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Old 02-10-2013, 04:58 PM   #1
ScubaruImpreza05
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Question Alternator or battery?

So randomly my car is dead in the driveway. Cranked fine yesterday morning when I moved it to shovel snow. Today completely dead. Haven't driven it since i moved it yesterday morning.

I have a Digital Multimeter and just checked the readings:

On a currently charging battery, AT IDLE:

At battery Terminals: 13.3V
At Alternator (ground on battery terminal): 13.55V


Now reading it may be the alternator is charging on the low side? I'll check the battery with the car off in a few minutes. Battery is somewhere between 4-5 years old (DieHard Platinum series battery).

Are those voltages indicative of bad alternator?
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Old 02-10-2013, 05:09 PM   #2
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After about 15 minutes charging, battery is at 12.1V and slowly discharging (Was at 12.11V and in 45 seconds went down to 12.07V)
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Old 02-10-2013, 06:38 PM   #3
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Pulled a fuse on my Audio System and the battery stopped discharging, I probably have a small short somewhere...yay
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Old 02-10-2013, 10:34 PM   #4
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Quote:
Originally Posted by ScubaruImpreza05 View Post
Pulled a fuse on my Audio System and the battery stopped discharging, I probably have a small short somewhere...yay

or a hack wiring job.....or yeah....
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Old 02-10-2013, 10:52 PM   #5
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The trouble isn't due to a short, it is from a small load on the circuit, possibly something like a power amp that isn't shutting off.
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Old 02-11-2013, 09:11 AM   #6
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Quote:
Originally Posted by Uncle Scotty View Post
or a hack wiring job.....or yeah....
Well the speakers and amps have been in the car working trouble free for roughly 5 years so I'm leaning towards a ground or as noted below, a piece of equipment going bad.

Quote:
Originally Posted by Cougar4 View Post
The trouble isn't due to a short, it is from a small load on the circuit, possibly something like a power amp that isn't shutting off.
Is there a way to test an amp to see if it has gone bad? I know you can use a DMM to test coils on speakers, can something similar work for an amp? Course, with the capacitors in it not sure if I want to be probing the amp too much. Can I test from the speaker wire outputs?

I guess I could also do process of elimination. It can't be the speakers themselves or wiring after the amp because the draw is happening with the car off. However, if an amp is staying in the 'stuck on' position, it could be sending power to the speakers and the speakers could have a bad ground somewhere. I guess Disconnect one amp, check for draw, then disconnect the other and check for draw. If the draw is still happening and both amps are off, could it be the main power line going to the trunk? That makes sense right?
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Old 02-11-2013, 10:27 AM   #7
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can you not just take it to auto zone or whatever is local and get the battery/ alternator tested? From what i know if the car is off the battery needs to read 12.5v (if not battery) and when vehicle is running i would say 13.5 and up.(If not alternator) But another thing could be the belt tension. not enough then the alternator isnt spinning enough..

As far as testing the amp srry i cant chime in on that.
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Old 02-11-2013, 11:06 AM   #8
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The best way to find a current drain problem is to set up your DMM meter to measure current and place the meter in series with the negative battery lead. You can then monitor the current drain on the battery while everything in the car is off and things have gone into the sleep mode. Normal current draw should be around 20 milliamps but if there is more 60 milliamps of draw I would start looking for a problem. It appears you have already pinned down the fuse circuit that has the problem on it so if you pull that fuse out then the current draw should go to the normal level. Once you verify the correct circuit the problem is on with the fuse then you can put the fuse back in and then start disconnecting things tied to that circuit and when the source of the trouble is disconnected the meter current reading will drop again to a normal level.
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Old 02-11-2013, 01:00 PM   #9
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You have a bad battery. The alternator is working but even with a small load the battery should not discharge that rapidly.
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Old 02-11-2013, 09:58 PM   #10
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Quote:
Originally Posted by LIChuck View Post
You have a bad battery. The alternator is working but even with a small load the battery should not discharge that rapidly.
.....unless there is something that is drawing power when the ign is OFF...which it seems there IS, in this case
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Old 02-11-2013, 10:08 PM   #11
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the draw from the stereo could have always been there, but now the battery is older and is less tolerant of the issue, meaning now you have symptoms of the a problem that could have been there all along.
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Old 02-11-2013, 10:29 PM   #12
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Quote:
Originally Posted by aerosaaber View Post
the draw from the stereo could have always been there, but now the battery is older and is less tolerant of the issue, meaning now you have symptoms of the a problem that could have been there all along.

THAT is another possibility....
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Old 02-23-2013, 01:02 AM   #13
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Get your battery fully charged with a battery charger, once fully charged wait a half hour for voltage to settle and hook a dmm on battery. Take periodic readings checking to see if battery is self discharging. Battery is old and should be replaced anyway. I would replace then do a parasitic draw test with stereo connected. See if the draw is within spec if not find the draw in the system.

Last edited by dollar5th; 02-23-2013 at 01:09 AM.
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