I have a 98 cherokee 4.0 that seemed to have a bad alternator. Voltage was running a little low, but after driving for a few minutes it would drop to zero. It would do this every time I restart the truck: show low voltage, then drop to nothing. Battery tested fine, and alternator tested fine. Since the voltage regulator for the alt is in the ECM I was told I might need to replace that (holding off until I know for sure what it is)
I pulled the codes and got:
P0118 Engine coolant temperature sensor circuit high voltage
P1492 Ambient battery temp sensor

Would this explain the drop in voltage? After driving a few times it drained my battery consistent with a bad alternator. Any help is appreciated. Just want to know what it is before I start throwing money at it. Thanks!

 

P1492 is the clue. There is a battery temp sensor in the battery tray, if it's sending wrong information to the ECM, like the battery temp is high, the ECM will cut the charge rate way down to avoid boiling the battery. Could be a bad sensor, could be a faulty ECM.

 

I just went through a similar thing with my 2000. I did not have any codes but my voltage meter symptoms were the same. I replaced the alternator and all is fine now. I looked into the battery sensor thing too. I saw it when I removed my battery box. It unplugs from the harness at a connector right next to the battery on the fender. I think if you unplug it it will tell the ecm that the battery is cold and it will charge normally, 14+ v. You might want to try that first and see if that fixes the problem. Its easy to do.

 

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Good info, thanks.

 

Thanks guys! I'll try running it without the battery temp sensor and see if that helps, if not go through my wires and see if that works. Thanks for the info!