Check your battery connections when buying used!!!

[Gladdybater]

First post here, hope im in the right place. I bought a 21 rubicon with 36k miles from a dealership. Fast forward two weeks, my truck auto stopped and didnt start (I usually disable this). The positive cable was completely off the battery terminal. I was able to start it and park it, but the high amp alternator fuse had blown. I replaced it and drove it 5 hours with no apparent issues... other than my battery voltage used to sit at 13.5-14.1 while driving and now it is at 12.7-12.9 (according to screen). It charges to 14.0 when letting go of gas. With a meter on cable connections (not terminal), im getting 12.8 with truck off and 13.0 idle even though screen shows 12.7. Could it be a weak but not dead alternator? I have warranty, but if I can test and replace the alternator in a day myself that will save me headache. I hear mixed opinions that this range actually indicates a healthy battery? I have the aux battery as well.

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[Hootbro]

Sounds about normal. The alternator on these is a demand base smart alternator with voltage regulated by the PCM. It is not a constant fix voltage output alternator which is now considered old school. You will have these fluctuations of voltage.

 

[jav_eee]

As stated above, that’s how these work.

 

[montechie]

That sounds normal for a modern alternator, I had my aux battery fail so watched my voltage a fair amount while driving.

2nd the check the battery cables, also after any shop touches your electrical. I discovered a negative cable for my winch disconnected after the aux battery replacement. To not be too judgy, when I sold a previous truck I had it not start on a test drive. Very embarrassing on a truck that never gave any issues for 150K miles, but I had cleaned the battery and posts when prepping to sell and didn't get a cable snug enough. Was still attached, but loose enough to cause a start failure

Always double check your own and others' work.

 

[Chasm]

It is not a constant fix voltage output alternator which is now considered old school.

That threw me for a while! Driving it home the first day I was like "crap, I already broke it".

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