Fault Code Guide

grounded feedback circuit from the closed pressure
switch indicates that the interaxle lock is activated
and pressure is being supplied to the interaxle lock.
An open feedback circuit indicates that the interaxle
lock is not activated, or there is a malfunction; the
interlock switch is turned on but the pressure switch
is still open.
On a vehicle with a two-speed tandem axle, the elec-
tronic controls of the interaxle lock need to take into
account the range position of the axles. Both axles
must be in the same state (either high or low range)
for the interaxle lock to be turned on or remain
activated.
The electronic controls of the interaxle lock incorpo-
rate axle range position input as well as a second
pressure-switch feedback to the CHM. Utilizing the
feedback from the axle and pressure switches, the
control logic provides a time delay to ensure proper
function and to prevent damage. Once the interaxle
lock is activated, any axle-range change may cause
the interaxle lock to deactivate. A shift made from
one range to another while the interaxle lock is acti-
vated requires that both axles reach the change state
quickly before feedback indicates that the axles are
not in the same drive range. For a description of the
axle shift feature, see the information under the "Axle
Shift" heading in this subject.
Axle Shift
Two-speed axle shift is an option available on a ve-
hicle with a single or tandem axle. This option is
available on a vehicle with a manual or automatic
transmission; however, nonmultiplexed controls are
only available on a vehicle with a manual transmis-
sion. A vehicle with a two-speed axle allows the op-
erator to shift the drive axle between high and low
gear ranges. At any time, the operator may change
axle speeds to take mechanical advantage of differ-
ent driving conditions.
NOTE: Since axle speed can be changed at any
time, it is the driver’s responsibility to ensure that
axle speed selection is not done under harmful con-
ditions, such as selecting low gear when the engine
is at high rpm.
Multiplexed Controls
Multiplexed controls for the two-speed axle are avail-
able on a vehicle with a manual or automatic trans-
mission. With the engine running, the two-speed axle
is shifted between high and low ranges by a momen-
tary, two-position smart switch. Press the upper half
of the axle shift switch to change the axle speed.
See Fig. 2. If the engine is turned off, then restarted,
the two-speed axle will default to low range.
When the axle shift switch is activated, the CHM
transmits a low-current output to a normally closed
air management unit (AMU) solenoid. On AAVA ve-
hicles, the solenoid current is about 1.5 amps. The
2010 CHM does not have the current sourcing ability
to drive these solenoids, so a relay in the chassis
PDM is added to the circuit. The energized solenoid
opens and supplies compressed air to the gear hous-
ing. The air sent to the housing shifts the axle into
high gear. Pressing the axle shift switch again directs
the CHM to remove the current supply to the AMU/
AAVA solenoid, which stops the air supply to the
gear housing. With a no-air condition at the housing,
the axle shifts into low gear.
Feedback to the CHM is provided by switches lo-
cated on each drive axle. When an axle shifts into
the low-speed range, the axle switch closes and
completes a ground signal to the CHM. The CHM
uses the feedback signal(s) to determine if the sys-
tem is functioning properly (axle shift switch confirms
axle shift switch selection), or if there is a malfunc-
tion (axle shift switch does not confirm axle shift
switch selection). Each rear axle of a tandem-axle
vehicle has its own AMU/AAVA solenoid and axle
switch. The CHM also monitors the feedback of both
axle shift switches to make sure that the rear axles
are in the same speed range.
Two-speed axle feedback is also provided to the En-
gine Control Module (ECM). For the MBE900 engine,
the system provides axle position input via the J1939
data line messages between the CHM, BHM, and
10/26/2001
f610594
Fig. 2, Axle Shift Switch
Chassis Electrical Control Systems 54.40
General Information
Business Class M2 Workshop Manual, Supplement 22, September 2012050/2