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High Amp Alternator issue... maybe B&B can add to this

Discussion in 'Chevy Trucks' started by FLASHMAN, Dec 23, 2008.


    FLASHMAN Member
    Messages: 78

    Now I'm sure there will be some that will think "duh" when I tell this story, but I didn't think of it until after the fact. Here it is:

    When my battery and alternator started to die, I decided to upgrade to a H/O alternator and dual battery system. I haven't added the second batt yet because of time constraints, and the fact that I needed my plow for the 15" we've gotten over the last 2 storms, but I did put in my 190 amp alternator. It was built for someone who never picked it up, and the rebuilding guy forgot about the pulley, and until today, I didn't know better.

    When you get a H/O alternator, you MUST change the stock pulley to a smaller one!!! For example, my stock 100/105 amp unit in my '98 GMC K1500 has a 2 3/8" pulley. For a HO alternator, it has to spin faster for the voltage drop to be the same at idle, so you need to drop it down to a 2 or 2.1" pulley. There is some kind of mathematical equation used to figure it out, maybe one of the experts on here can add it to the thread...

    With the stock pulley, the HO unit was only dropping 11.8 volts at idle, but as soon as RPM's went over 1k, BAM!!! 14.5 So I was almost sure the alt wasn't defective. Then I read about the pulley sizing.

    Sorry if this was obvious, but I'm sure there are plenty of amateurs out there like me that might find this info useful... Happy plowing!!!!! And Happy Christmahannakwannzakah to all!!! :drinkup:
  2. 2COR517

    2COR517 PlowSite Fanatic
    Messages: 7,115

    I put a 155 amp on my truck. The pulley that came with it is just a little too big. I need to put one of the smaller ones back on.
  3. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Alternators are generally rated at 2400 and 5000 RPM (alternator RPM not engine RPM) as far as their output at a given speed. So if the pulley ratios are not correct, especially with a HO alternator you get exactly what you found Flash, namely lower than expected idle output. as generally speaking the more amps the alt is capable of, the faster you need to spin it.

    Most applications you want at least a 3:1 pulley ratio to ensure the best low speed charging capability that the alternator can provide.

    But upwards of 4:1 works even better on a plow dedicated truck. ;)

    Same reason guys who plow mostly in 4 low have few power draw issues...since their spinning the alternator faster under hi demand conditions than those who plow in hi range.
  4. sweetk30

    sweetk30 PlowSite.com Addict
    Messages: 1,588

    our plow truck at work i installed a 1/4" smaller dia pulley on the alt. man did that help on a stock high option alt alone with out any other upgrades.

    FLASHMAN Member
    Messages: 78

    Ok, so now I'm going out of my mind..... Had my alt guy put on the smallest pulley he had in stock, which is 53mm. With my 197.25mm stock crank pulley size, that gives me a 3.72:1 ratio, and it's STILL dropping just over 12 volts at idle. There is only one smaller pulley, he says that is 47mm. That would put my ratio at 4.2:1. If that doesn't work, what then??? I don't want to change the crank pulley because that would throw off the ratios of everything else....GRRRRR
  6. ServiceOnSite

    ServiceOnSite Senior Member
    Messages: 928

    iive never heard of changing the pulley nor have i changed the pulley on my alternators with no problems. werid.:dizzy:
  7. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Still dropping when? When you operate the plow?
  8. CityGuy

    CityGuy PlowSite Fanatic
    Messages: 16,905

    Whats an HO?
  9. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    HO = High Output...as in alternator. :waving:
  10. CityGuy

    CityGuy PlowSite Fanatic
    Messages: 16,905

    ahh thanks little slow on the short hand

    FLASHMAN Member
    Messages: 78

    no, just at idle, even with nothing running, the voltage drop is bouncing between 12.2 and 12.5 volts. No load whatsoever. It is improved from when I originally got the alternator, but there's only one more size down in pulley that I can go, which is down to the 47mm... That puts my ratio (if I'm doing the math right) at 4.2:1...

    It may fix it, but I'm just concerned that if it doesn't, what then???

    At first, I had to bring the RPM's up to 1k before the voltage drop would go to normal, when he put the 53mm pulley on, I now have to go up to about 850... I'm HOPING that taking away an extra 5 mm will take it the rest of the way... What do you think???
    Last edited: Jan 6, 2009

    FLASHMAN Member
    Messages: 78

    Do you use stock alternators? I've had to do so much research lately, I'm starting to know a lot more than I ever wanted to...haha
  13. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Flash we'd you get this alternator? Is it something built and bought local? Most of those guys can't build them to put out good power at low speeds...you usually end up with LESS amperage at idle than your stock alternator...which sounds like the text book case in your account.

    It takes more than a few off the shelf generic replacement parts to build a good quality hi output alternator...and most local places can't deliver.

    FLASHMAN Member
    Messages: 78

    It is a local place, but it's a guy who did this large scale for almost 40 years, and came HIGHLY recommended by everyone in town with a plow, on top of every car dealer in the surrounding area. Is there a place you'd recommend???
  15. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Have one of those type of guys here also...his don't put out near enough amps at low speed either.

    DB Electrical or Alterstart is where I source mine.

    FLASHMAN Member
    Messages: 78

    Well, I'll be dipped in ****... I guess I'll have him give me a stockie, and refund me the difference if the smaller pulley thing doesn't work...

    Mental note: ask BB BEFORE buying expensive electrical stuff....

    Thanks again BB. I'll let you know how my last ditch effort works out...
  17. elite1msmith

    elite1msmith 2000 Club Member
    from chicago
    Messages: 2,762

    correction - mental note...ask B&B before buying anything ....
  18. 70monte

    70monte Senior Member
    Messages: 468

    B&B is definately knowledgeable. He has helped me out quite a few times in the past. I appreciate the help he gives on this board.