As someone said before running 20 minutes is no where near enough time to recharge a dead battery. Most auto parts places will recharge your battery and load test it for free. Do that and then drive the Jeep over there and have them test the alternator on the Jeep for free. You might have a parasitic drain that is slowly killing your battery, you could have a bad rectifier in your alternator and it's not fully charging, you may have a bad battery. Have you taken off all your battery cable ends and cleaned them? I'm talking both terminals, the ground on the engine (may be two), the ground to the fender, the wire to the alternator, the wire to the PDC, the wire to the starter, all of them.
Ill just buy a new battery. See if the problem presists. 25 dollars for a battery should have expect these results. A coworker of mine bought a used battery 3 months ago and he says its running pretty good. Just thought if try my luck. Never again. Ill buy a new battery Saturday since thats payday. Glad my job is only 4 blocks away walking distances. Ill report back in about 3 weeks after the purchase of the new battery. Since it was 2 weeks that the used battery lasted thanks again guys
I think the battery is bad too but I always have it tested first. Even if the battery is bad I always have the alternator checked once it's replaced to make sure there are no other issues. Anyway, the testing is free.
FWIW I works at a parts store for about ten years. I've seen people who kept bringing batteries back as "bad", getting them charged and tested good, getting them swapped under warranty as a courtesy only to find out that their alternator belt was loose, or the pulley was loose, or the cables were corroded away, or the alternator was bad. A lot of ways that system can get broken.
Originally Posted by zjosh93 View Post
Let's say the battery was good , alternator was bad . You remove a cable , it shuts down as the test works . Problem ! Yes , people will put the cable back on the terminal before shutting the ignition off . Now , you could fry the PCM .
That said , multi-meters are cheap enough to have that convenience on hand .
1995 Orvis build
1988 MJ build
1998 Slate 5.9 build
1998 Platinum 5.9 build
Originally Posted by newfieZJ View Post
The alternator supplies the power as the vehicle uses it but the battery buffers the system. The voltage regulator looks at the overall system voltage so it can regulate the alternator output. If you unplug the battery the regulator sees the voltage drop and cranks the alternator up to 100% charging. Without the battery as a buffer the voltage spike hits the PCM and fries it. A lot of the electric bits in the PCM are very sensitive to how clean the source voltage is. At least that's how they explained it back when I worked parts.
The other problem is the it's pretty common for one of the three diodes in the rectifier bridge to go bad. The alternator will still make electricity but can only put out like 12.4 volts. That'll keep your Jeep going but won't charge the battery. Saw tons of these on the alternator tester. This would pass your test but not solve the problem.
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