T-Roy-TX

Just purchased a battery for it thinking old one (10 yr old red top Optima was out). The gauge light started coming on a few days after I changed the radio I bought it with (clarion) to a cheapy pioneer I had on another car (has remote & reads mp3 cds). The optima was reading 11.75V with or w/o the jeep on. This led me to believe it was time to go. Battery gauge goes down to 9 with jeep running like a champ. No light dim or other symptoms other than check gauges light.

I drove it this morning with the new battery (interstate) running and started at close to 14 then worked it's self down close to 12V. It never goes over 14, ever! I've had her for 1.5 yrs. I got to work (25 miles later) and the volts dropped on the gauge meter and check gauges light came on. I left it on, had lights on only, and checked volts, which read 12.08 volts. I've taken the alternator off and comes out fine. I've been reading about the PCM, but I'd like some advice before I start dumping more $$$ in it. Because it's draining a perfectly new battery it would lead me to the alternator, but it checks out and doesn't dim lights either. Please help! 2000 Classic 4X4 4.0L automatic.

freegdr

Alt comes out fine ?

tjwalker

I will tell you this. I can't tell you how many alternators I've seen that have tested as "good" at a parts store, but are failing.

There are many opportunities for "escapes" on the parts store alternator testers. An escape is exactly what it sounds like. A part that is failing, that tests as good.

My bet is that your alternator is starting to fail. I'd replace it.

steelybill

I just replaced the alternator in my wife's Buick.
It tested fine at the local auto parts place, but would work for only a few minutes, and the voltage would drop off. I would have replaced the regulator in it, but needed to get the car fixed right away, so, with a lifetime warranty I swapped it out...
So my money is on your old alternator being the culprit

sycoglitch

What other maintenance have you done. How's the copper look. The ground etc. do the inexpensive test first and see your outcome that way you know or sure of its wiring or the alt. plus you said you swapped the radio out. Did you wire it yourself. You check the wiring ad the work you did. And honestly, my battery guage only reads 14v during the cold weather. Normally its between 13 and 14. Upgrading the wires to a later size is suppose to help with that but I've been lazy. Would also help with voltage drop of running several large usage items. But I digress now so go noggin poke you jeep some and report back

T-Roy-TX

Thanks for all the replies guys! I'm leaning heavy toward the alternator even when it's tested ok. Tonight after work all looked good and then after 5 miles or so it hit the floor and the light came on again. I stopped at the mall and ran some errands, 30 minutes later left and it was good for a minute and then down again. I drove her home about 20 miles and checked her when I got there. The voltmeter at the battery was 11.88 running. I believe the previous owner had a sound system and might have fried the old alternator. When I installed the radio the right side speaker died. When I pulled it to check it I saw some dynamat (sound proofing) type material. Any suggestions on where to buy? I have Autozone, Orileys, Advanced, and NAPA locally. This is my daily driver, so I need to get it done soon. I'm leaning toward Autozone because I have their rewards card and a $20 credit. I'm planning on a JL 10inch sub myself, so it might be a good idea anyway. Thanks again!

T-Roy-TX

The other wiring looks good. I did replace the fender ground last night, thinking that might be it. It was loose and had broken off the eyelet. All other wiring looks good. It did this a couple of times about a year ago. I cleaned the battery and tightened the fender ground and it went away. I did do the wiring myself. I bought a scosche harness and just crimped it to the replacement ratio harness. All matched. I did have two loose speaker wires when I installed. One was the right front the other a rear. The right front was the only one that died. Everything powers up fine except for the speaker that quit. Everything looks solid. That is why I was asking about the PCM and maybe some fuses. I just don't think that would cause it. It's either a loose wire or dying alternator. I'll double check again. Thanks for the advice.

aa7kn

I had the same problem on my 91. It was a bad wire in the harness that connecto to the PCM. The PCM does the regulating and every now and the my battery voltage would drop, and would read 11.8 volts at the battery with the motor running. I could wiggle the wiring harness at the PCM and the voltage would jump up to 12.8 to 13.0 volts and my volt meter in the dash would be almost to 14. I opened the harness and found a cracked frayed wire. I repaired it and it has been good ever since. Hope this gives you a idea.

T-Roy-TX

Excellent, I'll look at that as well before I dish out $140 for a new alternator. Any ideas or suggestions when going through the PCM? Thanks.

T-Roy-TX

New alternator and she is purring like a kitten again! Thanks for all the help!