Rear Gear Train Engines with Excessive Metallic Gear Train Noise During Air Compress... Page 1 of 4

Technical Service Bulletin Number
TSB140157

Technical Service Bulletin

Subject
Rear Gear Train Engines with Excessive Metallic Gear Train Noise During Air Compressor Operation

Issue
Product Affected





ISB6.7 CM2150
ISB6.7 CM2250
ISB6.7 CM2250 B101
ISB6.7 CM2350 B103

This document only applies to rear gear train equipped engines with the following low mount air
compressor options:




CP9573
CP9686
CP9574

Manufacturing variations can lead to excessive gear lash between the air compressor drive gear and
the camshaft gear. This gear lash can lead to excessive metallic gear train noise when the air
compressor is loaded.
Gear train noise is common because of the design of reciprocating air compressors. However on
specific units, variations in gear runout and air compressor mounting can introduce backlash in the
gear train which amplifies the noise.
Gear train noise during air compressor operation will not lead to progressive engine damage or
mechanical failure.

Verification



Turn the steering wheel to load the air compressor while the metallic noise is the loudest.
If the metallic noise is noticeably reduced, see the Resolution section below.

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Rear Gear Train Engines with Excessive Metallic Gear Train Noise During Air Compress... Page 2 of 4

The following audio link is a representation of an air compressor drive gear producing excessive
metallic gear train noise.
NOTE: http://tsb.cumminsvirtualcollege.com/audio/TSB140157_BadGearTrainNoise.mp4
The following audio link is a representation of an air compressor drive gear producing acceptable gear
train noise.
NOTE: http://tsb.cumminsvirtualcollege.com/audio/TSB140157_AcceptableGearTrainNoise.mp4
NOTE: A metallic noise can be heard when the air compressor is loaded, indicating that the
gears are impacting each other. This is not to be mistaken for the puffing sound of the air
compressor while it is pumping, which is caused by air pressure pulses.

Resolution
Gear lash can be reduced by timing the air compressor using the following instructions:
Remove the air compressor from the engine. See the corresponding Service Manual. Reference
Procedure 012-014 in Section 12.
Use the barring tool, Part Number 3824591, or equivalent, to rotate the crankshaft until the number 1
cylinder is at Top Dead Center (TDC).

Figure 1, Use of the Barring Tool
Align the vibration damper so that the TDC indicator on the vibration damper is at the 12-o'clock
position.

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Rear Gear Train Engines with Excessive Metallic Gear Train Noise During Air Compress... Page 3 of 4

If no TDC mark is present on either the vibration damper or the crankshaft speed indicator ring, align
the large gap in the crankshaft speed indicator ring to the 5-o'clock position. The dowel pin can also be
visible in the 9-o'clock position.

Figure 2, TDC Indicator
View the air compressor with the drive gear in front. Figure 3 has the top mounting hole (2) oriented so
that it is on top. The normal direction of rotation of the air compressor gear is counterclockwise. Align
the air compressor gear timing mark (4) with the timing notch (1) on the air compressor housing.
Locate the 5th drive gear tooth from the timing mark on the drive gear, in the clockwise direction (3).
Mark the 5th drive gear tooth with ink or paint.
Rotate the air compressor drive gear in the counterclockwise direction (5) to align the newly marked
tooth with the timing notch on the air compressor housing.
In this modified timing position, the factory set timing mark on the air compressor gear should point
towards the bottom mounting hole of the air compressor.

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Rear Gear Train Engines with Excessive Metallic Gear Train Noise During Air Compress... Page 4 of 4

Figure 3, Air Compressor Drive Gear Teeth
Install the air compressor. See Procedure 012-014 in Section 14 of the appropriate service manual.

Warranty Statement
The information in this document has no effect on present warranty coverage or repair practices, nor
does it authorize TRP or Campaign actions.

Document History
Date
2014-11-5
2016-3-1

Details
Module Created
Updated title. Added Product Affected section. Revised the Verification section.

Last Modified: 01-Mar-2016
Copyright © 2000-2010 Cummins Inc. All rights reserved.

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4/5/2016