Section 07: TRANSMISSION
PA1553
3
1. DESCRIPTION
XLII Series Bus Shells are provided with an
Allison automatic transmission.
1.1 ALLISON AUTOMATIC TRANSMISSION
The B500 or B500R (with retarder) Allison
Transmission has 6 speeds with two top range
(fifth and sixth) overdrives. Total coverage is
determined by dividing the highest gear ratio by
the lowest gear ratio. Total coverage expresses
the transmission gear ratio versatility.
Transmissions with larger total coverage number
have a wider variety of available ratios.
An electronic control allows the transmission to
shift at exactly the right point on the engine's fuel
consumption curve for best economy. Early
lockup maintains the highest possible mechanical
efficiency through the closely-spaced gear steps,
culminating in two overdrive ratios. This
combination allows progressive shifting
techniques, where engine speeds are reduced for
higher efficiency and lower fuel consumption.
FIGURE 1: ALLISON TRANSMISSION 07075
Gear selection and torque converter modes are
controlled by a microcomputer-based electronic
transmission management system. It is fed
information regarding throttle position, operator
range selection, engine speed, turbine speed,
transmission output speed and various system
pressures from special electronic sensors. With
this information, it computes shift points and
clutch pressures to meet immediate needs.
Using closed loop adaptive logic; the electronic
control looks at a number of parameters during
the shift, and makes minute adjustments to
match the shift to desired profile stored in its
memory. It then looks at these adjustments and
resets the parameters, which allow the
transmission to quickly compensate for
variations in load, terrain or environment and to
adjust for clutch wear and engine power changes.
A Diagnostic Data Reader can be connected to
the electronic control unit to provide a self-check
of all systems in the transmission. Four-digit
trouble codes greatly reduce the time it takes to
pinpoint potential problems. (Refer to heading
"10. TROUBLESHOOTING" in this section).
1.1.1 Retarder (if applicable)
This optional auxiliary braking device for the
automatic transmission is integrated into the basic
envelope of the transmission and transmits its
braking force directly to the propeller shaft. It
requires no additional length and adds only 75
pounds (34 kg) of weight. Operation of the
retarder is controlled electronically by the driver's
use of the brake and/or by hand control lever.
FIGURE 2: ALLISON TRANSMISSION CONTROL PAD 07025
When activated, fluid enters a cavity and provides
resistance to the turning of rotor blades revolving
with the output shaft. This effectively slows the
vehicle to the point where the service brakes are
needed only for final stopping. The retarder is fully
modulated and is compatible with ABS.
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