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Thread: Transmission Overheating

  1. #21
    Join Date
    Oct 2010
    Location
    Scottsbluff, NE
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    137

    Default

    Fred,
    How did your transmission experience end? (or has it)...

  2. #22
    Join Date
    Jul 2009
    Location
    Dallas
    Posts
    240

    Default

    Not that it would be significant in Fred's situation but I thought we were told at the seminar in OK City that Transynd could take 300 degrees without creating issues. Did i get that wrong?

  3. #23
    Join Date
    Oct 2010
    Location
    Scottsbluff, NE
    Posts
    137

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    Jim mentioned that you can check the oil level from the Allison pad. For those of you unfamiliar with this function it's a good idea to perform this after each day of driving.
    Depress both arrow buttons with the transmission in N (neutral) at normal idle speed.
    Here's some codes for everyone..........
    OL,OK - oil level is OK
    OL,LO,01 - oil level is low 1 quart - 02 is 2 quarts,etc.
    OL,HI,01 - oil level is high 1 quart - 02 would be high 2 quarts, etc.
    0X - settling time too short
    50 - Engine rpm too low
    59 - Engine rpm too high
    65- Neutral must be selected
    70 - Sump fluid temp too low
    79 - Sump fluid temp too high
    89 - output shaft rotation
    95 - sensor failure

    Here's an example of what you might see if you tried this without warming up your trans oil first - OL,70

  4. #24
    Join Date
    Jan 2006
    Location
    anytown
    Posts
    8,908

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    Patrick,

    300 degrees is the temperature at which the Allison computer starts to remove the retarder function. I can find no technical information regarding temperature limits or safe operating range for the fluids in our transmissions. If it is out there I have not found it, and our gauges do not have markings to indicate the normal operating range or red line.

    But Allison provides a huge clue about the effect of heat on the transmission fluid. In their maintenance schedule a retarder equipped vehicle has its fluid change interval cut in half. With the retarder off my transmission fluid temps measured at the outlet just prior to the intercooler track engine coolant temperature reasonably well. The temps will never get beyond about 210 (hard to read because the gauge is poorly marked). With the retarder on I can see the needle head towards the high temperature range very fast so it is not hard to imagine that the fluids are being subjected to much higher temps than normal. Personally I limit my temps to 230, not for any scientific reason other than my plane had an oil temperature limit of 240 (red line) regardless of whether I used conventional opil or synthetics. I chose to personally limit the temp because as stated above there is no real guidance on our gauges or the Allison technical data, other than their reduced maintenance interval.

    Limiting my temps to 230 requires some effort and anticipation. If I know I am approaching a steep downhill slope I gear down and reduce speed. Out west where long downhill runs of 6 to 8% are not that unusual I try to be just below 55 at the crest and in 4th gear (which on my coach 55 is the 4th gear upper limit) and the retarder in the second position. If the speed creeps up I go into the 3rd position, if I slow too much I go back to the first position. By doing that I can descend at no more than 55 MPH and my tranmssion temp gauge stays at 230 (clearly marked) or below.

    Braking with the retarder on, even with the control in the "0" position will drive up the temps because the retarder is employed proportionately with braking force. Again, like hills I anticipate the need to brake and can limit temps by taking longer to slow or stop. When I have to brake suddenly I ignore the temps because safety is more important.

  5. #25
    Join Date
    Nov 2010
    Location
    lake havasu city, az
    Posts
    74

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    My coach is equipped with an amber trans fluid temp warning light which, I believe, is supposed to come on @285F at the retarder output. I've seen it a few times. I try to limit my trans-synd temps to 250F @ retarder output which usually involves no more than level 3 in 5th or level 4 in 4th for any extended downhill run but I usually gross about 65-70,000#. I use the brakes to keep rpm's under 2000. My last trans-synd change at about 25,000 miles use came out pretty good; I could go a little longer. Obviously, this is severe use. I'm on the original trans @150,000 miles.

  6. #26
    Join Date
    Nov 2010
    Location
    lake havasu city, az
    Posts
    74

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    P.S. Even if you're not breaking down your fluid, plastic, rubber, and certain other parts of the trans will wear faster with elevated temps.

  7. #27
    Join Date
    Jul 2010
    Posts
    78

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    Here's an issue that I ran across over the past weekend. The bracket that holds the accumulator for my retarder equipped transmission broke again. As provided on my coach (1999 chassis) this is a 1/4" plate. It had broken before and was welded as a repair. The local DD/Allison shop says that there is a tremendous amount of torque generated through this valve when the retarder is actuated, the plate flexes causing failure. I have it off again and will be fabricating a replacement from 1/2" stock.


    P8230663_1.jpgP8230655_1.jpg
    P8250670_1.jpg

  8. #28
    Join Date
    Mar 2006
    Location
    ON THE ROAD IN THE SOUTH
    Posts
    2,825

    Lightbulb

    Orren, my $.02 on this. I personally would go with a new original part or one just like it, the reason being the chain break always goes to the weakest link. You presently have an escape valve of sorts and I would rather have it brake than the casting it is bolted to.

    As I am sure you know mechanical failures will just move down the line to the next point that will fail.

    The area of the components appears from the photo to be quite rusted and corroded, maybe due to road salt or storage in a damp environment, be that as it may, I would clean everything up as well as possible and use a new plate and high grade fasteners all torqued to specs. Also note the elongated bolt hole next to the failure point, this suggests to me that the assembly was loose and moving. This would in itself lead me to think that improper fastening and not the bracket/plate thickness/strength was at fault, but then again, who knows ....happens and your there and I am here.
    As an aside the welded plate probably was doomed to failure from the start and should have been only regarded as a temporary repair, just say'n, please take no offense.
    Hope it all works out!

    JIM

  9. #29
    Join Date
    Mar 2006
    Location
    ON THE ROAD IN THE SOUTH
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    2,825

    Question

    I am curious, how did you find out about the failure?

    Was it suspected by some indication in handling, or was it found in routine inspection??

    JIM

  10. #30
    Join Date
    Jan 2006
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    anytown
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    8,908

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    I'm going to look at that on my bus. I have the same situation but have not seen a failure of the bracket, but then again I may have missed it.

    It looks to me that the bracket as mounted is being stressed since the two broken halves are not lined up. Consider a bracket repair that uses the original attach points, but is attached to the accumulator as the accumulator now sits with no forces on it. The stress combined with what is likely a full range of vibrations is the probable cause of the failure.

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