Fault Code Guide

Background Information
Chassis electrical control systems are optional fea-
tures on a Business Class
®
M2 vehicle. These fea-
tures include:
• interaxle lock
• axle shift
• suspension dump
• fifth wheel slide
• tag/pusher axle lift
• differential lock
The chassis electrical control systems are similar in
their electronic operation and control. Most of these
systems are activated by dash-mounted smart
switches.
Smart switches contain internal resistors that com-
municate switch identification, location, function, and
activation positions. Smart-switch signals are sent
directly to the Bulkhead Module (BHM). The BHM
reads the smart-switch resistor codes and communi-
cates the necessary signals that request system op-
eration.
Each smart switch is equipped with two light-emitting
diodes (LED). One LED provides switch illumination
while the other indicates when the switch is on and
the system is activated. For more information con-
cerning smart switches, see Section 54.14.
The BHM transmits system control requests via the
J1939 data line to the Chassis Module (CHM). The
CHM uses low-current outputs and digital inputs to
control and monitor the different chassis electrical
control systems.
Interaxle Lock
The interaxle lock is available on a vehicle with a
tandem axle or a tri-drive axle. When activated, the
interaxle differential is locked. This essentially makes
the driveshaft a solid connection between the rear
axles. Power entering the forward axle is transmitted
straight through to the rear axle. Driveline torque is
now delivered equally between the rear drive axles.
In slippery conditions, without the interaxle lock acti-
vated, one drive axle receives the majority of the
driveline torque when its wheels lose traction; how-
ever, with the interaxle lock activated, the rear drive
axles spin equally and improve traction by turning all
rear wheels at the same speed.
NOTE: Operating the vehicle with the interaxle
lock activated under normal driving conditions
increases driveline and tire wear. The interaxle
lock should be used only when improved trac-
tion is needed.
Interaxle Lock Controls
With the engine running, the interaxle lock can be
activated using a momentary, two-position smart
switch. Press the upper half of the interaxle lock
switch to activate the interaxle lock. Press the upper
half of the interaxle lock switch again to deactivate
the interaxle lock. See Fig. 1. If the interaxle lock is
activated and the engine is then turned off, the sys-
tem will deactivate the interaxle lock.
When the interaxle lock switch is activated, the CHM
transmits a low-current output to a normally closed
air management unit (AMU) solenoid. On auxiliary air
valve assembly (AAVA) vehicles, 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 AMU/AAVA solenoid opens and supplies
compressed air to the forward rear axle differential
housing. The air sent to the housing applies a lock to
the interaxle differential causing all driveline torque to
be shared equally by the rear axles.
Feedback from the AMU/AAVA solenoid is required
for correct operation of the interaxle lock controls.
Feedback is provided by a ground input to the CHM.
The ground input is delivered when the pressure
switch closes. On AAVA-equipped vehicles, the pres-
sure switch is in the air line. On AMU equipped ve-
hicles, the pressure switch is in the AMU module. A
10/26/2001
f610595
Fig. 1, Interaxle Lock Switch
Chassis Electrical Control Systems 54.40
General Information
Business Class M2 Workshop Manual, Supplement 22, September 2012 050/1