought the alternator was bad as they weren't able to read any output, they offered to replace it but that alternator was less than 2 months old so I didn't want to for over the $600 for a new one. When I went to pick up the car from the mechanics I started it and was surprised to see the dash light was off, I went inside and asked what they did and they were as surprised as I was. At the battery I was measuring 14.4+ volts with A/C, radio, defrost etc. So I assumed the connector between the PCM and Alt. had wiggled loose and re tightened by the tech. The car drove perfectly for 2 weeks until the damn battery light came back on whist driving. At this point I knew something was wrong, after doing some googleing I tried some things to troubleshoot. I had the alternator and battery tested and they were both perfect, I checked all fuses and connections and everything was seated right, nothing was frayed or visibly damaged, I re-sat the PCM connections and made sure they were clean, checked continuity between pin 51 at the PCM and at the alternator. At this point I think the ECU in the PCM is bad, does anyone else have any other ideas of things to test/try?


TL;DR - My alternator wont charge the battery because, I think, the PCM is not sending signal to the Alt. and is therefore bad. Although the 2 weeks of drivability between the dash lights is puzzling.
almost identical situation. I replaced 2 alternators before the mechanic admitted that both alternators had been fine. I had approximately 3 months of driveability between the disasters.
 
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