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Thread: Dropping voltage at higher RPM's at the end of a run.

  1. #1
    Tuner in Training
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    Dropping voltage at higher RPM's at the end of a run.

    My voltage drops a good bit around upper 5800 rpms, I notice it the most in 3rd at the end of a run. Is there something that cuts the alternator off at full throttle or do I just need to look at swapping Alternators? Maybe something that cuts it off in the HP tables I'm overlooking???
    The alternator pulley has really good belt wrap, so I don't believe it's slipping...
    I also notice my A/F normally leans up a half point at the same time, so I'm thinking it's related. I just installed a fuel pressure sensor to see if the fuel pressure drops also.

    030422017holly3.hpl
    021917aa.hpt
    .png

    blue/green PCM, 5.3, 4L80E, S476, DWB, pump gas with Alky

  2. #2
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    I don't remember if there's any cutoff, but I can say that I broke 3 alternators taking the car over 6000 RPM. One of them, brand new AC DELCO not "remanufactured", broke in a day! The other 2, one lasted a year, the other I forget!

    Seems that they are geared quite high, at least in older cars like mine, ('99 TA) and the alt overrevs and throws a rotor winding connection.

    I fixed that by fitting a later AD244 alternator, they are a lot tougher, then later I fitted an underdrive pulley as well, lol, been ok ever since (several years)... if you are lucky, perhaps you already have one!

    kr D.
    99 TA, Texas Speed LS376, PRC heads, 233/239 cam, Fast 92mm, 95mm TB, card style MAF, Tick TR6060, Strange 4.11 12 bolt axle & clutchpack diff, Strano springs/dampers, Vette 18" wheels, Vette disks, CTS-V calipers, 16lb flywheel, long tube headers, no cats.

  3. #3
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    Quote Originally Posted by dermotw View Post
    I don't remember if there's any cutoff, but I can say that I broke 3 alternators taking the car over 6000 RPM. One of them, brand new AC DELCO not "remanufactured", broke in a day! The other 2, one lasted a year, the other I forget!

    Seems that they are geared quite high, at least in older cars like mine, ('99 TA) and the alt overrevs and throws a rotor winding connection.

    I fixed that by fitting a later AD244 alternator, they are a lot tougher, then later I fitted an underdrive pulley as well, lol, been ok ever since (several years)... if you are lucky, perhaps you already have one!

    kr D.
    I really don't know much about what cuts it off and on or if it's PCM controlled. I have a 2nd alternator laying around that I thought about trying and I thought about an under drive. Thought I'd pry around a little bit before swapping stuff out.
    Last edited by zboss86; 03-13-2017 at 01:51 PM.

  4. #4
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    The PCM does control it on <my> car, but under what conditions it switches it on/off I dont know, it would likely turn it off at high RPM though... HPT doesn't have any controls for that I think. Yours will be later type ECU but I believe it does control the alt also...
    99 TA, Texas Speed LS376, PRC heads, 233/239 cam, Fast 92mm, 95mm TB, card style MAF, Tick TR6060, Strange 4.11 12 bolt axle & clutchpack diff, Strano springs/dampers, Vette 18" wheels, Vette disks, CTS-V calipers, 16lb flywheel, long tube headers, no cats.

  5. #5
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    Quote Originally Posted by zboss86 View Post
    My voltage drops a good bit around upper 5800 rpms, I notice it the most in 3rd at the end of a run. Is there something that cuts the alternator off at full throttle or do I just need to look at swapping Alternators? Maybe something that cuts it off in the HP tables I'm overlooking???
    The alternator pulley has really good belt wrap, so I don't believe it's slipping...
    I also notice my A/F normally leans up a half point at the same time, so I'm thinking it's related. I just installed a fuel pressure sensor to see if the fuel pressure drops also.

    030422017holly3.hpl
    021917aa.hpt
    .png

    blue/green PCM, 5.3, 4L80E, S476, DWB, pump gas with Alky
    I'm also experiencing this and quickly looking into it on the internet seems to point at either 1.) Insufficient main ground (battery should be grounded to the engine block)
    2.) Insufficient alternator ground (ground alternator body to the block despite the bracket doing so already)
    3.) Try a non-ac delco alternator as apparently the voltage regulator or other components internally are responsible for the symptoms mentioned

  6. #6
    Advanced Tuner MikeGyver's Avatar
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    It is the brushes in the alternator bouncing. I got lucky and a new, not re manufactured alternator fixed it. I think I got a Bosch, it's been awhile. It is especially dangerous when you have bad injector data like I did. The voltage dropped and the engine went lean at a most critical time.
    Last edited by MikeGyver; 03-15-2017 at 09:41 AM.
    2000 GMC 2500 2-bar SD
    Stock LM7, LTs, TBSS intake manifold