GMC 2003 C4C042 Owner's Manual page 106

The accelerator and clutch pedals must be released in
order for the exhaust brake to engage. The exhaust brake
will not function when the cruise control feature is
active, and will automatically shut off when the
-
anti lock brake system is active.
CAUTION:
Be careful when using the exhaust brake or you
could lock the wheels and lose control of the
vehicle. Operators not familiar with an exhaust
brake should not use this brake on slippery
surfaces before learning to use it on dry surfaces.
If the vehicle's wheels begin to lock when using
the exhaust brake on slippery surfaces, switch the
exhaust brake off immediately. The exhaust
brake will turn off if the anti
is activated.
-
lock brake system
Two Speed Rear Axle Electric Shift
-
Control (If Equipped)
If your vehicle is equipped with this feature, you'll find
a label on the headliner above the windshield or in
another place near the driver that shows how to use it.
Be sure to follow the directions carefully.
Use the following directions based on the type of shift
control you have.
Manual Transmission
When the switch is up, the rear axle is in the high range.
Push the switch down to change to low range.
The rear axle shift control
switch for a manual
transmission is located
on the shift lever.
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