Hello all,

Jeep XJ 98 sport. Stock. 308k. I am going to have battery (fairly new), and alternator tested today but I would like your suggestions on what to look for after that if they test good.
Car starts well in the morning, battery gauge is about 14, but dims down to the left once I put the AC on (not necessarily just the AC but the output control **** - center **** on a 98 sport). Goes immediately back to 14 when I turn the AC off. Actually all the lights (including headlights) dim. A screatching sound starts (I believe it to be the belt tensionner pulley- listened with a stethoscope).
I will check the belt tension again but I replaced it last weekend and I believe it is good.
I know it is not much info to go by but would this type of behavior more alternator or compressor related?
Also, does autozone give you readings when they test the alternator or just a good/bad (not sure if you need readings for more info here).
I'll update this afternoon on the results of the battery and alternator testing.
Thank you.
PS: my "mechanical" skills are ok, however my "electrical" skills are close to zero - I can use a multimeter if you tell me exactly where to put the red lead and where to put the black lead... I know...sad.

 

You are on a good start for trouble shooting. The other thing you need to do is to clean all the grounding points. Make sure that you make the all nice and shiny. Also check the battery cable and terminals.

 

check your fuse box, you want to make sure that the wires are not getting hot. the lights dimming shows that there is a huge draw on your system. If its not the altinator then it is the voltage regulator. take your meter and check the batt when running and have someone turn on the AC see how much it drops. should go far.
Torrez was right, check your ground leads, and make sure that the batt cable is not cracking, if it is you will get voltage leak and that will do the same thing.

 

I replaced the belt tensioner pulley. Same issue with lights dimming and loud noise still coming apparently from the pulley. Tightened the belt and now all is well. Can't believe how tight the belt has to be. I guess I should have started with this; I really thought it was tight enough. Anyway, problem solved as far as I can tell.

 

Good job. Yes, the 4.0 does like the belt to be tight.