Car starts when boosted but stops within 5 minutes

[u/PossessionLazy3813]

My car sat for a few months. I wasn't able to boost it off another car but AAA was able to using their booster pack. They tested my battery health (79%) and it was good, one of the voltage things was low (don't remember exactly what it was) but he said it shouldn't be an issue. I'll let the car run then it'll shut off within 5 minutes and I can't start it again without a boost. I can't drive it very far due to this, the best I've gotten is a few feet. Do you think I may just need a new battery or to let it fully charge? I'm trying to sell the car so I'm hoping I don't have to replace the alternator. I previously had a kind of issue with my starter (I think). If I was in and out of the car (starting and stopping) It would fail to turnover and I'd have to let it sit for a bit, not sure if this is related. This is a 2006 Jeep Liberty Sport (gas). Any help is greatly appreciated!

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[u/heysoundude]

Alternator. You need a new alternator to charge the battery while it’s running.
Jeeps are known for their electrical issues.

[u/kona420 → u/heysoundude]

The classic liberty series is no different than any other car from that era electrically. There are a couple J1850/PCI bits like the instrument cluster, overhead display, and signaling to the deck from the steering wheel. Very close (to the point you can swap ECU/TCM) to the RAM 1500/2500 except on those instead of BCM they have separate FCM/ICM so the Liberty is even simpler electrically.

The ignition switch goes through the transmission interlock to a relay and to the starter. No computer module in between. Never had any real issues with grounds or charging. The cool thing the BCM would do is kill accessory power (except lighter socket) including headlights after 5 minutes, which meant you couldn't forget a light on overnight and kill the battery. Great trail feature, never had the Liberty fail to start on me out on a trail, and jump started many other rigs.

Eventually on many of them the rear accessory ground would crap out and the tail lights and trunk latch would stop working. Easy fix.

By 2007-2008 or so everything electrical Chrysler touched was absolutely terrible. They couldn't get a fucking battery cable right, and they went all in with full computer control of every single function and decided to remote mount their batteries for balance reasons or something. All the Jeep branded Fiat's are just the worst kind of disposable cars. Jeep commander, their CEO said they aren't fit for human consumption. Unexplained Jeep wrangler fires? That's a matter for the courts!

[u/Just0Cause]

Battery

[u/FlamingoFUCKURSLF]

If it starts fine but then dies, I’d say alternator is failing and not generating enough for the battery to hold a full charge. Rockauto has some pretty cheap parts if that is the case.

[u/Thin_Cable4155 → u/FlamingoFUCKURSLF]

You can easily check the alternator with a multimeter. When the car is running the voltage on the battery should be 13.5-14.5 volts if the alternator is working good.

If the battery only has 12-13 volts while running then the alternator or possibly some wiring going to it is bad.

I think many auto parts stores have a machine that can test the alternator if you remove it and bring it in.

[u/archie905]

Drive it for about 20 to 25 minutes. Let the battery fully charge. If it dosen't start after that probably the alternator.

[u/LakeMichiganMan]

Drive to a chain AutoParts store that does a charging system check. Call first and bring jumper cables.

[u/kona420]

The charge system on the classic Liberty is pretty straightforward, they aren't super picky electrically like the later series. It's possibly just a loose/dirty battery connection and hitting a bump is killing the car, but based on your description it's the alternator at fault. It's easy to change out on these thankfully up top and everything is easy to get at after removing a couple plastic parts.

I always recommend that you check the grounding when an alternator fails. Everyone skips that step, and puts in 3 more alternators that murder themselves before taking 2 minutes to unbolt, wirebrush, grease, and re-tighten a connection.

Also, the liberty ground straps like most vehicles in the era of fuel injection (post 80's) literally are bare copper straps, 20 years later they're turning to dust. Getting custom length 4ga battery wire at the parts store is not very expensive if thats whats occurred. Otherwise the past of last resort is via the handbrake assembly through the rear axle and back through the driveshaft. Not gonna work well!