Weird electrical problem, kills idle

My 2001 Jeep Cherokee 4x4 is having some weird electrical issues. This started two days ago, out of the blue. It will start just fine and idles like normal but as soon as there is any load on the electrical system (headlights, radio, brake lights, window down/up button) the idle gets real low and then dies. Also the interior lighting is very dim. Got the battery checked, its fine. I assume the alternator is good because i read 13.9v across the bat when running and only 11.8 across the bat when off. I checked the power distribution center and the interior fuse panel, no blown fuses. Not sure what to do next.

Battery voltage of 11.8 volts is not normal....
The at rest voltage should be 12.3-13.0.
How much time does the battery take to get from the 13.9v charging to 11.8 at rest?
You either have a constant draw on the system such as something left on.... or you have a ground in a circuit which hasn't blown a fuse.
If the 11.8 volts is almost immediate; then your battery is on its way out....

Ok, those voltage readings were what I remembered from a long day of trial and error. took it again this morning, reading 12.8. Still 13.9 while running, it gets there just as soon as I can start the car and get to the meter in sight.

Bigger new development, I was testing the voltages across the dome light the first reading was 8.7 then without doing anything differently it jumped back to 12.8, and everything works fine. Starts and idles with accessories running. Its good news but none that I'm happy about because it means it can just happen at any time and leave me stranded.

idunnogui said:

Ok, those voltage readings were what I remembered from a long day of trial and error. took it again this morning, reading 12.8. Still 13.9 while running, it gets there just as soon as I can start the car and get to the meter in sight.

Bigger new development, I was testing the voltages across the dome light the first reading was 8.7 then without doing anything differently it jumped back to 12.8, and everything works fine. Starts and idles with accessories running. Its good news but none that I'm happy about because it means it can just happen at any time and leave me stranded.

Click to expand...

That is good news...
Even though the voltages are good right now... I would check around for loose or dirty grounds.... Under the dash on either side near kick panels, engine compartment firewall are to name just a few.
You are correct; if it happened once it can happen again....

well it didn't take long, problem is back. process of cleaning the grounds, none really dirty yet. I don't understand why I only have 8.5ish volts available on all the interior accessories.

A bad ground will cause this problem, but when you say ALL interior accessories; which ones have you tested?
The door locks have a problem with the length of wiring that goes thru the vehicle before they get to the door locks or window regulators.
You should check the relays that supply the window regulators to verify what their voltage is....; this way you know in which direction your troubleshooting needs to go....

I have tested the dome light, power to the driver side window, radio power and auxillay power port. learned a little bit. with the ignition on but not running the voltage at the window and dome is 11.5, battery reading is 12.5 at that time. when running those voltages drop to 5-8. the aux port and radio always have 12.5. Also just to rule it out I got the alternator tested, it was fine, like i figured.

My suggestion would be to start cleaning grounds, battery cables and relay terminals; then check the voltages before and after to see when you make progress. There is a large voltage drop somewhere due to a dirty ground or terminal connections.