SERVICE DOCUMENT
Diagnostic and Repair Plan
FSC 24-02-6055 Allison Transmission Regulator Update
Version 1, Online
ISSUE DESCRIPTION:

Allison Transmission equipped buses were built without a pressure
regulator at the Solenoid inlet to the transmission retarder accumulator air
inlet. This bulletin provides retrofit instructions for installing a pressure
regulator to remedy leaks at the solenoid air exhaust.
SYMPTOMS:

Heavy oil leakage at retarder accumulator air solenoid exhaust hose. See
figure 1 for example.

TOOLS REQUIRED:

Basic Hand Tools, Hose cutters, Masking tape, Permanent Marker, Snoop
or Soapy water

PARTS REQUIRED:

02-56940-003

BRACKET, ACCUMULATOR MTG

1

04-56627-003

REGULATOR, AIR PRESSURE

1

54-11316-008
54-11324-004

BOLT, HEXHD, .2500-20 X 2.25, STL
GR5/ZN
BOLT, HEXHD, .5000-13 X 1.25, STL
GR5/ZN

2
2

54-12540-005

NUT, NYLOCK, .2500-20, STL GR5/ZN

4

54-12551-009

NUT, NYLOCK, .5000-20, GR8

2

54-12601-006

WASHER, FLAT, SAE, 0.25, STL GR8/YZ 4

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FSC 24-02-6055 Allison Transmission Regulator Update

54-12956-005

FITTING REDUCER, 0.375-0.25 MNPT,
BRASS

1

56-14037-024.00

HOSE, AIR, .375" BLACK. 24" LONG

1

54-12929-006

UNION, COMPRESSION .375” X .375”

2

54-44786-000
13-50024-001

FITTING TEE, F-TYPE, 0.25 MNPT-0.375
TUBE
LOOM, CONVOLUTED BLACK NYLON
.35 IDX24L

1
2

Other consumables:
Zip ties
PTFE tape or thread sealant
DIAGNOSTIC STEPS:

Verify significant oil leakage is evident at the accumulator solenoid
exhaust hose

REPAIR STEPS:

REPAIR STEPS:
Disassembly:
1. Turn off the battery switch
2.

Access the air tanks drain valves above the battery compartment and
relieve the air pressure from all tanks

3.

Use shop air to clean the area around the air connections on the
accumulator to avoid air system contamination

4.
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FSC 24-02-6055 Allison Transmission Regulator Update

4.

Disconnect the wiring harness connector from the Air solenoid

5.

With a permanent marker mark the air hose that’s closer to the
solenoid on the F-pipe so the hose will be reinstalled at the same
location

6.

Note: There will still be about 70psi of air pressure in the retarder
solenoid supply line due to the pressure protection valve at the air
tanks. Slowly loosen one of the compression fittings to bleed off the
remaining air pressure.

7.

Disconnect the two air lines from the F-pipe fitting at the solenoid inlet

8.

Cut the brass ferrules off the air hoses to avoid reusing them on the
new F-pipe fitting

9.

Cover the air hose ends with masking tape to avoid contamination

10.

Unscrew the hose barb fitting at the bottom of the solenoid to
disconnect the exhaust tube

11.

Wrap zip ties around the accumulator housing to loosely support the
accumulator body

12.

Remove the 2 bolts holding the Accumulator Mounting Bracket to the
transmission and lower the accumulator several inches to better
access the solenoid support bracket

13.

Remove and discard the solenoid support bracket, the (2) 2.5” long
solenoid mounting bolts and the nylock nuts. Save the bolts and
washers

14.

Remove and discard the 3/8 to ¼” NPT reducer on the solenoid air inlet

15.

If the accumulator is being rebuilt follow these steps below:
a.

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FSC 24-02-6055 Allison Transmission Regulator Update

15.
a.

Remove the accumulator mounting bracket from the accumulator

b.

Follow the published Allison Transmission procedure for replacing
the accumulator piston seals

Figure 2: Old/current accumulator brackets descriptions.

