Fault Code Guide

ECM. For other engine configurations, the system
controls a relay that provides a ground input as the
ECM feedback. With the system set for low range,
the relay coil receives no power and a ground input
is provided to the ECM through the closed switch
contacts of the relay. The operating power for this
relay is provided by a splice connection into the CHM
output that also controls the the AMU/AAVA solenoid.
Pressing the axle shift switch shifts the axle to high
range which causes the CHM to power both the the
AMU/AAVA solenoid and the relay. The energized
relay swings the switch contacts open and removes
the ground signal from the ECM, thus communicating
that the axle is in high range.
Nonmultiplexed Controls
On a vehicle with a manual transmission, the axle
shift switch that activates the two-speed axle is built
into the transmission shift knob. Because of the axle
shift switch location, the controls are nonmultiplexed.
The switch signals go directly to the AMU/AAVA sole-
noid that supplies the airflow to shift the two-speed
axle. The axle shift switch operates as a two-
position, latching switch with selections for low or
high speed ranges. If the engine is turned off, then
restarted, the two-speed axle defaults to low range.
With the engine running, selecting high range with
the axle shift switch within the transmission shift
knob closes the switch contacts and supplies power
to a normally closed AMU/AAVA solenoid. The ener-
gized solenoid opens and supplies compressed air to
the gear housing. The air sent to the housing shifts
the axle into high gear. Selecting low range with the
control switch opens the switch contacts and re-
moves the power supply to the AMU/AAVA solenoid,
stopping the air supply to the gear housing. With a
no-air condition at the housing, the axle shifts into
low gear.
For nonmultiplexed two-speed axle controls, axle
range feedback is supplied to the ECM. An axle
switch controls a relay that delivers a ground input to
the ECM. The absence or presence of this ground
signal indicates to the ECM what gear range the axle
is in. In low range, the axle switch closes and com-
pletes a ground path for the relay. Since the relay
receives power from a BHM-powered splice pack,
the completed ground path through the axle switch
allows the relay to energize. In high range, the relay
is not energized because the axle shift switch is
open and the relay is not grounded.
Suspension Dump
The suspension dump is available on a vehicle with
rear air suspension. When activated, the suspension
dump deflates the suspension air bags to lower the
rear of the vehicle. Most vehicles with a suspension
dump have an automatic refill that will inflate the rear
suspension when the engine is turned off.
NOTE: To protect the chassis, the suspension
dump is turned off when the vehicle speed
reaches approximately 5 mph (8 km/h).
The suspension dump is available in two options.
The first option has a lock solenoid that keeps the
suspension in the last selected state when the en-
gine is turned off. The second option does not have
a lock solenoid; when the engine is turned off, the
suspension inflates.
Suspension Dump Controls
With the engine running, the suspension dump can
be activated using a momentary, two-position smart
switch. Press the upper half of the suspension dump
switch to deflate the suspension air bags and lower
the rear of the vehicle. Press the upper half of the
suspension dump switch again to raise the suspen-
sion to its normal height. See Fig. 3. The rear sus-
pension also inflates when:
• Vehicle speed reaches approximately 5 mph (8
km/h);
• The engine is turned off and the autofill feature
activates.
When the suspension dump switch is activated, the
CHM transmits a low-current output to a normally
closed air management unit (AMU) solenoid. On
AAVA vehicles, the solenoid current is about 1.5
10/26/2001
f610597
Fig. 3, Suspension Dump Switch
Chassis Electrical Control Systems 54.40
General Information
Business Class M2 Workshop Manual, Supplement 22, September 2012 050/3