Bendix Commercial Vehicle Systems BENDIX 720CC COMPRESSOR - Manual (Page 7)

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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 and/or repair 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.
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 build-up 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 fix) 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 certified 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 required to raise the system
pressure a selected range (for example: from 90 to 120 psi,
or from 100 to 120 psi, etc.) in the vehicle’s maintenance
files. Subsequent build-up times throughout the vehicle’s
service life can then be compared to the first one recorded.
(Note: the 40 second guide in the test above does not
apply to this build-up time.) If the performance degrades
significantly over time, use the Advanced Troubleshooting
Guide for Air Brake Compressors, starting on page A-1 of
this document, to assist investigation of the cause(s).
Note: When comparing build-up times, be sure to make
an allowance for any air system modifications which would
cause longer times, such as adding air components or
reservoirs. Always check for air system leakage.
LEAKAGE TESTS
See the standard Air Brake System and Accessory Leakage
test on Page A-14, Test 2.
Note: Leakage in the air supply system (components
before the supply reservoir - such as the governor, air dryer,
reservoir drain cocks, safety valve, and check valves) will
not be registered on the vehicle dash gauges and must
be tested separately. Refer to the various maintenance
manuals for individual component leakage tests and the
Bendix “Test and Checklist” published in the Bendix Air
Brake System Handbook (BW5057) and on the back of
the Bendix Dual Circuit Brake System Troubleshooting
Card (BW1396).
CYLINDER HEAD
Check the cylinder head gaskets for air leakage.
1. With the engine running, lower the air system pressure
to 60 psi and apply a soap solution around the cylinder
head. Check the gaskets between the cylinder head
and the valve plate assembly, as well as the inlet reed
valve/gasket between the valve plate assembly and
crankcase for air leakage.
2. No leakage is permitted. If leakage is detected, replace
the compressor or repair the cylinder head using the
maintenance kit available from an authorized Bendix
parts outlet.