Operational and Leakage Tests
1. Check all lines and fittings leading to and from
the air dryer for leakage and integrity. Repair any
leaks found.
2. Build system pressure to governor cutout and
note that the dryer purges with an audible es-
cape of air. Watch the system pressure and note
the pressure fall-off for a 10-minute period. If the
pressure drop exceeds 1 psi/minute from either
service reservoir, inspect the vehicle air systems
for sources of leakage and repair them. Refer to
troubleshooting information in Bendix or Haldex
service literature.
3. Check for excessive leakage around the purge
valve with the compressor in the loaded mode
(compressing air). Apply a soap solution to the
purge valve exhaust port and observe that leak-
age does not exceed a 1-inch (25-mm) bubble in
1 second. If the leakage exceeds the maximum
specified, refer to troubleshooting information in
the Bendix or Haldex service literature.
4. Build system pressure to governor cutout and
note that the dryer purges with an audible es-
cape of air. Fan the service brakes to reduce
system air pressure to governor cut-in. Note that
the system once again builds to full pressure and
is followed by a dryer purge. If the system does
not follow this pattern, refer to troubleshooting
information in the Bendix or Haldex service litera-
ture.
5. Check the operation of the end cover heater and
thermostat assembly during cold weather opera-
tion as follows:
5.1 Electric Power to the Dryer: With the igni-
tion in the ON position, check for voltage
to the heater and thermostat assembly
using a voltmeter or testlight. Unplug the
electrical connector at the air dryer and
place the test leads on each of the con-
nections of the female connector on the
vehicle power lead. If there is no voltage,
look for a blown fuse, broken wires, or
corrosion in the vehicle wiring harness.
Check to see if a good ground path ex-
ists.
5.2 Thermostat and Heater Assembly Opera-
tion: These tests are not possible except
in cold weather operation. Turn off the
ignition switch and cool the thermostat
and heater assembly to below 40°F
(4°C). Using an ohmmeter, check the re-
sistance between the electrical pins in the
air dryer connector half. The resistance
should be 1.5 to 3.0 ohms for the 12-volt
heater assembly and 6.0 to 9.0 ohms for
the 24-volt heater assembly.
NOTE: Some models of the AD–9 may
have a resistance reading of 1.0 to 2.5
ohms.
Warm the thermostat and heater assem-
bly to approximately 90°F (32°C) and
again check the resistance. The resis-
tance should exceed 1000 ohms. If the
resistance values obtained are within the
stated limits, the thermostat and heater
assembly is operating properly. If the re-
sistance values obtained are outside the
stated limits, replace the heater and ther-
mostat assembly.
42–03 Air Dryer Desiccant and
Coalescent Filter
Replacing
NOTE: Desiccant change intervals may vary
from vehicle to vehicle. Typical desiccant car-
tridge life is three years. However, if experience
has shown that extended or shortened life has
resulted on a particular installation, then the in-
terval can be increased or decreased accord-
ingly.
Bendix AD-9
1. Park the vehicle on a level surface, shut down
the engine, and set the parking brake. Chock the
tires.
2. Completely drain all air reservoirs. Air pressure
gauges should read 0 psi (0 kPa).
3. Loosen the desiccant cartridge with a strap
wrench at the base of the cartridge. Spin off the
cartridge and discard it.
4. Remove the O-ring from the threaded neck and
discard it.
5. Remove the coalescent filter and discard it.
Brakes42
Recreational Vehicle Chassis Maintenance Manual, July 201842/2
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