Page 201 of 376

Trailer Connections
Heavy-Duty Trailer Wiring Package
The vehicle may be equipped with an eight-wire trailer
towing harness. This harness, with a seven-pin universal
heavy-duty trailer connector, is attached to a bracket
on the platform hitch.
The Center High-Mounted Stoplamp (CHMSL) wire is
tied next to the trailer wiring harness for use with a
trailer.
The eight-wire harness contains the following trailer
circuits:
•Yellow: Left Stop/Turn Signal
•Dark Green: Right Stop/Turn Signal
•Brown: Taillamps
•White: Ground
•Light Green: Back-up Lamps
•Light Blue: CHMSL
•Red: Battery Feed
•Dark Blue: Trailer Brake
Trailer Connections
(C7/C8 Models Only)
If the vehicle has these trailer connections, they are
located at the rear of the cab for a tractor and at the rear
of the chassis when towing a trailer. There will be an
air connection and an electrical connection.
Be sure that the couplings to the trailer are made
correctly.
Air Connections
If the trailer air line connections will not be connected to
another piece of equipment, be sure to replace their
protective caps.
Electrical Connections
This system is a seven-wire cable that will connect to
the trailer, supplying power to the trailer’s lighting
system. When not in use, it is stored in the trailer
air-hose storage bracket.
4-25
Page 202 of 376
Page 203 of 376

Service............................................................5-3
Accessories and Modifications..........................5-3
California Proposition 65 Warning.....................5-4
California Perchlorate Materials Requirements.....5-4
Doing Your Own Service Work.........................5-5
Engine Fan Breakage.....................................5-6
Fuel................................................................5-6
Gasoline Octane............................................5-6
Gasoline Specifications....................................5-7
Additives.......................................................5-7
Diesel Engine Fuel.........................................5-8
What Fuel to Use...........................................5-8
Very Cold Weather Operation.........................5-11
Water in Fuel...............................................5-11
Running Out of Fuel.....................................5-17
Fuel Filter Replacement.................................5-18
Fuels in Foreign Countries.............................5-19
Filling the Tank............................................5-20
Filling a Portable Fuel Container.....................5-22
Checking Things Under the Hood....................5-22
Hood Latches...............................................5-23
Engine Oil (DURAMAX Diesel Engine).............5-24
Engine Oil (Caterpillar Diesel Engine)..............5-27
Engine Oil (Isuzu Diesel Engine).....................5-28
Engine Oil (Gasoline Engine)..........................5-32
Engine Oil Life System (Gasoline Engine)........5-35Engine Oil Life System (DURAMAX/Isuzu
Diesel Engines).........................................5-36
Engine Oil Life System (Caterpillar
Diesel Engine)..........................................5-38
Engine Air Cleaner/Filter................................5-38
Automatic Transmission Fluid.........................5-42
Manual Transmission Fluid.............................5-42
Cooling System............................................5-43
Engine Coolant (Caterpillar Diesel Engines)......5-47
Coolant Surge Tank Pressure Cap..................5-47
Engine Overheating.......................................5-48
Engine Fan Noise.........................................5-49
Power Steering Fluid.....................................5-49
Windshield Washer Fluid................................5-50
Brakes........................................................5-52
Battery........................................................5-61
Jump Starting...............................................5-62
Rear Axle.......................................................5-67
Rear Axle Shift Motor....................................5-68
Four-Wheel Drive............................................5-68
Front Axle......................................................5-69
Noise Control System.....................................5-70
Tampering with Noise Control System
Prohibited.................................................5-70
Bulb Replacement..........................................5-71
Section 5 Service and Appearance Care
5-1
Page 204 of 376

Windshield Wiper Blade Replacement..............5-71
Other Service Items........................................5-72
Fuel Filter....................................................5-72
Primary Fuel Filter and Water Separator..........5-73
Secondary Fuel Filter and Water Separator/
Heater (Caterpillar Diesel)............................5-74
Front Wheel Bearings with
Oil-Filled Hubs..........................................5-75
Tires..............................................................5-76
Inflation - Tire Pressure.................................5-77
Wheel Loading.............................................5-78
Dual Tire Operation.......................................5-78
When It Is Time for New Tires.......................5-78
Buying New Tires.........................................5-79
Wheel Alignment and Tire Balance..................5-80
Tightening the Wheel Nuts.............................5-80
Wheel Replacement......................................5-84
If a Tire Goes Flat........................................5-85
Appearance Care............................................5-87
Interior Cleaning...........................................5-87
Fabric/Carpet...............................................5-88
Instrument Panel, Vinyl, and Other
Plastic Surfaces........................................5-89
Care of Safety Belts......................................5-89
Weatherstrips...............................................5-89
Washing Your Vehicle...................................5-90Cleaning Exterior Lamps/Lenses.....................5-90
Finish Care..................................................5-90
Windshield and Wiper Blades.........................5-91
Aluminum or Chrome-Plated Wheels
and Trim..................................................5-92
Tires...........................................................5-92
Sheet Metal Damage.....................................5-93
Finish Damage.............................................5-93
Underbody Maintenance................................5-93
Chemical Paint Spotting.................................5-93
Vehicle Identification
......................................5-94
Vehicle Identification Number (VIN).................5-94
Service Parts Identification Label.....................5-94
Electrical System............................................5-95
Add-On Electrical Equipment..........................5-95
Headlamp Wiring..........................................5-95
Windshield Wiper Fuses................................5-95
Fusible Links................................................5-95
Power Windows and Other Power Options..........5-96
Fuses and Circuit Breakers............................5-96
Instrument Panel Fuse Block..........................5-96
Underhood Fuse Block..................................5-99
Capacities and Specifications........................5-102
Normal Maintenance Replacement Parts.........5-108
Maintenance Replacement Parts...................5-108
Engine Drive Belt Routing............................5-110
Section 5 Service and Appearance Care
5-2
Page 205 of 376

