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Thread: Voltage fluctuation from 14.5V- 12v

  1. #1
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    Voltage fluctuation from 14.5V- 12v

    At a cold start voltage stays at 14.5volts . After it warms up it drops to 12v at idle and fluctuating to 14v. Lights dim down until I give it gas then voltage pops back up. While driving it drops to 12v when the trans shifts gears into a higher gear. Sitting at a red light it just fluctuates from 14.5v to 12v very quickly. With no loads on motor, no ac , no heat, wipers or radio. Itís getting pretty bad to where I canít drive it. I changed the alternator several times, belt, tensioner, positive and ground cables, alternator plug, battery several times, new pcm, IAC valve, tps. Still no change. And yes I only use FoMoCo parts

    any help would be appreciated

  2. #2
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    Quote Originally Posted by Broncokid97 View Post
    At a cold start voltage stays at 14.5volts . After it warms up it drops to 12v at idle and fluctuating to 14v. Lights dim down until I give it gas then voltage pops back up. While driving it drops to 12v when the trans shifts gears into a higher gear. Sitting at a red light it just fluctuates from 14.5v to 12v very quickly. With no loads on motor, no ac , no heat, wipers or radio. Itís getting pretty bad to where I canít drive it. I changed the alternator several times, belt, tensioner, positive and ground cables, alternator plug, battery several times, new pcm, IAC valve, tps. Still no change. And yes I only use FoMoCo parts

    any help would be appreciated
    Mine does something very similar to yours just not to a point where its undriveable. My volts usually are at like 12.7 - 13.5 never really 14 unless cold. My lights do dim down slightly as well. Same conditions. All I did was replace my alternator, the old one was failing and my positive lead because it was cording away, but I had the conditions regardless of my parts changed situation. Never really thought much about it but if your having that problem too, I'm open to hear everyone else too.

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  3. #3
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    Very frustrating, I can drive it but it has to be driven a certain way. If someone else drove it regularly like a normal person lol they’d kill the alternator within minutes. It’s my daily unfortunately until I find another truck.

  4. #4
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    Honestly, I had an issue with voltage drops just like this. Under load the car would be around 12.6-8v. I had two reman ford alternators that both did it. I swapped in a low(er) mile non-rebuilt original unit and that fixed my issue.

    Not saying that is your problem, but its how I solved mine.


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  5. #5
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    Quote Originally Posted by offroadkarter View Post
    Honestly, I had an issue with voltage drops just like this. Under load the car would be around 12.6-8v. I had two reman ford alternators that both did it. I swapped in a low(er) mile non-rebuilt original unit and that fixed my issue.

    Not saying that is your problem, but its how I solved mine.
    Right now, I went from the original ford unit to an all new acdelco unit. Sounds like this problem is a common thing, that makes 3 of us.

    Sent from my SM-G715U using Tapatalk

    2003 Black 300A #3545 DOB 8/14/2002

    BBK 65MM Twin Bore Throttle Body
    K&N Marauder Pipe/Cobra filter
    AM Underdrive Pulleys
    PS Z32 EVO Brakes
    Addco Rear sway bar
    ADTR Henious Arms
    TCE SS Brake Hoses
    Nitto 420s 255/55/18 Rears
    KYB`s all around
    SCT X4 Tune
    Superior Products Shift Kit & Tuff Plate
    For the Future 3.73's & Auburn HP LSD
    Restoration Project Car
    2004 Lincoln Aviator (so close to being complete)

  6. #6
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    Ford's reman units are junk. When my original went out years ago, I went through 5 reman units in 3 days. The dealership told me its pretty common for Ford reman units for our cars to fail. So I went with aftermarket and never looked back. I'm using DB Electrical, but there are others. Maybe you can find a local alternator shop that can rebuild your original with quality parts?
    2003 300A
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  7. #7
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    Quote Originally Posted by Peace2Peep View Post
    Ford's reman units are junk. When my original went out years ago, I went through 5 reman units in 3 days. The dealership told me its pretty common for Ford reman units for our cars to fail. So I went with aftermarket and never looked back. I'm using DB Electrical, but there are others. Maybe you can find a local alternator shop that can rebuild your original with quality parts?
    I just looked at DB, they got some nice upgraded units, same style too but much more amps up too 220! makes me wonder if these units are too little for the cars from factory only 130, which one did you get?

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    BBK 65MM Twin Bore Throttle Body
    K&N Marauder Pipe/Cobra filter
    AM Underdrive Pulleys
    PS Z32 EVO Brakes
    Addco Rear sway bar
    ADTR Henious Arms
    TCE SS Brake Hoses
    Nitto 420s 255/55/18 Rears
    KYB`s all around
    SCT X4 Tune
    Superior Products Shift Kit & Tuff Plate
    For the Future 3.73's & Auburn HP LSD
    Restoration Project Car
    2004 Lincoln Aviator (so close to being complete)

  8. #8
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    I got the 220 amp version because I am planning a bunch of stereo upgrades.
    2003 300A
    Widened Rear Wheels with 295 45 Nitto G2's
    Eaton Swapped by Curless
    Brake Upgrade
    Mo's Tune
    JLT Intake
    Addco Sway Bars
    Metco Control Arms
    4.10 Gears
    Aeroforce Gauges

  9. #9
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    I'm on my second DB Electrical alternator (Item #: AFD0147). First one had a loud whine. Second one is a little quieter. When idling warm, it's at about 13 volts, then when I give it a little throttle, it spikes to arounds 16 volts for a second then it settles to around 14 volts when driving. With lights and fan running, it has a hard time maintaining 12 volts at idle.

