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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.
 

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2019 Jeep Compass North Altitude
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Or check tour sensor modules.. those little suckers often times go unchecked and cause the biggest heartaches!
 

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Discussion Starter #6
Or check tour sensor modules.. those little suckers often times go unchecked and cause the biggest heartaches!
Hi,
Yeah I've checked the ground points. Everything has a solid connection. As for the 'tour sensor modules' I am not sure what those are. I did a google search and wasn't able to pinpont what that was, is there another name that they go by?

Also another update for the car. I went to a class the other day, I unplugged the battery charger and drove the 10 minutes to class. The whole time the battery dash light was on, and the battery voltage when I got there was 11.6v. However when I went to start my car 8 hours later, the battery light was off and I was reading 14.5v at the battery terminals. And everything has been working perfectly for the 3 days since. So this is very confusing to me as nothing under the hood was fiddled with or even touched. So if it was a bad PCM I highly doubt that it would break and then work again, once a computer component is fried it always stays that way.

Again thanks for the input so far!
 

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Could also be a wire short. If you replaced everything else that's the only thing I could think of. Something is drawing more than it should. If you have done any recent upgrades or repairs start there. Beyond that you are kinda chasing gremlins by the sound of it. Good luck.
 
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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.
Well you car is almost 12 yrs old....if your going to keep it, Your better off bringing it to the dealer, rather than a independent mechanic... who can make things worse and pay more in the long run....
 

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Just an update: The car and alternator have been working perfectly since that day. And nothing has changed since. Soooo it’s beyond me what was wrong. On the bright side I didn’t have to buy a sketchy aftermarket PCM
 

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Just an update: The car and alternator have been working perfectly since that day. And nothing has changed since. Soooo it’s beyond me what was wrong. On the bright side I didn’t have to buy a sketchy aftermarket PCM
Sounds like the connection was loose. I'd also suggest a little dielectric grease on the pins to protect against corrosion.
 
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