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.