GMC 2011 Sierra Owner's Manual page 411

If the vehicle is equipped with an
Integrated Trailer Brake Control
(ITBC) System, the blunt cuts
exist, but are not connected further
in the harness. If an aftermarket
trailer brake controller is installed,
the ITBC must be disconnected.
Do not power both ITBC and
aftermarket controllers to control
the trailer brakes at the same time.
Pressing this button at the end of
the shift lever turns on and off the
Tow/Haul Mode.
This indicator light on the instrument
panel cluster comes on when the
Tow/Haul Mode is on.
Tow/Haul is a feature that assists
when pulling a heavy trailer or a
large or heavy load. See
for more
information.
Tow/Haul is designed to be most
effective when the vehicle and
trailer combined weight is at least
75 percent of the vehicle's Gross
Combined Weight Rating (GCWR).
See Weight of the Trailer under
Tow/Haul is most useful under
the following driving conditions:
When pulling a heavy trailer or
a large or heavy load through
rolling terrain.
When pulling a heavy trailer
or a large or heavy load in
stop-and-go traffic.
When pulling a heavy trailer
or a large or heavy load in busy
parking lots where improved low
speed control of the vehicle is
desired.
Operating the vehicle in Tow/Haul
when lightly loaded or with no
trailer at all will not cause damage.
However, there is no benefit to
the selection of Tow/Haul when
the vehicle is unloaded. Such a
selection when unloaded may
result in unpleasant engine and
transmission driving characteristics
and reduced fuel economy. Tow/
Haul is recommended only when
pulling a heavy trailer or a large
or heavy load.
.