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Need help diagnosing dead battery

4K views 15 replies 6 participants last post by  JAPSTANG 
#1 ·
Hello... I have a 2001 Mustang GT convertible and my battery keeps dying on me. At first I thought the battery I bought a year and a half ago failed, so I took it back to Advance Auto under warranty and they swapped it out with a new one. Now two months later the new one is completely dead. Not sure what could be causing this. No lights are being left on... even turned the radio off... not sure what could be drawing all the power out of the battery. Any suggestions to help me troubleshoot this would be much appreciated... besides making sure things are not being left on, I have no idea where else to begin. Thank you.
 
#2 ·
Alternator - stop #1
Second potential problem - parasitic draw on the battery which could be anything from a slightly frayed wire touching the body to a loose connection causing the issue but I'd stake my money on the alternator. Pull it off and have it tell tested at the auto parts store. If it's bad, my personal advice would be buy the Crown Vic alternator for the same year as your stang and use it. Make sure it's the police model one so it'll crank higher amps to charge your system better.

Is the battery light on when you drive it? Could be the convertible top or a switch which appears off but still has a hot connection. For that matter, any switch in the car could cause that

Electrical problems are a bloody nightmare...id start with the Alternator though, that's easy to unbolt and have tested
Nate




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#3 ·
It would also help if you posted the voltage at the battery when the motor is running.

Is the top of the battery clean? If covered with oily "acid migration", this could be a source of the battery discharge. The usual cause of "acid migration" is over charging (bad alternator).

How about the battery terminals and battery cables? Are they clean and tight? Are the grounds around the radiator core support clean and tight?

Also have the battery AND alternator tested. Have the alternator tested for correct output and excessive AC ripple (bad diode). No excuse to skip this step as almost all auto parts stores will test for free. OBTW, no need to remove the alternator as it can be tested on the car.

When you are certain that the full charging system is in "tip top" shape then start trouble shooting. Locating a parasitic draw can be a real PIA. Be prepared for a ton of test with trial and error. Hint. Suggest staring with the sound system.
 
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#5 ·
I've had the battery and alternator tested 3 times by 3 different places and all checked good... but the battery still kept dying and they gave me a brand new one that I installed not even 2 months ago and it too died. Twice already. When I put the new battery in, I put in new positive and negative battery cables and connections, so I know those are good as well. When it first went dead, I jumped it and drove it around and had a good charge on it but then the next day it was dead. So I jumped it again - let it run for a good 20 minutes or so to let the alternator charge up the system... and then when I cut it off... the first thing I did was pull the radio head unit out... it's an aftermarket radio but I'm seeing if it's the parasitic source in case it's wired in wrong or something. The next day the car fired right up had no issues. So now I'm letting it sit a few more days to see if it still goes dead or not. If not, then I'll know it's the radio. (CROSSING FINGERS) LoL.
 
#6 ·
Shouldn't need to remove the entire head unit. Pulling the appropriate fuse should be sufficient.

Sounds like you are on track to narrowing down the problem, but I find it curious that the local parts store has replaced the battery three times. In my experience with a couple different places, they run tests to verify a defective (won't charge or hold a charge) battery before offering a warranty replacement. Having for instance the radio or other circuit draw and cause voltage discharge generally only causes the battery to be dead, not defective.

Assuming you get it narrowed down and otherwise confirmed to be the radio circuit, it is most likely a wiring fault and not an inherint fault of the radio itself. If car audio is not your thing, take it somewhere and have them doublecheck and correct as necessary any wiring faults.
 
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#8 ·
They've only replaced the battery once... it died 3 times before it was replaced. The first two times I had them test the battery and the alternator (while still in the car) and both times they both passed. The first time was Advance Auto (where I bought it from maybe a year ago) who tested it... the second time was Auto Zone - just because I wasn't trusting Advance Auto's results at the time as it was only a week apart. The 3rd time (another week later) it died so I pulled the battery out - took it back to Advance - it was so dead they couldn't get a reading and gave me the new one to replace it with. This was in early July that they replaced it. The new one has now died "3 times" as of today and that's with the radio head unit pulled... so I'm still scratching my head. LoL.

