towing GMC SIERRA 2015 Owner's Guide

Black plate (69,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
Driving and Operating 9-69
Warning (Continued)
When towing a trailer:
.Do not drive with the liftgate,
trunk/hatch, or rear-most
window open.
.Fully open the air outlets on
or under the instrument
panel.
.Also adjust the climate
control system to a setting
that brings in only outside air.
See“Climate Control
Systems” in the Index.
For more information about
Carbon Monoxide, see Engine
Exhaust on page 9-32.
Towing a trailer requires a certain
amount of experience. The
combination you are driving is
longer and not as responsive as the
vehicle itself. Get acquainted with
the handling and braking of the rig
before setting out for the open road. Before starting, check all trailer hitch
parts and attachments, safety
chains, electrical connectors, lamps,
tires, and mirrors. If the trailer has
electric brakes, start the
combination moving and then apply
the trailer brake controller by hand
to be sure the brakes work.
During the trip, check occasionally
to be sure that the load is secure
and the lamps and any trailer
brakes still work.
Following Distance
Stay at least twice as far behind the
vehicle ahead as you would when
driving the vehicle without a trailer.
This can help to avoid heavy
braking and sudden turns.
Passing
More passing distance is needed
when towing a trailer. The
combination will not accelerate as
quickly and is longer so it is
necessary to go much farther
beyond the passed vehicle before
returning to the lane.
Backing Up
Hold the bottom of the steering
wheel with one hand. To move the
trailer to the left, move that hand to
the left. To move the trailer to the
right, move your hand to the right.
Always back up slowly and,
if possible, have someone
guide you.
Making Turns
{Caution
Making very sharp turns while
trailering could cause the trailer to
come in contact with the vehicle.
The vehicle could be damaged.
Avoid making very sharp turns
while trailering.
When turning with a trailer, make
wider turns than normal. Do this so
the trailer will not strike soft
shoulders, curbs, road signs, trees,
or other objects. Avoid jerky or
sudden maneuvers. Signal well in
advance.

Black plate (70,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
9-70 Driving and Operating
If the trailer turn signal bulbs burn
out, the arrows on the instrument
cluster will still flash for turns. It is
important to check occasionally to
be sure the trailer bulbs are still
working.
Driving on Grades
Reduce speed and shift to a lower
gearbefore starting down a long or
steep downgrade. If the
transmission is not shifted down, the
brakes might get hot and no longer
work well.
Vehicles can tow in D (Drive). Shift
the transmission to a lower gear if
the transmission shifts too often
under heavy loads and/or hilly
conditions.
The Tow/Haul Mode may be used if
the transmission shifts too often.
See Tow/Haul Mode on page 9-39.
When towing at high altitude on
steep uphill grades, consider the
following: Engine coolant will boil at
a lower temperature than at normal
altitudes. If the engine is turned off immediately after towing at high
altitude on steep uphill grades, the
vehicle may show signs similar to
engine overheating. To avoid this,
let the engine run while parked,
preferably on level ground, with the
transmission in P (Park) for a few
minutes before turning the engine
off. If the overheat warning comes
on, see
Engine Overheating on
page 10-21.
Parking on Hills
{Warning
Parking the vehicle on a hill with
the trailer attached can be
dangerous. If something goes
wrong, the rig could start to move.
People can be injured, and both
the vehicle and the trailer can be
damaged. When possible, always
park the rig on a flat surface. If parking the rig on a hill:
1. Press the brake pedal, but do
not shift into P (Park) yet. Turn
the wheels into the curb if facing
downhill or into traffic if facing
uphill.
2. Have someone place chocks under the trailer wheels.
3. When the wheel chocks are in place, release the regular brakes
until the chocks absorb the load.
4. Reapply the brake pedal. Then apply the parking brake and shift
into P (Park).
5. Release the brake pedal.
Leaving After Parking on a Hill
1. Apply and hold the brake pedal.
2. Start the engine.
3. Shift into a gear.
4. Release the parking brake.
5. Let up on the brake pedal.
6. Drive slowly until the trailer is clear of the chocks.

