Allison Transmission Motorhome Series Operator's Manual page 29

There is no speed limitation on upshifting, but there is a limitation on
downshifting and for shifts that cause a direction change such as
D (Drive)-to-R (Reverse) or R (Reverse)-to-D (Drive).
Manual range downshifts will not occur until a calibration output speed (preset) is
reached. When a range downshift is manually selected and the transmission output
speed is above the calibration speed, the transmission will stay in the range it was
in even though a lower range was requested. Apply the vehicle service brakes or a
retarding device to reduce the transmission output speed to the calibration speed
and then the shift to the lower range will occur.
Directional shifts, D (Drive)-to-R (Reverse) or R (Reverse)-to-D (Drive), will not
occur if selected when throttle position, engine speed, or transmission output
speed is above the calibration limit for a calibration time period. The current
calibration time period for engine speed is 0.5 seconds and for throttle position
and output speed is three seconds.
Shifts from N (Neutral)-to-D (Drive) or N (Neutral)-to-R (Reverse) are also
inhibited when the TCM/ECU has been programmed (by input/output function) to
detect that auxiliary equipment is in operation and the shift should not be allowed.
When directional change shifts are inhibited, the TCM/ECU will put the
transmission in N (Neutral) and the digital display, if present, will flash the letter
of the range selected (D or R). To reselect D (Drive) or R (Reverse) when engine
throttle, engine speed, and transmission output speed are below the calibration
value:
• Pushbutton selector—Press the desired pushbutton again.
• Lever selector—Move the lever to N (Neutral) and then to the desired
range.
When a direction change shift is requested and engine throttle, engine speed, and
transmission output speed drop below the calibration value during the calibration
time interval, the shift to D (Drive) or R (Reverse) will occur.
For example, if the transmission output speed was just above the calibration limit
when R (Reverse) was selected, but dropped below the limit during the next three
seconds, the shift to R (Reverse) would occur (assuming the engine was at idle
and the throttle was closed).
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