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So the other day after I installed my amp and sub on the way home my battery light came on. I get home and head to work the next morning. On my way to work my car shuts down, I get it restarted by doing a rolling start since it is a manual. I get to work where I test the battery and the battery is passing the test just shows a little bit of a low voltage (11.8v) I chalk that up to the colder weather not really thinking. I then start the car with my tester still attached and notice that it isn't charging. I test the alternator and yep it is not producing a charge. I back the car out of the bay and proceed to do the jobs that I have on the board. At lunch I go buy a new alternator. I get back to work and go to pull the car into the bay and the battery is dead. I push it into the install bay and hook it up to a battery charger while I install the alternator. I install that and lo and behold now I am charging. I close everything up and back it out and head home. About an 30 minutes into my ride home the battery light comes back on again then goes off and then off and on a few times then stays on. I pull into an Advance auto and have them check my battery and my alternator and the guy says the battery is good it has 865 cca and the alternator is charging well at around 14v. He and I are both stumped so I head home. I get nearly home and I turn my radio on and my high beams on and while sitting at a stop sign the light goes out. I start moving again and it comes on. I then realize what the guy said about the CCA of the battery, the CCA on that battery should only have been 745. I then start playing with my lights and climate control and notice when I put a heavy electrical load on the car the light goes out then after a few seconds or so it comes back on. Could I have over charged the battery and now the alternator is keeping it too charged or do you think that I just have a bad battery/alternator? Any help would be appreciated
 

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I forgot to mention that this morning when I started the car to come to work it started fine. No battery light at all, No gauges and the radio wouldn't power on. Everything else works though, the heater, the lights interior and exterior, cruise control, turn signals all the warning lights worked when I started the car. After about 20 minutes of driving (headed to work) I tried the radio and sure enough it came on and acted like nothing had happened (ie. like it didn't lose power). 20 Minutes later I go to merge to my left and use the turn signal (like I had multiple times before) and the gauges flicked to life. After about another 10 minutes the battery light came back on and started acting like it did the night before. On while moving unless a full load was on then it would go off for a bit but, ultimately come back on.
 

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Now you just have to find what blew that fuse in the first place...
 

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I just changed timing belt and water pump and about a week later in a cold snap - -19 degrees - my battery light came on. As battery was 7 years old I changed it out but battery light stayed on. Did the hold trip reset in while starting and toggled through everything and got to battery voltage and it says 13.6 and with everything on it goes to 13.3. So I am assuming it is charging but is 13.6 the correct voltage for charging? Anyone think my alternator is failing or could the mechanic that did timing belt break my 3 wire pigtail on alternator for the smart charge? I am a disabled vet who still still tries to tinker so looking for any help.
 

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Have tried the F10 fuse, as I’m convinced it’s not the alternator as it’s charging checked wiring from alternator blue wire to pin 72 of ecm, checked and cleaned grounds - found nothing but while cycling through screens on trip screen all of a sudden battery voltage jumped to 14.5/14.7. Not sure what to think. Cluster going bad? Also am showing a P117 ECT but have no overheating. Think it’s time to pop a cold one and relax -
 

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The alts are supposed to put out a minimum of 14.0 volt at warm idle with no other loads on. The normal charging range is from 14-15 volt even and 13.6 is usually an indicator of a cracked diode which these alts do at the drop of a hat. They will check often as good on a tester and then suddenly the volts drop just like said.

I rebuild the alts, the OEM diode plate on them is utter crap and rebuilds generally change that part first thing as it causes so much trouble.
 
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