Quote Originally Posted by SideShow Bob View Post
Here's a quick couple of tests to check the alenator, while running, see if a wrech will stick to the back of the altenator. If not weak altenator. Then Start the jeep, pull the positive battery cable, then start turning on the electrical items..... Headlights, radio etc.... Until everything is on or the jeep dies. If the jeep dies, bad alternator.
With modern electronics, this is a bad idea... worked fine on older vehicles, but especially if there's a problem with your charging system (shorted diodes, etc), then you could fry the computer, etc... the battery acts like a giant capacitor, absorbing voltage spikes, and limiting the overall voltage. Without it in the system (even a bad battery), you could have voltage spikes that destroy electronics.

don't believe me? then ask this guy: http://www.w8ji.com/battery_and_charging_system.htm

Quote Originally Posted by GilpinGuy View Post
Wrench stuck to the alternator (not super tight, but there was obvious magnetism) and the Heep ran fine for 15 mins with everything on and the + battery cable off. Bad battery looks like. I got the charger on it for tonight just to be safe so I can get to work tomorrow.
I love the backyard methods, but they're subjective... and there are a lot of variables, if the battery isn't taking a charge, and there's not a large load on the alternator, then it won't stick well... There's nothing like the solid # you get from a Digital Multimeter... check the voltage at the battery right after starting, should be around 14.3-14.4V.. if higher, it will boil out the water, if lower, there's something wrong somewhere
so then you check the Alternator output with the DMM (Digital MultiMeter) ground on the alternator case. If you have around 14.4V there, then the problem is in wiring somewhere... check vs chassis ground somewhere and if that's lower, then you've got a bad ground somewhere- if it's the same, then the problem is in the + feed to the battery.

You have to think of all electrical circuits as a loop- the - has to get to the + eventually, weakest link will prevent it from working.

All that said, I'd put money on the battery still.