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When my 1999 Mustang 3.8 liter sits for more than two days, there's not enough power left in the 6 month old Interstate battery to turn the engine over.

All other electrics work, but I have to jump start the engine.

What's the most likely cuplrit that's shorting?
 

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When my 1999 Mustang 3.8 liter sits for more than two days, there's not enough power left in the 6 month old Interstate battery to turn the engine over.

All other electrics work, but I have to jump start the engine.

What's the most likely cuplrit that's shorting?
first and foremost, have your battery load-tested for voltage AND cca. CCA is more important than voltage when starting a car. it would also be a good idea to have your alternator tested. if the battery & alternator checks out ok, then what you need to do, is get yourself a DVOM and check your current draw.

the way to this, is as follows:

1. go over your car, make sure all doors are closed, the trunk is closed, all lights off, all cigarette lighter-powered accessories are unplugged, make sure everything is in the OFF position.
(if anything was found open or left on, that was likely your problem...)

2. disconnect the negative battery cable only.
3. set DVOM to DC milliAmps.
4. insert black dvom test lead into Common Ground port, and put the red dvom test lead into 200milliamp port.

5. put the black test lead on the negative battery post, and put the red test lead on the negative battery terminal. you should see no more than ~25 mA.
(my car, for example, draws 13-14mA).

6. if you see significantly more than 25mA, go into the car and start unplugging things, one by one. each time you unplug something, go back to your dvom, and see what your current draw is. if you unplug, for example, your radio, and then you check your current draw and it's within spec, you have a short in your radio. i would probably start with any aftermarket accessories. then start checking headlamps, taillamps, radio/cd player, the culprit can typically be pretty much anything in the car that runs on electricity; which is why excessive current draw is such a b**** to diag.
 

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btw, autozone/advance will load test your battery and alternator for free.
 

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im sure it could be a number of things but i also have a 99 that was doin that and it was one of the amps for the rear speakers. i had my dad take it to a ford dealer for me when i was in iraq and they just disconnect the rear amps but then i didnt have any bass so when i got home i took it to a shop that knows about all that stereo stuff and they told me that there is 3 amps in those cars, 2 in the back and 1 in the front behind the dash and the one in the front controlls the 2 in the back and the front one was the bad one cuz it wasnt turnin off the 2 in the back. o and they dont make replacements for the front ones anymore so $1500 for a whole new system if you want any bass
 

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im sure it could be a number of things but i also have a 99 that was doin that and it was one of the amps for the rear speakers. i had my dad take it to a ford dealer for me when i was in iraq and they just disconnect the rear amps but then i didnt have any bass so when i got home i took it to a shop that knows about all that stereo stuff and they told me that there is 3 amps in those cars, 2 in the back and 1 in the front behind the dash and the one in the front controlls the 2 in the back and the front one was the bad one cuz it wasnt turnin off the 2 in the back. o and they dont make replacements for the front ones anymore so $1500 for a whole new system if you want any bass
if he also has the Mach460 system, he can check current draw before and after unplugging the amplifiers.
 

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Also - be sure to clean your terminals and wire connectors! I had some rough starts on a brand new Bosch battery because my terminals were corroded from the old one... Just another helpful tip. :bigthumbsup
 

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a short to ground on the alternator regulator can make the battery die in a day.
a short to ground just about anywhere can drain the battery.
 
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