Allison Automatic Transmission; Retarder; Volvo I-Shift Transmission - PREVOST H3-41 Maintenance Manual

SECTION 07: TRANSMISSION
2. DESCRIPTION
H3 and X3 Series vehicles may be provided with
either an Allison automatic transmission or a

Volvo I-Shift transmission.

2.1

ALLISON AUTOMATIC TRANSMISSION

The Series 4000 Allison Transmission has 6
speeds with two top range (fifth and sixth)
overdrives.
Two variants are available: B500 for commercially
operated highway coaches and 4000MH for
private use motorhomes.
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
combination allows
techniques, where engine speeds are reduced for
higher efficiency and lower fuel consumption.
FIGURE 1: ALLISON TRANSMISSION
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
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PA1621 Maintenance Manual All Series | January 2017
overdrive ratios. This
progressive shifting
(07075)
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,
transmission to quickly
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 6
"allison transmission TROUBLESHOOTING" in
this section).

2.1.1 Retarder

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 PUSHBUTTON SHIFT SELECTOR
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.
2.2 VOLVO I-SHIFT TRANSMISSION
The Volvo I-Shift transmission is a single
countershaft transmission with 12 forward gears
and two reverse gears. The I-Shift is an
automated mechanical transmission.
which allow the
compensate for