  10. #10
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    Quote Originally Posted by crouse View Post
    I'm on my second DB Electrical alternator (Item #: AFD0147). First one had a loud whine. Second one is a little quieter. When idling warm, it's at about 13 volts, then when I give it a little throttle, it spikes to arounds 16 volts for a second then it settles to around 14 volts when driving. With lights and fan running, it has a hard time maintaining 12 volts at idle.
    You know, I had the same issue with my alternator back in the day, discovered the tuning can fix the charging setup at idle. I am taking mine to Mo's in a month or so for that to be programmed. It's surprising what tuning does: even in these 2003-4 cars! I did have the tune set up in my fist Marauder so I know it to be true...
    2003 300A
    Widened Rear Wheels with 295 45 Nitto G2's
    Eaton Swapped by Curless
    Brake Upgrade
    Mo's Tune
    JLT Intake
    Addco Sway Bars
    Metco Control Arms
    4.10 Gears
    Aeroforce Gauges

  11. #11
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    Quote Originally Posted by Peace2Peep View Post
    You know, I had the same issue with my alternator back in the day, discovered the tuning can fix the charging setup at idle. I am taking mine to Mo's in a month or so for that to be programmed. It's surprising what tuning does: even in these 2003-4 cars! I did have the tune set up in my fist Marauder so I know it to be true...
    Considering the voltage regulator of the alternator is PCM controlled it makes alot of sense on going to a tuner to change such a thing like that. They basically talk to eachother, i know of some alternators that communicate on the CAN BUS, its insane how techy things are. Everything talks to eachother now days!

    Sent from my SM-G715U using Tapatalk

    2003 Black 300A #3545 DOB 8/14/2002

    BBK 65MM Twin Bore Throttle Body
    K&N Marauder Pipe/Cobra filter
    AM Underdrive Pulleys
    PS Z32 EVO Brakes
    Addco Rear sway bar
    ADTR Henious Arms
    TCE SS Brake Hoses
    Nitto 420s 255/55/18 Rears
    KYB`s all around
    SCT X4 Tune
    Superior Products Shift Kit & Tuff Plate
    For the Future 3.73's & Auburn HP LSD
    Restoration Project Car
    2004 Lincoln Aviator (so close to being complete)

  12. #12
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    Quote Originally Posted by crouse View Post
    I'm on my second DB Electrical alternator (Item #: AFD0147). First one had a loud whine. Second one is a little quieter. When idling warm, it's at about 13 volts, then when I give it a little throttle, it spikes to arounds 16 volts for a second then it settles to around 14 volts when driving. With lights and fan running, it has a hard time maintaining 12 volts at idle.
    I oddly enough had a whine when idling around 5-600 rpms, AC Delco unit, I bumped my idle speed up to 740 which is in the range service data suggests the idle speed of the merc should be at and it for the most part went away. Still hear is everynow and then, sounds like a vortech unit whining away sometimes I wish! I dont have any issues holding 12V with loads though.

    Sent from my SM-G715U using Tapatalk

    2003 Black 300A #3545 DOB 8/14/2002

    BBK 65MM Twin Bore Throttle Body
    K&N Marauder Pipe/Cobra filter
    AM Underdrive Pulleys
    PS Z32 EVO Brakes
    Addco Rear sway bar
    ADTR Henious Arms
    TCE SS Brake Hoses
    Nitto 420s 255/55/18 Rears
    KYB`s all around
    SCT X4 Tune
    Superior Products Shift Kit & Tuff Plate
    For the Future 3.73's & Auburn HP LSD
    Restoration Project Car
    2004 Lincoln Aviator (so close to being complete)

  13. #13
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    I had this exact issue for years, spent a few thousand dollars over the years changing batteries, alternators, high output alternators, etc.... Various different mechanics and shops all doing the same lazy thing "ohh change the alternator" or "must be your battery". Even ran down the never ending rabbit whole of "the engine has a bad ground". I did the big 3 upgrade (or whatever it's called) and still for years continued with the issue. Mine was so bad, there would be times I drove, go inside a store, come out, and the car wouldn't turn over or start. Soon as i jumped it, drove fine for weeks until it would randomly do it again. To the point I kept cables and a jump box, knowing if it won't start, jump it and it's fine.

    Long story short, I finally took to an Auto/Marine electrical only repair shop. They diagnosed originally with the lazy "its the alternator", paid a few hundred for the high amp upgraded alternator and crossed my fingers. I drove out of the parking lot and immediately the volts were jumping, up/down, spike/normal. Took it right back and showed the mechanics. They said let us keep your car and run some test.

    They called me later that day, said its one of two things... Either its your main computer and will cost about $1,000 or a single wire that runs from your alternator to your computer (I believe). It goes from the alternator directly into the cab. They swapped that wire and voila!!! They inspected it and determined the wire had a brake in the cover and was being shorted out on the firewall intermittently. The firewall wasn't properly protected with a grommet or other material and over time the wire chaffed and caused it to touch the metal.

    Explains why it was intermittent! And once they ran a new replacement wire I have never had the problem again! Kindly enough they didn't charge me for the hours they put in to find the real culprit since they misdiagnosed originally and stuck me with a several hundred dollar high output alternator I didn't necessarily need.

    If you think this maybe the issue and want any additional info or a photo, just let me know. If it's not your issue, hopefully this info may save someone else a huge headache someday.

  14. #14
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    If going the above route of replacing that one wire, you cannot just double up the wire, you must replace the one wire and set aside the old one... or else the new one will also ground out.

    you can also unpin each end and test with a new wire, if the issue is still there, at least you didn’t cut up the oem harness.

    ohming out the wire really doesn’t help, 17 wires could be broken out of 18 wires and you will still get a decent ohm reading and obviously continuity.

    ive bought used alternators and no issues
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