Weirdest Alternator Problem Ever(or is it the alt?)

Hey guys, kinda new here so take it easy on me.

Here's the deal, my jeep's battery has been running at 10-11 volts lately(about 2 weeks now). It's enough to start the vehicle, but just barely. I had the battery and alternator checked out at several places and even checked them myself and everything read like it should. I had the battery fully charged and put back in the jeep. As soon as I started it, the gauge on the dash went from ~14V to the usual ~10-11V. Here's the WEIRD part.

Once when I cranked it, it didn't have enough power to start. I have an Emergency Jump Starter(I'll call it the EJS). One of those battery shaped things with the positive and negative cables that hook up to the battery. I hooked it up, cranked the engine and it started fine. At this time, the battery was running at ~14V. I walked over and unhooked it and checked the dash gauge again and sure enough, 10-11V. With the switch turned off on the EJS, I hooked the cables back up to the battery, checked the gauge, ~14V again.

With no power running out of the EJS, just the pos to pos and the neg to neg, the voltage read correctly.

I unhooked everything, shut the engine off and restarted it.....10-11V

With the engine running, I hooked up a voltmeter to the battery, pos to pos and neg to neg, set it down and went to check the gauge. 14V!!!!!!!!!

What the hell is this? It does not make any sense to me. The battery refuses to charge under normal conditions with an alternator and battery that are supposedly OK, but when a connection is made like the aformentioned, the voltage shows at the proper reading.

Has anyone ever heard of this before? What is causing this?

1988 Jeep Cherokee, 4x4, 100k miles, 4.0L I6

Red top battery?

Sorry, i should have thought to include that info. lol. No, its an AutoCraft Silver, 550 CCA

1. less than par battery.
2. dirty corroded battery terminals.

Id bet on 2, those are the symptoms

battery terminals are spotless, battery is only like a week old. and Auto Zone, Napa, and O'Reillys all put a load on it and it tested ok....i dunno wtf

what symptoms? the low voltage? I'm gonna slap another battery in there tonight and try it out. but i'm still curious as to why connecting something between the terminals with no power to the object is boosting the voltage?

A load test can't show all Battery problems.

Hey Langer1 I have the same problem with my 96. I changed the bat and the terminals and it still did the same thing after a day or so. But when I went to check things out again, I noticed that the cable going from the bat to the starter had corrosion under the sheathing of the cable. So I replaced the cable and no more problems.

clean where the cables lead too and all the other things hhoked up to them clean clean clean.

kodiax2000 said:

Hey Langer1 I have the same problem with my 96. I changed the bat and the terminals and it still did the same thing after a day or so. But when I went to check things out again, I noticed that the cable going from the bat to the starter had corrosion under the sheathing of the cable. So I replaced the cable and no more problems.

Click to expand...

Yep that will sure cause problems with any battery and can be hard to see some times.

I had trouble with corrosion in the wire coming from the alt to the battery (common problem on pre '91 XJs and MJs) I added a new wire direct from the alt main terminal to the battery + and a heavier ground wire from battery negative to the body. Whenever I turned on my lights or heater, the volt gauge would dip to 11 volts.

Also check the ground strap from the back of the engine block to the firewall. That's another common problem. If you want to replace the whole set of cables, 5-90 sells a high quality set for what I consider to be a good price. Check it out at http://www.geocities.com/JeepI6Power/electric.html

Disconnect the wire from the alternator to the power distribution block. Start engine. Should run and have 12+ volts. Based on my experiences, you need a new alternator. Hooking up the voltmeter helps absorb the AC pulses generated by the generator.

Another test, Start engine ( all cables in normal position ) and disconnect positive terminal at battery (loosened before you start the engine so that you can manually pull it off once the engine is running. Should run, but with low voltage displayed in your case and possibly run rough.