PREVOST X3-45 2016, Руководство по обслуживанию

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Section 07: TRANSMISSION
PA1593
2
1. DESCRIPTION
X3 Series coaches featuring Metropolitan
Transportation Authority (MTA) specifications are
provided with an Allison automatic transmission

ALLISON AUTOMATIC TRANSMISSION
The 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
07136
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. Five-digit
trouble codes greatly reduce the time it takes to
pinpoint potential problems. (Refer to paragraph
"8. TROUBLESHOOTING" in this section).

Retarder
This 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
pedal.
FIGURE 2: ALLISON TRANSMISSION CONTROL PAD
07142
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.
2. WELDING PROCEDURES
These procedures are intended only for vehicles
equipped with transmission electronic controls.
When frame or other welding is required on the
vehicle, precautions are to be taken to protect the
electronic control components. Refer to section