NHTSA ID Number: 11006923
Manufacturer Communication Number: CBR-343 202408
TSB/Document Date: 2024-09-04
Summary
CBR: Mack
chassis with an mDRIVE transmission, there have been reports of complete shift assemblies and TECUs being replaced due to technicians being unable to pinpoint the source of the air leak.
9/3/24, 9:20 AM
Article
Preview Solution CBR-343-7
AMT Gear Box Air Leaks
Published 28 August 2024
Valid for all Mack and Volvo chassis with an AMT
On Volvo chassis with an I-Shift and Mack chassis with an mDRIVE transmission,
there have been reports of complete shift assemblies and TECUs being replaced due to
technicians being unable to pinpoint the source of the air leak.
Complaint:
Air Leak from the TECU Vent
AMT-D: An air leak is present at all times from the left or right side vent of the TECU
(Transmission Electronic Control Unit). Refer to Figures 2 and 3.
AMT-C: An air leak is present at all times from the gearbox case vent tube. Refer to
Figure 5.
Cause:
AMT-C & AMT-D: One or more TECU shift valves may be stuck open, allowing air to
escape through the venting solenoid. This issue can be intermittent. Testing has
shown that exercising the valves with TT by forcing shifts may temporarily resolve the
leak, but the valve may become stuck again.
TECU to Control Housing Seals: Leaks from these seals can cause similar
symptoms. These should be inspected before considering the replacement of major
components.
Internal Leaks: A leak between ports in the TECU due to a failed gasket can cause
erratic or undesired shifting, as air bleeds into the wrong ports. The seals should be
pliable with a peaked appearance; flattened sealing surfaces indicate a bad seal.
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Repair:
Replace faulty TECU to Control housing seals and retest.
Replace the TECU if necessary.
Complaint:
; Live UI
Air Leak During Shifting or Incomplete Gear Engagement
Symptoms:
Air leak occurs only during shifting.
Certain gears are unavailable during operation.
Slow shifting accompanied by an air leak.
Causes:
1. Shift Control Housing Leaks:
These leaks are typically due to screws backing out of the shift piston covers
(see Figure 4), allowing the end cap to vent air during a shift.
This issue usually affects only one shifter and in one direction.
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It may result in random gear unavailability, causing the TECU to avoid these
gears during operation.
A key cycle typically resets this, requiring the unit to relearn the affected gears.
2. Other Issues:
Broken shift forks, piston rods, or cracked internal pistons can also cause air
leaks.
A broken rod can be detected by manipulating the shift forks after removing the
control shift housing; one fork will feel unattached.
AMT-C: With external vent tubes, these failures will vent air from the tubes on
the right side of the gearbox.
AMT-D: Internally vented through the TECU, with leaks appearing at the TECU
vents.
Note: Do not transfer external vent tubes from an AMT-C to an AMT-D gearbox
during repair.
3. Shift Piston Failures:
Broken or cracked internal shift pistons will cause air leaks during shifting,
accompanied by slow shifts.
These failures will vent through the TECU vents on all AMT versions.
Leaks can also be caused by a leaking countershaft brake assembly, CVU, or
CCA. Testing is needed before replacing the control housing assembly.
Repair:
Replace the shift control housing, unless other damage is found.
; Live UI
Complaint:
Slow Gear Engagement from Neutral or Poor Shift Quality
Symptoms:
Delayed gear engagement from neutral.
Slow shifting between gears.
Harsh shifting.
Air leak heard from the clutch/flywheel housing.
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Causes:
1. Clutch Control Actuator (CCA) Leak:
Air leak from the Clutch Control Actuator inside the clutch housing.
2. Countershaft Brake Assembly Leak:
Perform clutch release tests using Tech Tool (TT) and listen for excessive air
escape.
Some air venting is normal, but prolonged blasts of air indicate an issue.
Repair Procedure:
1. Inspect and Replace:
Identify and replace the source of the leak.
2. Investigate the Leak Using Tech Tool:
Use Test 4111-08-04-01 - PWM Valves Clutch Activation, Transmission
Removed.
Or use Test 4111-08-03-06 - PWM Valves Clutch Activation, to pressurize the
clutch system and check for leaks.
Internal AMT-D venting
Vent Location: The internal venting of the AMT-D is provided in the bearing retainer plate
for the rear main shaft bearing. This allows venting and oil separation for return back to the
gearbox.
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Bearing Retainer Removed: The AMT-D gearbox vents through a port in the main case,
leading into the control housing and TECU, and is vented through the ports on the left side
of the main case.
The vent port is cast and drilled in the main case, leading to the control housing and
TECU.
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Internal ports in the control housing and the air path for gearbox case venting.
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TSB/Document ID: CBR-343 202408
Replacement Service Bulletin Number:
MFR Communication Date: 2024-08-28
MFR Internal Campaign ID/Software Version:
Communication Type: Service Bulletin/Repair Instructions
NHTSA Components: POWER TRAIN
MFR Component System:
MFR Component Subsystem:
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