7
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 and
proper unloader mechanism operation. Repair or replace
parts as necessary.
COMPRESSOR UNLOADER & GOVERNOR
Test and inspect the compressor and governor unloader
system for proper operation and pressure setting.
1. Check for leakage at the unloader port. Replace leaking
or worn o-rings.
2. Make certain the unloader system lines are connected
as illustrated in Figure 5.
3. Cycle the compressor through the loaded and unloaded
cycle several times. Make certain that the governor
cuts-in (compressor resumes compressing air) at a
minimum of 105 psi (cut-out should be approximately
15–20 psi greater than cut-in pressure). Adjust or
replace the governor as required.
4. Note that the compressor cycles to the loaded and
unloaded conditions promptly. If prompt action is not
noted, repair or replace the governor and/or repair the
compressor unloader.
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 following compressor operating and leakage tests
need not be performed on a regular basis. These tests
should be performed: a) when it is suspected that leakage
is substantially affecting compressor buildup performance;
or b) when it is suspected that the compressor is “cycling”
between the loaded (pumping) and unloaded (non-
pumping) modes due to unloader 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
this test is a useful guide for in-service vehicles.
Optional Comparative Performance Check
It may be useful to also conduct the previous 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 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, 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 modifications which would
cause longer times, such as adding air components or
reservoirs. Always check for air system leakage.