Black plate (71,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
Driving and Operating 9-71
7. Stop and have someone pick upand store the chocks.
Maintenance when Trailer
Towing
The vehicle needs service more
often when pulling a trailer. See
Maintenance Schedule on
page 11-3. Things that are
especially important in trailer
operation are automatic
transmission fluid, engine oil, axle
lubricant, belts, cooling system, and
brake system. It is a good idea to
inspect these before and during
the trip.
Check periodically to see that all
hitch nuts and bolts are tight.
Trailer Towing
If the vehicle has a diesel engine,
see the Duramax diesel
supplement.
If the vehicle is bi-fuel, see the
bi-fuel supplement. Do not tow a trailer during break-in.
See
New Vehicle Break-In on
page 9-23 for more information.
{Warning
The driver can lose control when
pulling a trailer if the correct
equipment is not used or the
vehicle is not driven properly. For
example, if the trailer is too
heavy, the brakes may not work
well or even at all. The driver and
passengers could be seriously
injured. The vehicle may also be
damaged; the resulting repairs
would not be covered by the
vehicle warranty. Pull a trailer
only if all the steps in this section
have been followed. Ask your
dealer for advice and information
about towing a trailer with the
vehicle.
{Caution
Pulling a trailer improperly can
damage the vehicle and result in
costly repairs not covered by the
vehicle warranty. To pull a trailer
correctly, follow the advice in this
section and see your dealer for
important information about
towing a trailer with the vehicle.
See Vehicle Load Limits on
page 9-15 for more information
about the vehicle's maximum load
capacity.
To identify the trailering capacity of
the vehicle, read the information in
“Weight of the Trailer” later in this
section.

Black plate (78,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
9-78 Driving and Operating
a weight-distributing hitch, make
sure not to exceed the RGAWR
before applying the weight
distribution spring bars.
Weight of the Trailering
Combination
It is important that the combination
of the tow vehicle and trailer does
not exceed any of its weight
ratings—GCWR, GVWR, RGAWR,
Trailer Weight Rating, or Tongue
Weight. The only way to be sure it is
not exceeding any of these ratings
is to weigh the tow vehicle and
trailer combination, fully loaded for
the trip, getting individual weights
for each of these items.
Towing Equipment
Hitches
The correct hitch equipment helps
maintain combination control. Many
trailers can be towed with a
weight-carrying hitch which simply
features a coupler latched to the
hitch ball, or a tow eye latched to a
pintle hook. Other trailers may
require a weight-distributing hitch
that uses spring bars to distribute
the trailer tongue weight among the
two vehicle and trailer axles.
Fifth-wheel and gooseneck hitches
may also be used. See “Weight of
the Trailer Tongue” underTrailer
Towing on page 9-71 for rating limits
with various hitch types. If a step-bumper hitch will be used,
the bumper could be damaged in
sharp turns. Make sure there
is ample room when turning to avoid
contact between the trailer and the
bumper.
Consider using sway controls with
any trailer. Ask a trailering
professional about sway controls or
refer to the trailer manufacturer's
recommendations and instructions.
Weight-Distributing Hitch and
Adjustment
A weight distributing hitch may be
useful with some trailers. Use the
following guidelines to determine if a
weight distributing hitch should
be used.

Black plate (79,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
Driving and Operating 9-79
Vehicle SeriesTrailer WeightWeight Distributing Hitch
Usage Hitch Distribution
1500 Up to 3175 kg (7,000 lb) OptionalRefer to trailer
manufacturer ’s
recommendation
1500 Over 3175 kg (7,000 lb) Required50%
2500/3500 Up to 8165 kg (18,000 lb) OptionalRefer to trailer
manufacturer ’s
recommendation
1. Body to Ground Distance
2. Front of Vehicle
When using a weight-distributing
hitch, measure distance (1) before
coupling the trailer to the hitch ball.
Measure the height again after the
trailer is coupled and adjust the
spring bars so the distance (1) is as
close as possible to halfway
between the two measurements.
Fifth-Wheel and Gooseneck
Trailering
Fifth-wheel and gooseneck trailers
can be used with many pickup
models. These trailers place a
larger percentage of the weight (kingpin weight) on the tow vehicle
than conventional trailers. Make
sure this weight does not cause the
vehicle to exceed GAWR or GVWR.
Fifth-wheel or gooseneck kingpin
weight should be 15 to 25% of the
trailer weight up to the maximum
amount specified in the trailering
chart for the vehicle. See
“Weight of
the Trailer” underTrailer Towing on
page 9-71.
The hitch should be located in the
pickup bed so that its centerline is
over or slightly in front of the rear
axle. Take care that it is not so far

