In Service Operating Tests; Leakage Tests; Cylinder Head; Inlet, Discharge & Unloader - BENDIX BA-921 COMPRESSOR-STD-CLOSED ROOM Service Data Sheet

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 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 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.

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 Air Brake
System Handbook (BW5057) and on the back of the Dual
Circuit Brake System Troubleshooting Card (BW1396).

CYLINDER HEAD

Check for cylinder head gasket air leakage.
1. With the engine running, lower air system pressure to
60 psi and apply a soap solution around the cylinder
head. Check the gasket between the cylinder head and
valve plate assembly and 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 a
genuine Bendix maintenance kit available from an
authorized Bendix parts outlet.
INLET, DISCHARGE & UNLOADER
In order to test the inlet and discharge valves and the
unloader piston, it is necessary to have shop air pressure
and an assortment of fi ttings. A soap solution is also
required.
1. With the engine shut off, drain ALL air pressure from
the vehicle.
2. Disconnect the inlet and discharge lines and remove
the governor or its line or adapter fi tting.
3. Apply 120-130 psi shop air pressure to the unloader
port and soap the inlet port. Leakage at the inlet port
should not exceed 50 sccm.
4. Apply 120-130 psi shop air pressure to the discharge
port and then apply and release air pressure to the inlet
port. Soap the inlet port and note that leakage at the
inlet port does not exceed 20 sccm.
If excessive leakage is noted in Tests 3 or 4, replace or
repair the compressor using genuine Bendix replacements
or maintenance kits available from any authorized Bendix
parts outlet.
While it is possible to test for inlet, discharge, and unloader
piston leakage, it may not be practical to do so. Inlet and
discharge valve leakage can generally be detected by
longer compressor build-up and recovery times. Compare
current compressor build-up times with the last several
recorded times. Make certain to test for air system leakage,
as described under In Service Operating Tests, before
making a determination that performance has been lost.
Unloader leakage is generally exhibited by excessive
compressor cycling between the loaded and unloaded
condition.
1. With service and supply system leakage below the
maximum allowable limits and the vehicle parked,
bring system pressure to governor cut-out and allow
the engine to idle.
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