I'll start with some context: I replaced the stereo in my 01 XJ with an aftermarket unit. I crossed some incorrect stereo wires at first and blew a fuse located under the hood killing all my gauges. Fixed wires, replaced fuse, Jeep works fine for a couple of months and almost 1000 miles.

Two months later, I get in my Jeep after its been sitting for a week and plug the AUX stereo adapter into my phone for music. It sounds terrible, I can hear a buzz changing with RPMS and the battery Guage drops to zero. I use the car again, and with the just the radio on it seemed fine...no aux or phone, which caused the mayhem previous. So back to normal, then after driving for a bit I notice the headlamps and air blowers on make the battery Guage dip again. Not to zero, but not in a good range, under 10. I guess I'm not sure if the stereo job I did two months ago is causing problems as the AUX option kills the battery Guage, or if my alternator is going again. I last replaced it about 3 years ago with a reman unit.

Any thoughts or opinion? I think it's the alternator but I'd like a second opinion. Maybe a bad ground on the stereo? Seems it would of showed symptoms sooner though. Also battery should be good, it's a red top that's about 5 years old, tested good a month ago.

 

An alternator shouldn't be going out after three years unless you drive a lot of miles or spend a lot of time with it idling. I replaced the original alternator in my '00 last year, it had over 200,000 miles on it and was still fine. I replaced it because I already had it off for other work I was doing and the brushes were worn to about 1/3 of their original length....in other words, I didn't want to crawl back under there in a year or two to take it off again.

The buzzing is probably due to having power wires run too close to the AUX wire. My guess on your gauge dipping is that you've somehow created a loop or interference problem with the wiring to the gauge....assuming you're not seeing the headlights/dash lights dimming. I would verify voltage with a multimeter not at the battery, but at the connection to the PDC under the hood since that should be where the voltmeter is pulling the reading from.

All of that assumes you wired the stereo correctly. If you didn't, then all bets are off. I would pull it all back apart to double check against a service manual and the radio manual to see that everything is correct. If it's all correct, then your problem is unrelated to the radio. Did you use an adapter harness to connect the stereo or hack the factory wiring?

 

Bad diode probably, test and replace the alternator.