OPERATION
5-44 / Driving the Vehicle
© 2018 Pierce Manufacturing Inc. All Rights Reserved.
Driving the Vehicle
5-26. Transmission
5-26.1 Automatic Transmission
The Allison automatic transmission supplied with most Pierce Custom Chassis will only start in
N
(Neutral). Select
D
(Drive) mode for normal driving. Completely stop the vehicle before shifting from any forward range to
R
(Reverse). Turn off the vehicle high idle switch before attempting to shift from
N
(Neutral) to
D
(Drive) or
R
(Reverse).
For further information refer to the Allison Transmission Operator’s Manual.
5-26.2 Transmission Operating Temperature
NOTE:
The transmission temperature gauge does not indicate the sump temperature. The gauge indicates
the temperature of the oil as it exits the torque converter and before the oil enters the transmission
cooler. This reading provides a much more immediate indication of temperature rise due to heavy
use or retarder applications. If the sump fluid temperature reaches 250°F, the Electronic Control
Unit (ECU) will inhibit operation in the higher ranges.
Maximum sump operating temperature is 250°.
On multiplexed vehicles, this information is monitored by the Command Zone™ system. During a high temperature
condition, a text message, caution light, and chime tone are displayed on the Command Zone™ Information Center
(CZIC).
5-26.3 Neutral to Drive Shift Growl - Allison MD Transmission
A growling noise may sometimes be heard in the cab when shifting the transmission from
N
(Neutral) to
D
(Drive).
This is caused by electric solenoids within the transmission that control (modulate) the clutch hydraulic pressure.
These solenoids modulate clutch pressure by turning on and off very fast, similar to the operation of an alternator
voltage regulator. Increasing or decreasing the time the solenoid spends in the “on” state controls the clutch
pressure.
The rapid on-off operation of the solenoids results in pressure oscillations at the clutch. These pressure oscillations
result in a cyclical piston load on the clutch, which can cause torque oscillations. These torque oscillations are
transferred through the transmission and driveline, into the axle, axle shafts, and any suspension members attached
to the drivetrain.
If vehicle components such as body panels, drivelines, and frame members are excited by the frequency of the
torque oscillations, an audible noise may be heard inside the cab. These torque oscillations (and resulting noise)
are well below the design limits of the transmission and do not affect the life of the transmission or other vehicle
driveline components.
Whenever it becomes necessary to leave the vehicle, even momentarily, while the engine is running,
place the transmission shift selector in Neutral, set the parking brake, and chock the wheels. Failure to
follow this procedure may lead to loss of vehicle control resulting in possible property damage,
personal injury, or death.
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