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Old 12-18-2014, 03:22 AM   #1
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Volt meter bouncing / battery light lighting

My car has a new alternator and red top battery from AM. They were changed because there was an obvious issue as I could watch the meter go down, lights dim, accessories fail. After the change the meter would still drop some but not as bad. Now after the engine warms up (about 5 minutes) the meter starts bouncing around and the battery light will come on when the meter drops. Doesn't seem to be any correlation to rpms. Jumps around regardless if I'm steadily driving or at idle. Lights don't dim and no accessories seem affected by this. Just started two days ago.


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Old 12-18-2014, 06:35 AM   #2
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go to advance or autozone and get it tested. alternator could be a dud, or it could be the voltage reading or the gauge is just bad. the voltage regulator is built into the alternator, so replacing the alt changes that. was this a remanufactured alternator, or did you get it from AM also? If its a reman, take it back and ask for a swap. I swapped em out for people all the time at advance auto.
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Old 12-18-2014, 05:09 PM   #3
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Both the battery and alt came from AM. Worked fine for a couple of months. Then this started.


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Old 12-18-2014, 08:37 PM   #4
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It sounds like the voltage regulator, inside of the alternator, has malfunctioned. If you can, test the voltage at the battery with a multimeter. It should read 14-15 volts with the engine running, and the output should be reasonably steady. If the output is erratic, then it is your alternator. If it's steady, then you probably have a wiring problem somewhere.

Good luck Steve!
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Old 12-21-2014, 11:41 PM   #5
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Voltage at battery today was 10.49 and with the multimeter hooked up gradually and consistently reduced. Car is now dead.


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Old 01-03-2015, 11:25 PM   #6
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Replaced power wire with 4gauge power wire sold by AM. Made no change. Replaced alternator. Engine starts but voltmeter is sitting low. Turn headlights on and it drops. Open/close windows it drops. Press the brakes it drops. Returns and sits somewhat steady around the "r" of normal. Mods to car:

Changed by me:
Steeda under drive pulleys (alternator pulley still stock)
Accufab 75mm throttle body/plenum
Aftermarket CAI
MSD Coil Packs
Power stop rotors/pads set
Tokico struts/shocks
MM caster/camber plates
Side mirrors replaced with aftermarket mirrors with turn signals spliced into stick turn signal wiring (gay Japanese rule here so only temporary)
Additional aftermarket horns (spliced into stock horn system).

Changed by previous owner:
Steeda springs
Borla Catback

Don't have common stateside auto parts stores here. Went to a local Japanese garage. After explaining everything done they suggested the thermostat was stuck open and pulling on the alternator.

Any other suggestions?


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Old 01-03-2015, 11:50 PM   #7
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Lol at the thermostat.
You need to re-badge that car as a Subaru or something before you take it in for service.


Check the voltage at the battery and see if it reads at least 14 volts with all accessories OFF.
Then check it again with the lights or the heater fan, something to put a load on it.

Now it shows symptoms of a bad ground somewhere, so start at the battery and check every ground cable and wire that you can find.
Also, I'm not a big fan of splicing into the factory wire harness to install aftermarket goodies. Carefully check all of that wiring as well
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Old 01-03-2015, 11:58 PM   #8
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Maybe just bad connection at post.... Corrosion
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Old 01-05-2015, 01:54 PM   #9
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Try the main grounding strap. Goes from the driver's motor mount to the frame. Would not be the first one to be trashed. Also any other grounds like behind the headlights etc... Last one I did I just replaced the OEM crudded strap with an 8 gauge cable and then added one onto the other side as total overkill. Make damn sure to clean all your grounding contact points too, don't just replace wires/straps/cables.
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Old 01-06-2015, 03:49 AM   #10
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Love the "Subaru" comment. You have no idea how hard it is to get help with this car over here. As soon as you say Mustang they act like they were smacked in the face with kryptonite. Getting parts is expensive. Almost all the "free shipping" sites charge ridiculous rates to send stuff. Basically the discount we get from being members here only dents the shipping cost. Buying parts locally are obscenely expensive. I knew the thermostat was way off and that is why I mentioned it being suggested. Thank you for all the advice. I will try and get the car on a lift with time to check and clean grounds.


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Old 01-06-2015, 07:11 AM   #11
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Hmmm wow you can't do MilAir out there? I guess that is only going to be for deployed locations because I shipped a bunch of stuff out to myself in Qatar for pretty cheap via MilAir. Can't send a Rockauto order to someone in the states and then have them ship it the cheapest way possible out to you?
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Old 01-06-2015, 11:35 PM   #12
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I got an APO/FPO address but sites like AM charge for shipping. They also are unable to ship anything large. StangMods will ship for free but limited in parts and materials.


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