2018FA07
August, 2018
Page 1 of 8
SUBJECT:

H 40/50 EPTM Drive Unit C1/C2 Pressure Switch

MODELS AFFECTED:

H 40/50 EPTM Drive Unit

Introduction:
Allison Transmission, Inc. (ATI) has identified an issue with the pressure switch (P/N 29506484) used in a specific
population of H 40/50 EP Drive UnitsTM. The Drive Units, within the serial number range specified below, were built
with pressure switches that can fail in an either open or closed state. The failure can cause diagnostic trouble
codes (DTCs) to set which may disable the corresponding clutch, locking the Drive Unit into the unaffected
range with reduced performance, or disable propulsion. Additionally, a subset population of Drive Units within the
same serial number range may have debris in valve bodies or internal passages that could prevent proper valve
operation. The purpose of this Field Action is to proactively replace the two (2) pressure switches used in each
of the affected Drive Units, and inspect and remove debris in the subset population according to the procedure
included in this Field Action.
Prior to working on vehicles, please read SIL 21-TR-18, Defective Normally Open Pressure Switch P/N 29506484,
for parts information. All replacement pressure switches installed in the affected Drive Unit serial number ranges
must be stamped with a date code of 1918 or later.

Serial Number Range:
The serial number range included in this Field Action is 7110008855 through 7110009460. All serial numbers in this
range will require the two (2) pressure switches to be replaced.
The serial numbers of the subset population requiring the additional debris inspection and removal are 7110009294
through 7110009445.

Repair Process Overview:
All buses equipped with a Drive Unit serial number 7110008855 through 7110009460 will need to have the oil
drained into a clean container for reuse. The oil pan will be removed and the two (2) pressure switches replaced.
Refer to Allison Electric Drives Service Manual SM3602 Section 5-12 for pan removal and Section 6-15, C1/C2
Relay Valve Body, for switch replacement.
For buses equipped with a Drive Unit serial number 7110009293 through 7110009445, valve bodies will need
removed, inspected for debris, and compressed air blown through the C1 and C2 pressure taps in the rear cover to
remove any potential debris, followed by pressure switch replacement. Instructions for Drive Unit serial numbers
7110009293 through 7110009445 are provided at the end of this document. Refer to Allison Electric Drives Service
Manual SM3602 Sections 5-13 through 5-17 for disassembly and Sections 6-13 through 6-15 for installation.

ML / FA8182EN

4267846

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2018FA07
August, 2018
Page 2 of 8
Table 1. ATI Required Service Parts
Part Description

Part Number

Quantity Per Bus

Pressure Switch, N/O

29506484

2

O-Ring, Switch

29501439

2

Gasket, Oil Pan

29538847

1

O-Ring, Drain Plug

23019664

1

O-Ring, Filter

23018950

1

Claim Submittal Information:
Table 2. Claim Information
Claim Type

04

Special Activity Indicator

2018FA07

Primary Failed Part

29506484

Complaint Code

AT02

Failure Code

CC03

Table 3. Labor Codes
Labor Code

Labor Hours

Labor Operation

00094100

0.5

R&R Oil Pan and Fluid

00096100

0.2

Switch Replacement

00094700

2.8

Debris Inspection

00096400

0.5

Final Test Drive

Any additional parts, labor, net items, or travel costs deemed necessary to complete this Field Action must be
described in detail in the claim narrative. Unexplained costs will be deducted from the claim.
Authorized Allison service outlets must follow the published Allison Transmission current labor time guide for
work performed when adding any additional charges.

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2018FA07
August, 2018
Page 3 of 8

Procedure for Inspection and Removal of Debris:
1. With the Drive Unit installed, drain the fluid into a clean container for reuse, and drop the oil pan from the
Drive Unit.

Figure 1.

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2018FA07
August, 2018
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2. Remove the suction filter and unplug all connectors on the internal harness.

Figure 2.

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2018FA07
August, 2018
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3. Remove the oil level sensor, C1/C2 valve body, shift relay valve body, control main valve body, and separator
plate.

Figure 3.

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2018FA07
August, 2018
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4. Remove the C1 and C2 plugs (shown below) on the backside of the rear cover.

Figure 4.

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2018FA07
August, 2018
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5. Starting with the C2 pressure circuit, plug the 4mm orifice on the stator housing, as shown below. Pressurize
the C2 pressure tap on the rear cover with air to blow out any debris. Check the previously plugged 4mm
passage for loose debris and remove if any is found.

Figure 5.

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2018FA07
August, 2018
Page 8 of 8
6. To inspect the C1 pressure circuit, plug the 4mm orifice on the opposite side the stator housing, as shown
below. Pressurize the C1 pressure tap on the rear cover with air to blow out any debris. Check the previously
plugged 4mm passage for loose debris and remove if any is found.

Figure 6.
7. Inspect the removed valve bodies and the separator plate for any machine chips or debris. Verify proper
valve movement. If any debris is found, additional valve body teardown should be performed. Replace
any components that may have damage.
8. Reassemble the valve bodies and install two (2) new pressure switches (P/N 29506484) and O-rings (P/N
29501439). New switches must be stamped with date code 1918 or later on the metal retainer ring.

Figure 7. Date Code Format
9. Reassemble remaining parts, refill the Drive Unit, and test drive to verify proper operation.
Copyright © 2018 Allison Transmission, Inc. All Rights Reserved.