Battery loses power overnight

Tiny
MORNINGSTAR
  • MEMBER
  • 2000 GMC JIMMY
  • 4WD
  • AUTOMATIC
  • 165,000 MILES
2000 GMC Jimmy SLE battery loses power overnight. Current flows between battery and body when the ground cable's disconnected. We pulled every fuse one by one when the ground was disconnected and nothing interrupted the current. The alternator checks out fine on a load tester. Someone suggested the diodes in it are bad. There's a post-factory installed toggle switch in the inside fuse panel and we don't know what it's for (2nd-hand vehicle), but the current flows whether it's on or off. Alternator's charging fine while running.

I've been disconnecting the hot cable on the battery at night and re-attaching it in the mornings. Effective, and a nuisance.
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Sunday, April 3rd, 2011 AT 6:11 AM

8 Replies

Tiny
RASMATAZ
  • MEMBER
Check it again
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Sunday, April 3rd, 2011 AT 7:06 AM
Tiny
MORNINGSTAR
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Yup, that's what we did. Full charge flowing from the battery to the body all through the test, and we pulled every single fuse and nothing made the draw drop.
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Sunday, April 3rd, 2011 AT 7:30 AM
Tiny
MORNINGSTAR
  • MEMBER
Yup, that is exactly what we did, pulling one fuse at a time in a very deliberate pattern so we made sure we didn't miss any. The full charge was flowing throughout the test; nothing made it drop at all.
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Sunday, April 3rd, 2011 AT 7:32 AM
Tiny
RASMATAZ
  • MEMBER
Disconnect the alternator and recheck for draw-if it drops replace the alternator
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Sunday, April 3rd, 2011 AT 7:33 AM
Tiny
MORNINGSTAR
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Thanks.
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Sunday, April 3rd, 2011 AT 7:50 AM
Tiny
RIVERMIKERAT
  • MEMBER
If I read your question correctly, you're saying there is a current draw even with the battery negative disconnected. The only way this can happen is if there is a path to the chassis caused by corrosion. Most likely what is happening is that the current drain for such items as computer and radio memory coupled with a shorted battery cell is causing the problem. Have the battery properly charged and tested. I would guess it won't accept a full charge or pass a current draw test.
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Thursday, June 2nd, 2011 AT 4:15 AM
Tiny
MORNINGSTAR
  • MEMBER
Nope, because I have two batteries, and I'd trickle charge the flat one after I put the fresh one in, and they both did it. One was 15 months old when the problem started, and I bought the other cuz I thought it was the battery.

The problem turned out to be that the previous owner had attached a cooling fan in the back of the radio (who knows why -- I live north of the 60th parallel and it never gets too warm in there), and it was wired up to the battery. It was turning itself on and off at random, and that's what was the draw. We disconnected it six weeks ago, and it's been fine ever since.

Thanks for all the help, everyone. I learned a lot about what's under the hood of my first vehicle, this winter.
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Thursday, June 2nd, 2011 AT 4:46 AM
Tiny
RIVERMIKERAT
  • MEMBER
Wow. I never would have thought of that one, and one of my favorite things to is stereo installations. I've heard of fans on amps, but not on head units. Glad we were able to help at least a little.
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Friday, June 17th, 2011 AT 8:17 AM

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