under drive pulleys

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by 00chevy53, Oct 31, 2010.

  1. 00chevy53

    00chevy53 Rockstar 100 Posts

    so i buoght the asp ud pulley kit that comes with the crank pulley and alt. pulley. after doing so the voltt gauge indicated that it was only pushing about 10 volts whens its supposed to be pushing 14 volts... i was told get a bigger alt. which i did, i went from a 105amp to a 145amp alt. but am still having the same problem. basically im looking for the solution, preff. someone thats run into this problem.... not sure if it matters but its on a 2006 vortec maxx 6.0 motor
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    Don't know who told you that upping the amperege on your alternator would raise your voltage reading to 14 V at idle but......??
    The higher amp alternator will do better at recharging your battery if you are useing a lot of consumers at the same time, but I don't think it will ever show 14Volts at idle with an underdrive set. Mine never has. I put the March three pulley set (crank, wp, alt) on mine but it was so long ago I don't remember what the underdrive percentage is. It keeps my 6 year old battery charged with the 105 amp alternator if I don't let it sit for more than a week. My voltmeter has read as low as 8 volts at idle lol

    Do you know what underdrive % is?
    At what RPM does your voltmeter come back up to 14V?
  3. 00chevy53

    00chevy53 Rockstar 100 Posts

    its a 25% kit... but im not sure as to the exact rpms but durring the day when i get up to about 20mph and that brings it to 14v... night time at 20mph its only reading about 12v, im guessing thats due to the small system and 8k hids...
  4. ejohnson03

    ejohnson03 Epic Member 5+ Years 1000 Posts

    I have never heard anything good about under driving the Alt.
  5. stephan

    stephan Rockstar 4 Years 5000 Posts

    more power, better mileage, aluminum pulleys for less rotating weight. 25 to 35% isn't much loss for the alternator.
  6. ejohnson03

    ejohnson03 Epic Member 5+ Years 1000 Posts

    Under driving can give an increase in performance, but I don't think leaving the Alt. pulley will compromise the overall benefits.

    If you are running any additional lights or a large audio system, 25% loss at the alternator will not be a pleasant expierience.
  7. stephan

    stephan Rockstar 4 Years 5000 Posts

    Point taken EJ, but I only have bare bones factory stereo, & standard lights. Still everyone who's considering underdrive pulleys should do an inventory of all the electrical add ons that they have accumulated before diving in.
  8. RobRoberts

    RobRoberts New Member

    just put an over drive pully on the alternator, and you will be fine.
  9. 00chevy53

    00chevy53 Rockstar 100 Posts

    with the asp kit i have it came with the over drive pulley. the hype is its supposed to work but truth is that it dont do jack
  10. chevybulldog6

    chevybulldog6 Rockstar ROTM Winner 100 Posts

    Where were you able to find the pully kit? I cant seem to find a set for my 05 1500 5.3L. I am still researching and trying to decide if Iwant to go this route or not but I cant make that decision if I cant even find them to price em.

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