GMC 2010 Sierra 1500 Crew Cab Owner's Manual page 451

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 display.
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 20 to 25 mph
(32 to 40 km/h) and fully apply the Manual Trailer
Brake Apply lever.
Adjusting trailer gain at speeds lower than 20 to
25 mph (32 to 40 km/h) 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.
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