View Full Version : Alternator dummie light is on.

February 26th, 2012, 20:49
Ok So I have searched and I found nothing that helps me.

I have a 1998 XJ I6 AW4 231

Friday as I was leaving work my Dummie light came on(the one with the picture of the battery). So I figured on of my battery cables came loose. I pulled over and looked. All the cables were connected. Drove the rest of the way home. Later that evening I drove to autozone to have the battery tested, and the guy said my battery was good so I drove home. The next morning I drove to work, and when I went to get in the jeep to go to lunch it wouldn't start. It sounded like the battery was completely dead, when I turned the key everything went dead. After I got off work I went out to my jeep and went to go see if it would start, and it did. Meanwhile the dummie light is still on from Friday, I start my drive home, get 2/3's the way home and the jeep dies, and wont start at all. I got it towed home. Took the alternator off, and took it down to auto zone to have it tested. The guy said it was good too. Seeing as how my battery cables were really corroded and one of my grounds was missing, I went out this morning and replaced all of the battery cables. Started the jeep to see if the light would come back on and almost instantly it did! Not really trusting the guy at autozone I decided to swap the battery out of my dads explorer with mine. And took it for a test drive and again the light came back on. Next thing I figured I would buy a new alternator and try that(since I don't really trust the guy at autozone). but before I go and spend anymore money on an alternator I wanted to see what you guys thought might be causing this.

Thanks in advance for any and all help.

February 26th, 2012, 21:02
Do you have a multimeter? You can test if your Jeep is charging properly. With it off your battery should read a little about 12V and running it should read right above 14V.

February 26th, 2012, 21:08
Those are the readings I got when I tested it.

February 27th, 2012, 09:22
Alternators can test good at the parts store and still not function when installed. If you really want to, you can partially disassemble the alternator and inspect the brushes. You will probably find they are worn and nearly at the end of their service life.

O-Gauge Steamer
February 28th, 2012, 07:47
^^What happens is that as the brushes ner the end of thier service life, they loose the tension that is applied to them by a spring above. This causes the brush to lift off of the slip ring which, in turn, causes the alternator output to drop to zero thus triggering the Charge Failure Indicator. The thing is, as this is happening, the brushes will occasionally make contact and you could read a correct voltage at that point in time.

Check it again but this time load up the alternator. Put the headlights up on high, run the AC with the fan on high and turn on the radio. Leave the Driver's door open to put on the interior lights.

Bet the voltage is not the 14.2 ideal... If the tension is marginal, the brush will fail to conduct the current and the voltage should drop right along with it.

February 28th, 2012, 13:12
I went ahead and replaced the alternator today and now the light is staying off. Thanks for all the advice guys.