8
A. If the pressure drop in the front axle reservoir ex-
ceeds 8-14 psi, check the air brake system for
excessive leakage and repair and re-test. If the ex-
cessive pressure drop persists, replace the AD-SP
™
air dryer.
B. If the pressure drop in the rear axle reservoir
exceeds 2 psi, check the air brake system for
excessive leakage and repair.
Build up system pressure to governor cut-out, wait 30
seconds for completion of the purge cycle, then "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 an AD-SP
™
air dryer
purge.
4. Check the operation of the SC-PR
™
single check
protection valve. Build system pressure to governor
cutout, wait 30 seconds for completion of the purge cycle,
then drain all air pressure from the supply reservoir while
observing the front axle reservoir dash gauge. Reservoir
pressure should decrease to approximately 95 psi and
stop. If front axle reservoir pressure continues to drop
replace the SC-PR
™
valve. Do not attempt to adjust
the SC-PR
™
valve. This valve is factory preset and mix-
adjustment can result in insufficient air pressure in the
front axle reservoir in the event of a line or component
failure. The rear axle reservoir pressure should remain
constant.
FIGURE 8 - AD-SP
™
SYSTEM PURGE AIR DRYER EXPLODED VIEW
O-RING
DESICCANT
CARTRIDGE
AD-SP
™
AIR
DRYER BODY
DELIVERY PORT
SUPPLY PORT
PURGE VALVE
O-RING
PURGE PISTON
SPRING
EXHAUST COVER
RETAINING RING
HEATER & THERMOSTAT
ASSEMBLY
CONNECTOR
RETAINING RING
RETAINING RING
CAP
PISTON
O-RINGS
CONTROL PORT
5. Check the operation of the heater and thermostat
assembly in the body during cold weather operation (if
possible) as follows:
A. Electric Power to the Heater and Thermostat
With the ignition or engine kill switch in the ON
position, check for voltage to the heater and
thermostat assembly using a voltmeter or test light.
Unplug the electrical connector at the air dryer and
place the test leads on each of the pins of the male
connector. 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.
B. Thermostat and Heater Operation
Turn off the ignition switch and cool the body
assembly to below 40 degrees Fahrenheit.
Note: If this test is performed in warm weather (above 30
deg. F) it may be necessary to remove the heater and
thermostat assembly and cool it in a freezer.
Using an ohmmeter, check the resistance between the
electrical pins in the connector. The resistance should be
1.5 to 1.7 ohms for the 12 volt heater assembly and 6.0 to
6.9 ohms for the 24 volt heater assembly. If the resistance
is higher than the maximum stated, replace the heater and
thermostat assembly.
Warm the heater and thermostat assembly to over 90
degrees Fahrenheit and again check the resistance. The
resistance should exceed 1000 ohms. If the resistance
values obtained are within the stated limits, the thermostat
and heater assembly is operating properly. If the resistance
values obtained are outside the stated limits, replace the
heater and thermostat assembly.