freightliner M2 Plus Maintenance Manual page 121

Brakes
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
escape 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 for a single vehicle; or 3
psi/minute for a tractor trailer, inspect the vehicle
air systems for sources of leakage, and repair
them. Refer to troubleshooting information in
Bendix 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
leakage 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 service literature.
4. Build system pressure to governor cutout, and
note that the dryer purges with an audible
escape 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 service literature.
5. Check operation of the end cover heater and
thermostat assembly during cold weather, as
follows:
5.1 Electric Power to the Dryer: With the
ignition or engine kill switch 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 connections 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 exists.
5.2 Thermostat and Heater Assembly
Operation: These tests are not possible
except in cold-weather operation. Turn off
the ignition switch, and cool the
Business Class M2 Plus Maintenance Manual, 02/10/2023
thermostat and heater assembly to below
40°F (4°C). Using an ohmmeter, check
resistance between the electrical pins in
the air dryer connector half. 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
assembly to approximately 90°F (32°C)
and again check the resistance.
Resistance should exceed 1000 ohms. If
resistance values obtained are within
stated limits, the thermostat and heater
assembly is operating properly. If
resistance values obtained are outside
stated limits, replace the heater and
thermostat assembly.
6. On the AD-IS/dryer reservoir module (DRM) only,
observe the pressure gauges of the vehicle as
system pressure builds from zero. The primary
gauge should rise until it reaches approximately
109±5 psi (751±34 kPa), then level off (or a
momentary slight fall) as the second pressure
protection valve opens supplying the secondary
reservoir. When the secondary pressure gauge
passes through approximately 55±5 psi (379±34
kPa) and then 85±5 psi (586±34 kPa) there
should be an associated levelling off (or
momentary slight fall) of pressure as the third
and fourth pressure protection valves open.
Finally, both the primary and secondary gauges
should reach their full pressure of approximately
130±5 psi (896±34 kPa). If the AD-IS does not
perform within the pressure ranges described
above, recheck using gauges known to be
accurate. If readings remain outside the ranges
outlined above, replace the AD-IS.
There are no kits available for servicing the
pressure protection valves.
WARNING
Do not attempt to adjust or service the pressure
protection valves. Incorrect pressure protection
valve settings can result in automatic application
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