So I did jump it again today and took some voltage readings. With the car running, I'm getting 13.48 volts at the battery. With the car off I'm getting 12.30 volts.
 
#7 ·
take your 12v test light and put it inline between ground cable and battery.....if with key off it lights there is current draw. pull fuses 1 at a time to isolate where the draw is.

DO NOT try to start it like this but you can do this to isolate the problem
 
#12 ·
I think we found the problem. Had a friend come over and help diagnose using the method of connecting a multimeter between the negative terminal and negative battery cable, reading the amps, we found that when we pulled the radio fuse, it had a significant drop in amps on the multi-meter. However the radio itself was still removed from the car... so we were wondering how it was still pulling amps with it out of the car... then we thought maybe the radio-amplifiers mounted behind the convertible top were drawing power as they were still hooked up, so we put the radio fuse back in and proceeded to unplug the radio-amplifiers one at a time... and played around with them (plugging back in / unplugging) in a variety of combinations that we found the passenger side amp is the one drawing juice.

We are assuming the radio-amplifiers shouldn't be drawing any current at all when the car is off. The driver side does not but the passenger side does... so we are also assuming the passenger side radio amp is bad. Do these go bad and would cause this? Maybe shorted out inside or something?

So we also plugged back in the radio and tried the system out without the amps plugged in - it had no effect on the current draw. Then we plugged in the driver side amp while the radio was on, and you could tell the extreme difference in the sound of the music playing. We then unplugged the driver side and plugged in the passenger side... it didn't do anything... the music sounded the same as if both amps were unplugged, again, leaving us t believe the passenger side amp is jacked up.

Any thoughts or experiences with the radio-amps?

Additional info: With the passenger radio-amp plugged in, the amperage measurement we were reading on the multimeter was .340. With it unplugged we were reading .120. We also pulled all the other fuses as well just to make sure nothing else was drawing current and found no other fuse caused a drop in amperage... with exception that he GEM fuse did drop it some but from reading other forums - the current it was drawing was normal and we'd have to wait for it to fall asleep for it to cut current completely off. (About 45 minutes)
 
#13 ·
It's best to mention the fuses by ID. Then everybody will know WHICH fuse is being discussed.

1999-2004 fuse panel schedule:
http://www.allfordmustangs.com/foru...r-swap-wont-fire-please-help.html#post2669271

Do I understand correctly that this car does not have a factory head unit. Are you aware there's a signal that the head unit sends to the amps that tells the amps to turn on? So if the head unit isn't interfaced correctly this could be the source of your problem.

How long as this head unit been installed and how long has this problem been going on?

Yes electronics can and do go bad.

How long can the car set before the battery dies? Hours, days, weeks?

Does the battery still die with the amps disconnected?
 
#14 ·
Your answers in short: Fuse F2.27 (RADIO)... yes it does "not" have a factory head unit... don't know how long it's been in it... was in it when I bought the car 3 years ago... I'm aware the amps get a signal but from what I understand "shouldn't" get anything when the car is "off". Problem has been going on the last few months - and goes dead after a day or so as described in the original posts... Does it die with the radio-amps disconnected yet? Not sure yet as we just assumed it was them yesterday after our tests and currently letting the car sit a couple days with the passenger side amp disconnected to see if it is indeed the source of the issue. Will indeed give a shout out if it works or not.
 
#16 ·
With the passenger side radio amp unplugged, the battery has yet gone dead and has stayed at just above 12 volts. 12.03 volts to be exact at last reading. This is over a 5 day period. So now I'm pretty sure the amp was the parasite that was killing the battery. So looks like I will be replacing that one amp. Maybe it got wet at one time and caused it to short out or something? The top did have a slow leak at the back window before we put a new top on it earlier this year.
 
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