Black plate (80,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
9-80 Driving and Operating
forward that it will contact the back
of the cab in sharp turns. This is
especially important for short box
pickups. Trailer pin box extensions
and sliding fifth-wheel hitch
assemblies can help this condition.
There should be at least 15 cm (6 in)
of clearance between the top of the
pickup box and the bottom of the
trailer shelf that extends over
the box.
Make sure the hitch is attached to
the tow vehicle frame rails. Do not
use the pickup box for support.
Safety Chains
Always attach chains between the
vehicle and the trailer. Cross the
safety chains under the tongue of
the trailer to help prevent the tongue
from contacting the road if it
becomes separated from the hitch.
Instructions about safety chains
may be provided by the hitch
manufacturer or by the trailer
manufacturer. If the trailer being
towed weighs up to 2 271 kg
(5,000 lb) with a factory-installedstep bumper, safety chains may be
attached to the attaching points on
the bumper; otherwise, safety
chains should be attached to holes
on the trailer hitch platform. Always
leave just enough slack so the
combination can turn. Never allow
safety chains to drag on the ground.
Trailer Brakes
A loaded trailer that weighs more
than 900 kg (2,000 lb) needs to have
its own brake system that is
adequate for the weight of the
trailer. Be sure to read and follow
the instructions for the trailer brakes
so they are installed, adjusted, and
maintained properly.
Do not tap into the vehicle's
hydraulic brake system.
Auxiliary Battery
The auxiliary battery provision can
be used to supply electrical power
to additional equipment that may be
added, such as a slide-in camper.
If equipped, this relay will be on the
driver side of the vehicle, next to the
underhood electrical center.
Be sure to follow the proper
installation instructions included with
any electrical equipment that is
installed.
{Caution
Leaving electrical equipment on
for extended periods will drain the
battery. Always turn off electrical
equipment when not in use and
do not use equipment that
exceeds the maximum amperage
rating of 30 amps for the auxiliary
battery provision.
Trailer Wiring Harness
The vehicle is equipped with one of
the following wiring harnesses for
towing a trailer or hauling a slide-in
camper.

Black plate (81,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
Driving and Operating 9-81
Basic Trailer Wiring
All regular, double cab, and crew
cab pickups have a seven-wire
trailer towing harness.
For vehicles not equipped with
heavy-duty trailering, the harness is
secured to the vehicle's frame
behind the spare tire mount. The
harness requires the installation of a
trailer connector, which is available
through your dealer.
Heavy-Duty Trailer Wiring Harness
PackageFor vehicles equipped with
heavy-duty trailering, the harness
connector is mounted in the bumper.
The seven-wire harness contains
the following trailer circuits:
.Green/Violet: Left Stop/Turn
Signal
.Yellow/Gray: Right Stop/Turn
Signal
.Gray/Brown: Taillamps/Parking
Lamps
.White: Ground
.White/Green: Back-up Lamps
.Red/Green: Battery Feed
.Dark Blue: Trailer Brake
If charging a remote (non-vehicle)
battery, press the Tow/Haul Mode
button, if equipped, at the end of the
shift lever. This will boost the
vehicle system voltage and properly
charge the battery. If the trailer is
too light for Tow/Haul Mode, or the
vehicle is not equipped with Tow/ Haul, turn on the headlamps as a
second way to boost the vehicle
system and charge the battery.
Camper/Fifth-Wheel Trailer Wiring
Package
The seven-wire camper harness is
under the rear bumper, attached to
the frame near the rear
crossmember. A connector must be
added to the wiring harness that
connects to the camper.
The harness contains the following
camper/trailer circuits:
.Green/Violet: Left Stop/Turn
Signal
.Yellow/Gray: Right Stop/Turn
Signal
.Gray/Brown: Taillamps/Parking
Lamps
.White: Ground
.White/Green: Back-up Lamps
.Red/Green: Battery Feed
.Dark Blue: Trailer Brake

