GMC 2011 Sierra Owner's Manual page 416

Use the following procedure to
correctly adjust Trailer Gain for
each towing condition:
1. Make sure the trailer brakes are
in proper working condition.
2. Connect a properly loaded
trailer to the vehicle and make
all necessary mechanical
and electrical connections.
See
for more information.
3. After the electrical connection is
made to a trailer equipped with
electric brakes:
A TRAILER CONNECTED
message will be briefly
displayed on the DIC.
The Trailer Brake Display
Page will appear on the
DIC showing TRAILER
GAIN and TRAILER
OUTPUT.
In the Trailer Output display
on the DIC, - - - - - - will
disappear if there is no
error present. Connecting
a trailer without electric
brakes will not clear the
six dashed lines.
4. Adjust the Trailer Gain by using
the gain adjustment (+ / -)
buttons on the Trailer Brake
Control Panel.
5. Drive the vehicle with the
trailer attached on a level road
surface representative of the
towing condition and free of
traffic at about 32 to 40 km/h
(20 to 25 mph) and fully apply
the Manual Trailer Brake Apply
Lever.
Adjusting trailer gain at speeds
lower than 32 to 40 km/h
(20 to 25 mph) may result
in an incorrect gain setting.
6. Adjust the Trailer Gain to just
below the point of trailer wheel
lock-up, indicated by trailer
wheel squeal or tire smoke
when a trailer wheel locks.
Trailer wheel lock-up may not
occur if towing a heavily loaded
trailer. In this case, adjust the
Trailer Gain to the highest
allowable setting for the towing
condition.
7. Re-adjust Trailer Gain any time
vehicle loading, trailer loading or
road surface conditions change
or if trailer wheel lock-up is
noticed at any time while towing.