CAUTION
Repair any air leaks. A leaking air filter or air lines
can cause slow or hard shifting of the transmis-
sion, and eventual transmission damage.
26–05 Allison Transmission
Fluid and Filter
Changing
AT Series
1. Park the vehicle on a level surface and apply the
parking brakes.
2. Run the engine until the transmission fluid
reaches the operating temperature of 160° to
200°F (71° to 93°C). Shift the transmission to
neutral (N) and shut down the engine.
CAUTION
To prevent dirt from entering the transmission,
use only clean containers and fillers for the trans-
mission fluid. Do not use fillers or containers that
have been used for water or antifreeze. Dirt, water,
or antifreeze could damage the transmission.
3. Clean the area around the drain plug. While the
transmission fluid is warm, remove the drain plug
and drain the fluid. Disconnect the fill tube from
the oil pan.
IMPORTANT: Examine the used transmission
fluid for dirt, coolant or water, and metal par-
ticles. If any of these contaminants are present,
a problem may exist within the transmission
system.
4. Support the oil pan, then remove the twenty-one
washer-head screws that attach the oil pan to
the transmission case. Remove the pan, and dis-
card the pan gasket.
5. Clean the oil pan with mineral spirits.
6. Remove the washer-head screw that attaches
the internal fluid filter to the filter spacer, then
remove the filter and discard it. See Fig. 5. Re-
move the fluid intake tube, and then remove the
seal ring from the intake tube, and discard the
seal ring.
7. Remove, clean, and install the governor feed line
screen, located in the control valve body. See
Fig. 6.
8. Install a new seal ring into the fluid intake tube.
IMPORTANT: Avoid twisting the intake tube
when installing the internal fluid filter, intake
tube, and seal ring; the seal ring could be
pinched, cut, or deformed. An air-tight seal must
be maintained.
9. Install the intake tube, with the seal ring installed,
into the main housing. Install the internal fluid
filter on the intake tube, making sure the grom-
met in the filter fits the intake tube snugly.
10. Install the washer-head screw to retain the fluid
filter, and tighten it 10 to 15 lbf·ft (14 to 20 N·m).
11. Submerge a new oil pan gasket in transmission
fluid for five minutes. Then place the gasket on
the pan; do not use any substance as a gasket
retainer.
IMPORTANT: Do not use gasket-sealing com-
pounds any place inside the transmission or
where they might get washed into the transmis-
f260133a
1
2
3
4
5
6
10/05/94
1. Fluid Intake Tube
2. Filter Spacer
3. Internal Fluid Filter
4. Washer-Head Screw
5. Governor Pressure
Tube
6. Governor Feed Tube
Fig. 5, Allison AT Series Transmission Filter
Transmission26
Business Class Trucks Maintenance Manual, February 200326/6
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