5
SERVICE TESTS
GENERAL
The compressor operating and leakage tests listed below
need not be performed on a regular basis. These tests
should be performed when it is suspected that leakage is
substantially affecting compressor buildup performance, or
when it is suspected that the charging system is “cycling”
between the loaded (pumping) and unloaded (charging
system stops delivering air to the brake system reservoirs)
modes due to unloader system leakage.
IN-SERVICE OPERATING TESTS
Compressor Performance: Build-up Test
This test is performed with the vehicle parked and the
engine operating at maximum recommended governed
speed. Fully charge the air system to governor cut-out (air
dryer purges). Pump the service brake pedal to lower the
system air pressure below 80 psi using the dash gauges.
As the air pressure builds back up, measure the time
from when the dash gauge passes 85 psi to the time it
passes 100 psi. The time should not exceed 40 seconds.
If the vehicle exceeds 40 seconds, test for (and fi x) any
air leaks and then re- test the compressor performance.
If the vehicle does not pass the test the second time,
use the Advanced Troubleshooting Guide for Air Brake
Compressors,
starting on page A-1
of this document to
assist your investigation of the cause(s).
Note:
All new vehicles are certifi ed using the FMVSS
121 test (paragraph S5.1.1) by the vehicle manufacturer,
however the above test is a useful guide for in-service
vehicles.
Optional Comparative Performance Check
It may be useful to also conduct the above test with the
engine running at high idle (instead of maximum governed
speed), and record the time taken to raise the system
pressure to a selected range (for example, from 90 to
120 psi, or from 100 to 120 psi, etc.) and record it in the
vehicle’s maintenance fi les. Subsequent build-up times
throughout the vehicle’s service life can then be compared
to the fi rst one recorded. (Note: the 40 second guide in
the test above does not apply to this build-up time.) If the
performance degrades signifi cantly over time, you may
use the Advanced Troubleshooting Guide for Air Brake
Compressors, starting on page A-1 of this document, to
assist in the investigation of the cause(s).
Note:
When comparing build-up times, be sure to make
an allowance for any air system modifi cations which would
cause longer times, such as adding air components or
reservoirs. Always check for air system leakage.
symptoms and corrective action to be taken. In addition,
Bendix has developed the “Bendix Air System Inspection
Cup” or BASIC kit to help substantiate suspected excessive
oil passing. The steps to be followed when using the BASIC
kit are presented in APPENDIX B, on page A-16.
COMPRESSOR DRIVE
Check for noisy compressor operation, which could indicate
excessive drive component wear. Adjust and/or replace
as necessary. Check all compressor mounting bolts and
retighten evenly if necessary. Check for leakage. Repair
or replace parts as necessary.
CHARGING SYSTEM UNLOADING &
GOVERNOR
Note:
The Bendix
®
360cc (DLU)-style single cylinder
compressor does not contain components to unload
the compressor. Therefore, the compressor pumps
continuously. In most systems supplied by International,
a turbo cut-off style air dryer, governor and discharge line
unloader valve are used to unload the system (i.e. air is
not being delivered to the brake system reservoirs). When
system unloading occurs, air from the compressor will fl ow
out the exhaust port of the discharge line unloader valve.
Refer to Figure 6.
Test and inspect the unloading system (i.e. air dryer and
governor) for proper operation and pressure settings.
1. Make certain the unloader system lines (illustrated in
Figure 6) are connected and leak free.
2. Cycle the charging system between the loaded and
unloaded mode several times. This can be achieved by
applying the brakes to bleed down the system pressure.
Make certain that the governor cuts-in (charging
system resumes compressing air) at a minimum of
105 psi. Governor cut-out (charging system stops
delivering air to the brake system reservoirs) should be
approximately 15 - 20 psi greater than cut-in pressure.
Adjust or replace the governor as required.
3. Note that the charging system cycles to the loaded and
unloaded conditions promptly. If prompt action is not
noted, repair or replace: the governor; the discharge
line unloader valve; and/or the air dryer purge valve
assembly.
IMPORTANT NOTE
Replacement air governors must have a minimum
cut-in pressure of 100 psi.
The cut-in pressure is the
lowest system pressure registered in the gauges before
the compressor resumes compressing air.
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