I left the keys in the ignition for several days, and drained the battery. Ooops. I recharged the battery from a wall charger, but now the alternator doesn't seem to be working. Why would this be?

Here's the full story (and clues):

After finding the battery dead, I put it on a wall charger for an hour and then restarted the car. It ran fine, but as I was putting away the charger, the engine started stuttering and then died. It wouldn't start again. Hmm, I tested the battery voltage. and it was only 8.4v.

So I put it back on charge for another hour, got it running, and this time watched the battery voltage across the terminals..... 11.30v and dropping with the engine running rough. I would have expected it to be high with the alternator pumping in juice.

I left it charging for for another 4 hours, got the battery voltage back up to 13.2v, and restarted again. Now with the engine running, I did the following tests

• Voltage across the terminals is 12.3v and dropping
• Voltage between +ve on alternator and ground, also ~12.3v
• Voltage between +ve on alternator and case of alternator, also ~12.3v
• Revving the engine seems to make no difference,

So it seems like the alternator is no longer charging, and if I go driving around town, it'll stall again at some point?

I find it hard to believe the alternator is dead. There were no problems before? Could I have blown a fuse or some other electrical item when trying to charge the battery?

Thoughts appreciated before I try a new alternator.

Thanks

James

 

Sounds more like your battery is shot. A few days of sitting dead will kill a lead-acid battery. Take it to Autozone and have them test both alternator and battery. Should take about 5 minutes and it's free.

 

SOLVED.

After much head scratching, it literally solved itself. I left the car running for another 5mins, and came back and found the voltage up at a happy 13.9v.

I believe the ECU might reset after a completely dead battery. In the past the engine will often run rough for the first five mins after a jump start, until it figures itself out. I'm speculating, but maybe the voltage regulator for the alternator doesn't start doing its thing until the ECU has recalibrated.

Anyhow, sorry for the false post. Thanks for your help.

 

Yep, you are correct. I should have remembered that wrinkle. It comes up often enough!

I still suggest getting your battery tested.

 

Glad it seems to have fixed itself. You are right about the ECU thing, but it probably would not be a bad idea to get the battery and alternator tested just to make sure, especially if they are older. Jeep's need to have a charging system in as perfect order as possible.

 

Im new to the cherokees but im having a similar problem I believe. Not sure sure what’s happening I believe it’s definitely something electrical. I was driving it for 2 hours today on a trip and then the speedometer and tachometer dropped and went back up a few times then 3 mins later it died. I jumped it and it ran fine back home luckily i was only 15 mins away.

 

Start your own thread.