GMC 2003 C4C042, Owner'S Manual

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On the highway, with the axle in low range and the
transmission in top gear, you may shift the axle to high
range by raising the switch and releasing the accelerator
momentarily. Reapply the accelerator after you shift.
Under light load conditions, first shift the axle into the
high range with the vehicle at a standstill and then
operate the transmission normally.
Air Shift Control (Automatic Transmission)
The switch for the rear axle
shift control is located in the
instrument panel
switchbank.
Press the bottom of the switch to get to the low range.
Press the top of the switch to return to the high range.
The two
-
speed axle should be in low when you park
the vehicle.
You’ll need to engage the transmission, even moving the
vehicle a little, to be sure the axle is in low range. Then
shift into PARK (P), apply the parking brake and shut
off the engine.
Also, always start the vehicle in motion with the
two
-
speed axle in the low position. To shift up from
low to high, press the top of the rear axle shift control
switch and press down on the accelerator. When you
shift the axle from high to low, press the bottom of the
switch, release the accelerator and then quickly press it
down again.
Air Suspension (If Equipped)
Your vehicle may be equipped with the Hendrickson
HAS Series single
-
axle air suspension which is designed
for single
-
axle on
-
highway use. The feature is available
in 19,000 lb. (8 618.3 kg), 21,000 lb. (9 525.4 kg) and
23,000 lb. (10 432.6 kg) capacities for operations which
require extra cubic capacity and a reduced deck height.