Hey guys, I had my 93 Xj running when I unhooked my battery. After like 1 or 2 full seconds with battery unhooked the jeep died. I hooked the battery back up and tried to start but all it does it turn over now. What the heck happened?

 

Your battery does more than just provide electricity. It also shorts AC spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, endangering every semiconductor circuit in your car. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and even the car's stereo.

Even if your computers and stereo remain intact, in a great many cases removing the battery burns out the diodes in the alternator, necessitating a new alternator. If disconnecting the battery interferes with the voltage regulator's control voltage input, it's possible for the alternator voltage to go way over the top (I've heard some say hundreds of volts), frying everything.

Even the initial premise was wrong. If you disconnect the battery and the car conks out, you don't know if it conked out due to insufficient alternator current, or whether the resulting transients caused your ECU (the car's computer, which controls fuel mixture, timing, and much more) to spit out bad data, shutting down the car.

Nobody should EVER run your engine without a battery.

quoted from: https://mechanics.stackexchange.com/...car-is-running

 

well after letting it sit for a while it fired right up so I guess it just needed a pcm reset

 

Sometimes you get lucky.

 

Keep this in mind when using a battery quick disconnect switch used for preventing battery drain while jeep is in storage. Make sure the switch, the screw or what ever makes contact cannot work its way loose do to engine vibration. I removed my quick battery disconnect after realizing the damage that can occur if it came disconnected while running
For me, I unbolt the battery clamp for preventing battery drain while storing nowdays. No more janky disconnect switch.

 

When you unhooked the battery while it was running, it could have caused a hiccup in the electrical system. Sometimes, that can mess with the engine control module and other vital components. Try resetting the system by disconnecting the battery again for a few minutes and then reconnecting it.