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Car guy question

Old 09-20-2010, 05:46 PM
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Default Car guy question

I've got a 91 Mustang LX 5.0 convertible that is draining the battery while parked. I replaced the battery thinking that I just fried the last one. I removed the lightbulbs from the trunk, under the hood, and glove box - Interior lights are not on. it drained a brand new battery in about 36 hours maybe sooner - I didn't get around to checking it until then.

The car does not have power seats and is totally stock except a code alarm that appears to be totally disconnected from any power source and the cigarette lighter is out of the socket so its not draining from there either.

I am at a loss of what to check next, any ideas??;?
Old 09-20-2010, 05:49 PM
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have the ingition switch in the column ( not the tumble the key goes in. There is a slide switch in the column) checked. had one melt on me back in the day and drain my batt. Lucky for mre it didn't catch fire.
Old 09-20-2010, 05:57 PM
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Make sure the trunk light doesn't stay on....

Or any other light in the car when parked...
Old 09-20-2010, 06:16 PM
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I'd check the main ground
Old 09-20-2010, 07:01 PM
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Is this your daily driver? If so, it may not be charging the battery well. I've had two other cars with those symptoms and they were okay while driving but wouldn't start in the morning. I went through the typical things of checking the battery, starter, alternator, etc. In both cases, the main battery cable were the culprits.

Oh, almost forgot about an SUV where the previous owner removed something and left a hot 12v unswitched power wire dangling in the engine bay!

Good luck.
Old 09-20-2010, 08:03 PM
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If you disconnect the neg battery terminal and connect a test light in between the -battery post and the cable end it will show you how much current is flowing (in brightness), if it is bright start pulling the fuses one by one until it dulls, Your culprit is on that circuit.

To check the battery cables, set you multi meter to ohms and measure from the - battery terminal to the engine block and to ground locations on the body. You shouldn't have much over 1 ohm. Make sure your meter leads have clean connections.

Measure the battery voltage w/ engine off nothing on, should be over 12V then start the car should jump up to 13.5V ish if the alternator is working.


Good luck
Old 09-20-2010, 08:06 PM
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For it to draw down that quickly it probably is going to be a bad alternator.You can trace out a short by unhooking the ground cable from the battery, leaving the positive hooked up.Use a test light or volt meter and connect it to the negative battery post and the cable end.If it has a draw, the light will be fairly bright.You can pull each fuse,1 at a time, and see if the light dims.If none dim then you have to start pulling relays or unhooking different conponents.(starter solenoid,alternator,relays,etc.)But I have seen a lot of alternators charge perfectly but be shorted internaly and kill the battery.Hope this helps.
Old 09-20-2010, 08:07 PM
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OOPS,250 types faster than I do.Great advice.
Old 09-20-2010, 08:08 PM
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first check if your alternator feels warm when everything else under hood is cool. if thats ok disconnect the pos. battery cable and put a 12 test light in line with cable and pos terminal, I know the experts are going to say to use a volt meter, but this works just as well and not everyone has a meter. now with the ignition off, and everthing turned off how bright is the light glowing? remember to have the door closed so interior lights stay off. if the light has more than a slight glow start taking out fuses to isolate the draw. a slight draw is normal, you didnt say if you have an amplifier?,, seen them stay on.
i see i dont type fast enough either, all good advice given.
Old 09-20-2010, 08:28 PM
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Originally Posted by 250 sportsman View Post
To check the battery cables, set you multi meter to ohms and measure from the - battery terminal to the engine block and to ground locations on the body. You shouldn't have much over 1 ohm. Make sure your meter leads have clean connections.
Most battery cables are stranded so it will read close to one ohm even if most of the strands are broken or open. The battery cable needs to have the proper capacity to carry the amperage from the battery to the starter via the solenoid Usually if the cables are defective and barely carrying any continuity, they will feel hot to the touch while the engine is running due to causing too much resistance.

Electrical problems are the worst, IMO.
Old 09-20-2010, 08:37 PM
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from my experience the cables with green buggies and black strands have more then an ohm.

Maybe a better test is to measure the voltage from the + terminal to the engine block vs the battery voltage?

AND I'm a slow typer just got a major head start on you
Old 09-20-2010, 09:18 PM
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Do you maybe have pulleys on that motor?
Old 09-22-2010, 03:56 PM
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I had a similiar problem with a Grand Prix, ended up the switch that was supposed to turn off the trunk light was broken and the light would stay on when the trunk was shut. The only way I figured out the problem was when I parked the car outside in the dark I could see a little light coming out around the trunk lid. The car was a 1982 and they were not well known for quality back then. The gap around the trunk lid was so bad I could actually see the light in the gap. Actually it was a good thing because I would have NEVER figured out that was the problem. How would you know if your trunk light went out when you closed the trunk?

Russ
Old 09-22-2010, 04:05 PM
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x2 Alternator - battery voltage should increase when running - appprox. 1-2 volts.

Many cars now don't have a voltmeter in the dash - even the guage package on my Chevy SS has a damn vacuum/pressure guage (for the supercharger) instead of a voltmeter - what were they thinking....... There isnt even a good palce for an extra guage in the dash so mine is hooked into the cigarette lighter plug....
Old 09-22-2010, 04:22 PM
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Had a chevett once that was draining the battery, turned out to be a short in the dash circut board, took the dealer 4 days to track it down ( it was under warranty and every mechanic in the shop worked on it when they had the spare time ).
Old 09-22-2010, 04:50 PM
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Just went thru this with my Mother's Ford. Turned out to be the radio. ripped it out and put an after-market unit in it and no more problem.
Old 10-21-2010, 10:29 AM
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It was the alternator - Thanks for your help

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