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FSC 24-02-6055 Allison Transmission Regulator Update

Figure 3: Comparison of air hose routing with and without the pressure regulator

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Installation

Figure 4: Pressure regulator part descriptions

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Figure 5: Pressure regulator information

1.

If it was removed, reinstall the accumulator and the solenoid support
bracket to the accumulator mounting bracket

2.

Apply PTFE tape or thread sealant to both ends of the ¼ to 3/8” NPT
adapter. Keep the first 2 threads clean to avoid air system
contamination

3.

Insert the NPT adapter into the solenoid and finger tighten. Then turn
an additional 2 turns

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4.

Thread the pressure regulator air outlet onto the ¼” NPT adapter end
on the air solenoid. Tighten 2 turns past finger tight and then continue
tightening to align it vertically with the accumulator housing. Do not
loosen (back off) the threads to align the parts

5.

Apply PTFE tape or thread sealant to the NPT threads of the F-pipe.
Keep the first 2 threads clean to avoid air system contamination

6.

Insert the F-pipe into the air pressure regulator air inlet. Tighten 2 turns
past finger tight and then continue tightening until the compression
fittings are facing 180 degrees away from the regulator air outlet. Do
not loosen (back off) the threads to align the parts

7.

Loosen the two ½” bolts holding the solenoid support bracket to the
accumulator and the accumulator mounting bracket

8.

Attach the regulator support bracket to the solenoid alignment bracket.
Do not tighten the bolts yet

9.

Attach the pressure regulator to the regulator support bracket using the
new 2.25” bolts. Do not tighten the bolts yet

10.

Tighten the 6 bolts holding the brackets and regulator evenly to ensure
proper alignment

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FSC 24-02-6055 Allison Transmission Regulator Update

Figure 6: Bolts highlighted for proper alignment of air system parts

1.
2.
3.

4.

5.

Reinstall the accumulator mounting bracket to the transmission
Reinstall the solenoid exhaust tube to the solenoid
Trace the two air lines that were attached to the F-pipe at the solenoid
back to the air lines bundles inside the frame rail and cut zip ties and
rearrange the hoses to increase the free length of these two hoses
towards the accumulator as much as possible without kinking or
pinching anything
Determine if the air hoses have enough length to reach the F-pipe
without creating a sharp bend or kink in the air lines
Follow these steps only if the air hoses need to be extended

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5. 24-02-6055 Allison Transmission Regulator Update
FSC

a.

Insert the air line extensions into the F-pipe and determine a good
place to add the female compression union fitting to extend the air
lines about 12” back from the regulator so that the fittings can be
secured to other air lines or harnesses

b.

Tighten the compression fittings at the F-pipe and the female union.
Do not reuse the compression fittings to avoid air leaks.

6.

Follow these steps if the air lines do not need to be extended:
a. Refer to the permanent marker markings to reinstall the air hoses
back in the same locations

7.

Using zip ties secure the F-pipe air lines to the nearby air line bundle
Reconnect the solenoid wiring harness connector
Turn on the battery disconnect switch and start the engine to allow the
air compressor to build air pressure
Once the air system is above 120psi use Snoop or soapy water to
check for air leaks around the pressure regulator fittings
Tighten any loose fittings causing air leaks. Remove and re-apply
PTFE tape or thread sealant if the leaking fitting is tight
Using Allison DOC or the transmission shifter verify that there are no
fault codes
Report the VIN to GILLIG service for documentation?

8.
9.

10.

11.

12.

13.

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LABOR INFORMATION:
1.5 to install pressure regulator; 1.5 to R&R Accumulator seals (if needed)
WARRANTY CLAIMS:
Repair is reimbursable through GILLIG Warranty only if replacement occurs during warranty
coverage period. Include the bulletin number in the “Describe Specific Repairs Section” (3C’s) of
GILLIG Application for Warranty Form. Populate the Multiple buses s
PARTS RETURN DISPOSITION:
N/A

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