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Gearbox

Discussion in 'Props, Shafts & Seals' started by Cloudman, Aug 17, 2012.

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  1. Cloudman

    Cloudman New Member

    Joined:
    Mar 31, 2007
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    Location:
    Hong Kong
    Have Allisons gearbox on 8V92Ti when either forward gear or reverse is engaged the oil pressure and RPM drop significantly. 200-250 RPM
    Quite a time delay before the prop seems to move the boat. Any ideas welcome
    Gear Was overhauld last year but problem remains. Other engine has no problems and prop moves the boat as soon as engaged
  2. Kafue

    Kafue Senior Member

    Joined:
    Nov 29, 2006
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    Location:
    Gold Coast Australia
    Do you have Hydreco oil Pressure pumps? Check the pressure of the GB oil when you engage it.
    There is a pressure adjustment nut on the side of the Hydreco. I have 12V71n's and Allison M20's. The pressure at idle should be 150psi.
    I had similar problem on my Allison M20 (port) but only in forward and it turned out to be the pressure pump PLUS this then caused the GB piston & clutch to fail, I had to get a new piston shipped from the US.
    From what I understand (which is limited but growing), if the forward gear is the only problem, then the GB can likely be repaired without removing the GB but just pulling it back to access the flywheel, piston and main seal. However if it is the reverse then the GB needs to be removed and rebuilt.
    In your case, as the gearbox was overhauled last year, let's hope it is only the pressure pump. It can be rebuilt but a better solution is to buy a new one $1080 and easy to install. They are available in the US, I bought 2 rather than rebuild old ones.
    Good luck.
  3. Capt J

    Capt J Senior Member

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    If you are talking about engine oil pressure and rpm's dropping, check to make sure your shaft spins relatively freely and is not binding due to a swollen cutlass bearing or bad alignment. Next would be the engine itself, either it has an internal issue and is not making power, or it needs a tuneup (valve adjustment, injector timing). But I've seen lots of detroits drop a lot of rpm's when you put them in gear........
  4. Bill106

    Bill106 Senior Member

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    Beaufort NC
    It does sound like Kafue has the most likely explanation, the pressure pump. Dropping rpm's usually does cause a pressure drop in the gear which will further delay engagement. Since it sounds like the problem was evident before the gear rebuild, the engine could be at fault as well. Have a good Detroit mech check the buffer spring adjustment, it can also show the same symptoms.
  5. Cloudman

    Cloudman New Member

    Joined:
    Mar 31, 2007
    Messages:
    14
    Location:
    Hong Kong
    Thanks for the replies. I will check out the pump, both engines are overdue for a major overhaul and am planning to take them out in November after our fishing season costs.