Service
For service and parts needs, visit your dealer/retailer.
You will receive genuine GM parts and GM-trained and
supported service people.
Genuine GM parts have one of these marks:
Accessories and Modifications
Adding non-dealer accessories to the vehicle can affect
its performance and safety. Such things as airbags,
braking, stability, ride and handling, emissions systems,
aerodynamics, durability, and electronic systems like
anti-lock brakes, traction control, and stability control
could be affected. Some non-dealer accessories could
even cause malfunction or damage to parts and systems
and would not be covered by the vehicle warranty.
Damage to vehicle components resulting from the
installation or use of non-GM certified parts, including
control module modifications, are not covered under the
terms of the vehicle warranty and may affect remaining
warranty coverage for affected parts.
GM Accessories are designed to complement and
function with other systems on the vehicle. Your GM
dealer can accessorize the vehicle using genuine
GM Accessories. When you go to your GM dealer and
ask for GM Accessories, you will know that GM-trained
and supported service technicians will perform the
work using genuine GM Accessories.
Also, seeAdding Equipment to Your Airbag-Equipped
Vehicle on page 1-58.
5-3
Page 206 of 376

Aftermarket Engine Performance
Enhancement Products and
Modifications
Some aftermarket engine performance products and
modifications promise a way to increase the horsepower
and torque levels of the vehicle’s powertrain. You should
be aware that these products could have harmful effects
on the performance and life of the engine, exhaust
emission system, transmission, and drivetrain. The
engines, transmissions, and drivetrains have been
designed and built to offer industry leading durability and
performance in the most demanding applications. Engine
power enhancement products may enable the engine to
operate at horsepower and torque levels that could
damage, create failure, or reduce the life of the engine,
engine emission system, transmission, and drivetrain.
Damage, failure, or reduced life of the engine,
transmission, emission system, drivetrain, or other
vehicle components caused by aftermarket engine
performance enhancement products or modifications
might not be covered under the vehicle warranty.
California Proposition 65 Warning
Most motor vehicles, including this one, contain and/or
emit chemicals known to the State of California to cause
cancer and birth defects or other reproductive harm.
Engine exhaust, many parts and systems (including
some inside the vehicle), many fluids, and some
component wear by-products contain and/or emit
these chemicals.
California Perchlorate Materials
Requirements
Certain types of automotive applications, such as airbag
initiators, seat belt pretensioners, and lithium batteries
contained in remote keyless transmitters, may contain
perchlorate materials. Special handling may be
necessary. For additional information, see
www.dtsc.ca.gov/hazardouswaste/perchlorate.
5-4
Page 207 of 376

Doing Your Own Service Work
{CAUTION:
You can be injured and the vehicle could be
damaged if you try to do service work on a vehicle
without knowing enough about it.
•Be sure you have sufficient knowledge,
experience, the proper replacement parts, and
tools before attempting any vehicle
maintenance task.
•Be sure to use the proper nuts, bolts, and
other fasteners. English and metric fasteners
can be easily confused. If the wrong fasteners
are used, parts can later break or fall off. You
could be hurt.If doing some of your own service work, use the proper
service manual. It tells you much more about how to
service the vehicle than this manual can. To order
the proper service manual, seeService Publications
Ordering Information on page 7-10.
This vehicle has an airbag system. Before attempting to
do your own service work, seeServicing Your
Airbag-Equipped Vehicle on page 1-58.
Keep a record with all parts receipts and list the mileage
and the date of any service work performed. SeePart
D: Maintenance Record on page 6-33.
5-5
Page 208 of 376

Engine Fan Breakage
{CAUTION:
Changing the Fan Drive Ratio or Engine Governed
Speed:
If you change the fan drive ratio or increase the
governed speed of the engine, you might increase
stress and the fan could eventually fail. If the fan
breaks apart while rotating, pieces can cause
severe injury to anyone — such as a service
technician — who is nearby. And, of course, the
pieces can severely damage the vehicle. Do not
change the fan drive ratio or increase the
governed speed of the vehicle without getting the
necessary information from your dealer/retailer.
Winter Fronts, Grille Covers, or Obstructions:
Winter Fronts, grille covers, or other add-on
equipment causing obstructions in front of or
behind the fan should not be used on this vehicle.
If this causes the fan to eventually break apart
while rotating, the pieces can cause severe injury
to anyone nearby, such as a service technician
working on the engine, and, of course, the pieces
can severely damage the vehicle.
Fuel
For diesel engine vehicles, seeDiesel Engine Fuel on
page 5-8.
For vehicles with gasoline engines, please read this.
Use of the recommended fuel is an important part of the
proper maintenance of this vehicle. To help keep the
engine clean and maintain optimum vehicle performance,
we recommend the use of gasoline advertised as TOP
TIER Detergent Gasoline.
Gasoline Octane
Use regular unleaded gasoline with a posted octane
rating of 87 or higher. If the octane rating is less than 87,
you might notice an audible knocking noise when you
drive, commonly referred to as spark knock. If this
occurs, use a gasoline rated at 87 octane or higher as
soon as possible. If you are using gasoline rated at
87 octane or higher and you hear heavy knocking, the
engine needs service.
5-6