
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

If you get the overheat warning with no sign of steam,
try this for a minute or so:
1. In heavy traffic, let the engine idle in N (Neutral)
while stopped. If it is safe to do so, pull off the
road, shift to P (Park) or N (Neutral) and let
the engine idle.
2. Turn on your heater to full hot at the highest fan
speed and open the windows as necessary.
3. If climbing a hill, downshift to raise engine and fan
speeds.
If you no longer have the overheat warning, you can
drive. Just to be safe, drive slower for about 10 minutes.
If the warning does not come back on, you can drive
normally.
If the warning continues and you have not stopped, pull
over, stop, and park your vehicle right away.
If there is still no sign of steam, you can push down the
accelerator until the engine speed is about twice as
fast as normal idle speed for at least three minutes while
you are parked. If you still have the warning, turn off
the engine and get everyone out of the vehicle until it
cools down.
You may decide not to lift the hood but to get service
help right away.Engine Fan Noise
Your vehicle has a clutched engine cooling fan. When
the clutch is engaged, the fan spins faster to provide
more air to cool the engine. In most everyday driving
conditions, the clutch is not fully engaged. This improves
fuel economy and reduces fan noise. Under heavy
vehicle loading, trailer towing and/or high outside
temperatures, the fan speed increases when the clutch
engages. So you may hear an increase in fan noise.
This is normal and should not be mistaken as the
transmission slipping or making extra shifts. It is merely
the cooling system functioning properly. The fan will
slow down when additional cooling is not required and
the clutch disengages.
You may also hear this fan noise when you start the
engine. It will go away as the fan clutch disengages.
Power Steering Fluid
SeeScheduled Maintenance on page 6-5to determine
when to check your power steering fluid.
5-49

Fuse Usage
1 Stoplamps
2 Not Used
3 Parking Lamps
4 Powertrain Control Module
5 Auxiliary Wiring
6 Heater/Air Conditioning
7 Hazard Warning Flashers
8 Power PostFuse Usage
9 Courtesy Lamps
10Warning Lights, Gages and
Indicators
11 Starter
12 Rear Axle/Four-Wheel-Drive
13Trailer Turn Signals/Hazard Warning
Flashers
14 Radio/Chime
15 Daytime Running Lamps
5-97

Fuse Usage
16 Airbag System
17 Exterior/Interior Lamps
18 Parking Brake
19 Accessory Power
20 Ignition 4
21 Sidemarker Lamps
22 Turn Signal/Backup Lamps
23 Transmission
24 Hydraulics/Air Brake
A Spare
B Spare
Fuse Usage
Blank Not Used
BRK Brake Warning Lamp
RT PRK Passenger Side Parking Lamps
Blank Not Used
Blank Not Used
LT PARK Driver Side Parking Lamps
RT REAR
TRN/STOPPassenger Side Rear Turn
Signal/Stoplamp
LT REAR
TRN/STOPDriver Side Rear Turn
Signal/Stoplamp
Fuse Usage
RADIO Radio
Blank Not Used
Blank Not Used
PWR WNDW Power Windows
Relay Usage
ECU/PTO*Engine Control Unit/Power Take-Off
*Diesel 7.8 Duramax
BRK LAMPC4/C5 Brake Lamps,
C6/C7/C8 Tractor/Trailer Wiring
DRL Daytime Running Lamps
IGN-4 Ignition
CHMSL Center High Mounted Stoplamp
MRK LTS Sidemarker and Clearance Lamps
HTD/MIRR Heated Mirrors
HTR Diesel Heated Fuel
RT TRN TRLR Passenger Side Trailer Turn Signal
Blank Not Used
LT TRN TRLR Driver Side Trailer Turn Signal
Blank Not Used
Blank Not Used
Blank Not Used
5-98