Black plate (82,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
9-82 Driving and Operating
If the vehicle is equipped with the
heavy-duty trailering option, see
“Heavy-Duty Trailer Wiring Harness
Package”earlier in this section.
When the camper-wiring harness is
ordered without the heavy-duty
trailering package, a seven-wire
harness with a seven-pin connector
is at the rear of the vehicle and is
tied to the vehicle's frame.
Electric Brake Control Wiring
Provisions
These wiring provisions are
included with the vehicle as part of
the trailer wiring package. These
provisions are for an electric brake
controller.
The harness should be installed by
your dealer or a qualified service
center.
Tow/Haul Mode
Pressing this button at the end of
the shift lever turns on and off the
Tow/Haul Mode.
This indicator light on the instrument
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
Tow/Haul
Mode on page 9-39.
Tow/Haul is designed to be most
effective when the vehicle and
trailer combined weight is at least
75% of the vehicle's Gross
Combined Weight Rating (GCWR).
See “Weight of the Trailer” under
Trailer Towing on page 9-71. 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.

Black plate (85,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
Driving and Operating 9-85
.A trailer without electric brakes
is connected (no DIC message
will display).
.A trailer with electric brakes has
become disconnected (a
CHECK TRAILER WIRING
message will also display on
the DIC).
.There is a fault present in the
wiring to the trailer brakes (a
CHECK TRAILER WIRING
message will also display on
the DIC).
.The ITBC system is not working
due to a fault (a SERVICE
TRAILER BRAKE SYSTEM
message will also display in
the DIC).
Manual Trailer Brake Apply
The Manual Trailer Brake Apply
Lever is used to apply the trailer ’s
electric brakes independent of the
vehicle’s brakes. Sliding the lever to
the left will apply only the trailer
brakes. Use this lever to adjust
Trailer Gain to properly adjust the
power output to the trailer brakes. The trailer's and the vehicle's brake
lamps will come on when either
vehicle brakes or manual trailer
brakes are applied.
Trailer Gain Adjustment Procedure
Trailer Gain should be set for a
specific trailering condition and must
be adjusted anytime vehicle loading,
trailer loading, or road surface
conditions change.
{Warning
Trailer brakes that are
over-gained or under-gained may
not stop the vehicle and the trailer
as intended and can result in a
crash. Always follow the
instructions to set the Trailer Gain
for the proper trailer stopping
performance.
Use the following to adjust Trailer
Gain for each towing condition:
1. 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.
2. Adjust the Trailer Gain, using the Trailer Gain Adjustment Buttons,
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.

Black plate (86,1)GMC 2015i Sierra Denali Owner Manual (GMNA-Localizing-U.S./Canada/
Mexico-8431500) - 2015 - crc - 6/20/14
9-86 Driving and Operating
3. Readjust Trailer Gain anytimevehicle loading, trailer loading,
or road surface conditions
change or if trailer wheel lock-up
is noticed at any time while
towing.
Other ITBC-Related DIC Messages
In addition to displaying TRAILER
GAIN and OUTPUT through the
DIC, trailer connection and ITBC
system status are displayed on
the DIC.
TRAILER CONNECTED: This
message will briefly display when a
trailer with electric brakes is first
connected to the vehicle. This
message will automatically turn off
in about 10 seconds. This message
can be acknowledged before it
automatically turns off. CHECK TRAILER WIRING: This
message will display if:
.The ITBC system first
determines connection to a
trailer with electric brakes and
then the trailer harness becomes
disconnected from the vehicle.
If the disconnect occurs while
the vehicle is stationary, this
message will automatically turn
off in about 30 seconds. This
message will also turn off if it is
acknowledged or if the trailer
harness is reconnected.
If the disconnect occurs while
the vehicle is moving, this
message will continue until the
ignition is turned off. This
message will also turn off if it is
acknowledged or if the trailer
harness is reconnected.
.There is an electrical fault in the
wiring to the trailer brakes. This
message will continue as long
as there is an electrical fault inthe trailer wiring. This message
will also turn off if it is
acknowledged.
To determine if the electrical fault is
on the vehicle side or trailer side of
the trailer wiring harness
connection:
1. Disconnect the trailer wiring harness from the vehicle.
2. Turn the ignition off.
3. Wait 10 seconds, then turn the ignition back to RUN.
4. If the CHECK TRAILER WIRING message reappears, the
electrical fault is on the
vehicle side.
If the CHECK TRAILER WIRING
message only reappears when
connecting the trailer wiring
harness to the vehicle, the
electrical fault is on the
trailer side.
SERVICE TRAILER BRAKE
SYSTEM: This message will display
when there is a problem with the
ITBC system. If this message