1990 school bus

Discussion in 'Class A / Diesel Pushers' started by candyman, Apr 20, 2009.

  1. candyman

    candyman New Member

    hey guys need someone's help I have a 1990 school bus that has been converted to a RV it had a 7.3 non turbo diesel in it with a 4 speed allison tran in it I change the diesel to a 6.0 gas engine I need to know how to change the shifting points on the trans which means I need for the trans not to change so quick thanks can SOMEONE please HELP :question:
  2. Triple E

    Triple E Senior Member

    Re: 1990 school bus

    Look under "General RV'ing" at the top of the "RVUSA Main Forum" menu. There you will find a place to ask question about the Allison Transmission. Or change you question from 1990 school bus to "Shift Points on Allison Transmission. Good Luck.
  3. dbarton291

    dbarton291 Senior Member

    RE: 1990 school bus

    FIRST: You have changed this vehicle to a much higher RPM engine that it was originally. You must perform a driveline analysis to make sure the driveline configuration you have will not be torsionally active at this higher RPM. Dana has a good driveline calculator online called Dana Expert. If you do not do this, you risk driveline vibration, or worse consequences. If you don't know how to do this, or don't own an inclinometer, get some help from a qualified driveline tech.

    Here is what is in my opinion, the safest and most cost effective method for this conversion after you make sure your driveline can handle the increased maximum speed it will be turning:

    You need to look at the data tag on the transmission and record:
    1. Trans Model number. Should be AT-545 or MT-643
    2. Serial Number
    3. Assembly (or part) number

    Go to a transmission rebuilder, or an Allison outlet that sells ReTran transmissions. Use your transmission for a core and buy a transmission that is built to go behind a gas engine that matches the full load RPM of your engine. That way you make sure you have the right torque converter to match the engine and the correct shift points.

    There are other ways to attempt this. They involve choosing the right torque converter, valve body calibration, output governor and governor drive gear on the output shaft of the transmission. It is time consuming, risky if you miss something, and may end up costing nearly the same as buying a remanufactured transmission.

    If you just experiment with different springs for the shift signal valves, you may or may not end up with satisfactory results, and the question of proper torque converter will not be addressed. There are adjusting collars on the shift signal valves. They will not have enough adjustment in them to raise the shift points to match a gas engine without changing springs and/or valves.

    Done properly, you will end up with a powertrain that performs properly and stays cool. Short cutting this process can result in any of a myriad of problems ranging from annoying to potentially serious.
  4. dbarton291

    dbarton291 Senior Member

    Re: 1990 school bus

    Don't forget to get a modulator system hooked up and properly set if you haven't already. That will also adversely affect the shift points if it's not correct.

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