Voltage fluctuations

Discussion in '2005+ Mustang GT 4.6L Tech' started by DiMora, Aug 27, 2015.

  1. DiMora

    DiMora More Is Better

    938
    26
    I have the following symptoms:

    Idle voltage 11.5-12.5 fluctuating.

    Running voltage @2500 RPM and up: 12.5-13.5 fluctuating.

    I am assuming my alternator is going out. Fortunately, I have two spares...so I'll probably just swap it out, but is there anything else to check first? Any voltage regulators external to the alternator, or anything in the under-hood fuse box or SJB to check? Any fuses that blow which can cause those symptoms?

    I am not throwing any codes.
     
  2. dysan

    dysan Dis-Member

    3,902
    9
    How high are you spinning your engine? I ended up having a custom oversized pulley made by innovators west to slow my alternator down because I was over-spinning mine and killing them. If you have been over-spinning it I don't think a new regulator will fix it since it's the internal wiring that starts losing connection causing the lower voltage.

    I need to have a new pulley made for mine at some point since it was made with the offset for the reverse mounted setup with the E-force blower and I have since put my alternator where the a/c compressor used to be.

    As for the voltage regulator I believe you can replace it and it's on the back of the alternator. http://www.americanmuscle.com/paperformance-6gw-repair-kit.html
     
  3. 702GT

    702GT S197 Fanatic

    2,060
    50
    Yup, dead alternator. Our alternators are internally regulated. Just best to replace.

    You can isolate the battery from the alternator just to be sure the battery isn't dragging the alternator, but honestly if you had a dead battery, when you go to hit the starter it is going to drop that battery like a rock. Starter wouldn't even try. Just ditched a battery in the g/f's mustang. Her battery would sit at 12.6 and as soon as you hit the starter it was maybe 5v, wouldn't even click.

    If you can start your car, do so. Once the car is running, remove the positive lead from the battery, hit the cable with a multi-meter and check voltage. If it's still shit, you have your absolute confirmation. Just the fact that you can start the car confirms the battery is not dead.

    No fuses are going to cause low voltage, except maybe a direct ground, and that will send smoke signals lol.
     
  4. 01yellerCobra

    01yellerCobra forum member

    2,122
    122
    Removing the battery leads while the engine is running is no longer recommended. It's possible to fry the ECU that way. Just FYI.
     
  5. 702GT

    702GT S197 Fanatic

    2,060
    50
    From arc out? Never considered it. The surge happens at the point of arc, but if it's a potential possibility, then you're right.

    You could take the alternator to an auto parts store and have them test it, but TBH I have had them test some of mine before and they say they pass, but they are still jacked up. I don't really trust their testers. I'm the type that would just rather replace it than wonder if it's working or not.
     
  6. Saleen4971

    Saleen4971 forum member

    202
    0
    pulling a battery terminal with the engine runnign is bad news on a newer vehicle.

    what should these things put out anyway? mine is seeing 13.5v, where as every other vehicle i ahve owned was above 14.....
     
  7. 702GT

    702GT S197 Fanatic

    2,060
    50
    My crown vic alternator sits at 14.4.
     
  8. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Age:
    56
    691
    89
    The minimum should be 13.0v (0.5v above battery voltage with engine off) with no electrical loads but at that level, it's going to be very marginal. At 13.5v your alternator's still OK but only just.
    Better have a spare one ready just in case your existing unit goes south. I just recently replaced my alternator and I'm going to rebuild my old one with new bearings/brushes/regulator to have as a spare.
     
  9. DiMora

    DiMora More Is Better

    938
    26
    Thanks, guys...I'll install a new one and report back the results.
     
  10. BruceH

    BruceH BBB Big Bore Boss 322

    13,802
    7
    It should be 14.2 at idle. I just changed an alternator out yesterday and the new one is crap. I'm getting 11.x to 15.x. I bought the new alternator over a year ago so it's going to be hard to send back.
    The old alternator with ground off ears is going back on today. What a waste of time and money.
     
  11. Saleen304

    Saleen304 forum member

    450
    1
    Voltage is also controlled by the PCM and is lowered to 13.5V when IATs are high. It does this to protect the battery since it thinks high IATs mean high engine bay temps.
     
  12. BruceH

    BruceH BBB Big Bore Boss 322

    13,802
    7
    This post was a result of operator error. Upon disassembly I noticed the connector wasn't pushed all the way in. Pushed it in until it clicked, reassembled, and I'm seeing 13.8-14.1.

    If anyone is looking for a good working condition alternator with the ears ground off for Roush or DOB use I have one.
     
  13. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Age:
    56
    691
    89
    Depends on the outside temperature. In the winter the voltage needs to be at the higher end (14.0-14.5) at idle whereas in the summer, a voltage of 13.5-13.9 would be acceptable.
    In DiMora's case, it seems the alternator voltage regulator is failing so he's wise to change his alternator before it fails altogether.
     
  14. BruceH

    BruceH BBB Big Bore Boss 322

    13,802
    7
    I got to thinking about it and a better statement for me would of been, "mine has always been 14.2 at idle".

    I can't recall off hand what the tune calls for.
     
  15. BruceH

    BruceH BBB Big Bore Boss 322

    13,802
    7
    From the stock tune:

    [​IMG]
     
  16. JeremyH

    JeremyH 3V Fuel Guru S197 Team Member

    20,682
    100
    That checks 13.5-14.5volts is generally in the healthy range.
     
  17. Saleen4971

    Saleen4971 forum member

    202
    0
    Yeah, it's a new reman....

    Le sigh
     
  18. mr_exon

    mr_exon Junior Member

    9
    0
    If your running a cheap under drive pully set it will cause voltage issues
     
  19. Pentalab

    Pentalab forum member

    2,927
    153
    12.4 vdc with eng off. 14.3 to 14.4 vdc with eng on..and idling. The 05-09 alternator's are single phase types. The 2010 alternator is a 3 phase type. A 2010 style will drop right in on any 05-09 car. Single phase alternator's have long gone the way of the do-do bird. I'm surprised ford used em on the 05-09 cars, since most other car maker's used 3 phase types.
     
  20. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Age:
    56
    691
    89
    BBK, Steeda, Real Speed, and ASP UDPs cost around $250 while those from SR Performance cost a mere $157. I haven't heard of issues with any of them on the 05-10 GT.