❑Air brake automatic slack adjuster service (or every
500 hours, whichever occurs first).(33)
❑Check air brake relay valve operation and check for
leaks (or every month, or every 300 hours, whichever
occurs first).
❑Clean air brake application valve (or every 3 months,
or every 300 hours, whichever occurs first). Lubricate
linkage.
❑Air brake chamber service (or every 2 months,
whichever occurs first).(34)
100,000 Miles (160 000 km)
❑Diesel Only: Inspect engine drive belt; replace as
necessary.
❑Gasoline Engine Only (Vehicles with GVW of
16,000 lbs or less): Spark plug service.(2) (25)
❑Change power steering fluid (or every 36 months,
whichever occurs first).(12)
❑Replace power steering reservoir filter element
(or every 24 months, whichever occurs first).
❑Wheel bearing (oil type) service (and whenever hubs
are removed).(22) (36)
❑Except four-wheel drive: Front axle service.(31)
❑Four-wheel drive only: Front axle service.(30)❑Four-wheel drive only: Transfer case service.(30)
❑Rear axle service — Eaton
®, Rockwell®, Spicer®axles
(or every 12 months, whichever occurs first).(30)
❑Exhaust brake service, if equipped. Check for
excessive spindle free play and smooth operation.
Lubricate ball joint cap.
❑Inspect air compressor discharge port (or every
6 months or every 1,800 hours, whichever occurs
first).
❑Manual transmission (ZF 6-speed only) fluid
replacement (or every 48 months, whichever occurs
first).
❑Trailer brake hand control valve service (or every
12 months, or every 3,600 hours, whichever occurs
first).(35)
120,000 Miles (192 000 km)
❑C600, C700, and C800 Gasoline Engines Only:
Replace engine drive belts.
❑C400 and C500 Gasoline Engines Only: Engine drive
belts service (or every 12 months, whichever occurs
first).(17)
❑DURAMAX Diesel Only: Diesel Particulate
Filter (DPF) service. Then every 100,000 miles
(160 000 km).(7) (38)
6-19

150,000 Miles (240 000 km)
❑Cooling system service (or every 60 months since
last service, whichever occurs first).(2) (29)
❑Diesel Engine Only: Inspect engine drive belt;
replace as necessary.
❑Isuzu Diesel Only: Diesel Particulate Filter (DPF)
service (or every 4,500 hours, whichever occurs
first). Then every 150,000 miles (240 000 km),
or every 4,500 hours, whichever occurs first.(7)
200,000 Miles (320 000 km)
❑Rear axle service — Eaton®, Rockwell®, Spicer®
axles (or every 12 months, whichever occurs
first).(30)
❑Remove, disassemble, clean, and inspect the air
brake trailer supply valve (or every 2 years, or
every 7,200 hours, whichever occurs first).
❑Diesel Engine Only: Inspect engine drive belt;
replace as necessary.
250,000 Miles (400 000 km)
❑Four-wheel drive only: Front axle service.(30)
❑Four-wheel drive only: Transfer case service.(30)
❑Manual transmission (except ZF 6-speed) fluid
replacement (or every 60 months, whichever occurs
first).
300,000 Miles (480 000 km)
❑Rear axle service — Eaton®, Rockwell®, Spicer®
axles (or every 12 months, whichever occurs
first).(30)
❑Remove, disassemble, clean, and inspect the air
brake air dryer (or every 3 years, or 10,800 hours,
whichever occurs first). Replace desiccant.
❑Isuzu Diesel Only: Diesel Particulate Filter (DPF)
service (or every 4,500 hours, whichever occurs
first). Then every 150,000 miles (240 000 km),
or every 4,500 hours, whichever occurs first.(7)
Footnotes
† = The U.S. Environmental Protection Agency or the
California Air Resources Board has determined that the
failure to perform this maintenance item will not nullify
the emission warranty or limit recall liability prior to
the completion of the vehicle’s useful life. We, however,
urge that all recommended maintenance services be
performed at the indicated intervals and the
maintenance be recorded.
(1)= If your vehicle has an Allison Transmission
®,
your owner manual is supplemented by an Allison
Transmission
®Operator’s Manual. Always refer to
these manuals for related maintenance services.
(2)= An Emission Control Service.
(3)= A Noise Emission Control Device.
6-20

•Replace the air compressor filter element, mounted
on the air compressor. For remote air compressor
intake service, refer to engine air cleaner filter
replacement.
•Remove, disassemble, clean, and inspect the safety
valve, service brake chambers, quick release valves,
quick release/double check valve combinations,
parking brake control valve, double check valves,
pressure protection valves, and ether injector
(if equipped).
•Remove, disassemble, and clean the application
valve; replace parts showing wear.
•Remove, disassemble, clean, and inspect the spring
brake control valve; replace rubber parts.
(33)= Clean and lubricate air brake automatic slack
adjuster. Check pushrod travel and auto adjustment
operation. Have serviced if needed.
(34)= Air brake chamber service: Check operation,
mounting clamps, and air lines and check for leaks.
(35)= Trailer brake hand control valve service: Check
operation; lubricate cam and follower.(36)= Tighten the wheel stud nuts to the specified torque
values at 100 miles (160 km). Thereafter, tighten them
1,000 miles (1 600 km) after each time the wheel is
removed. SeeTightening the Wheel Nuts on page 5-80.
(37)= Inspect and clean any accumulated dirt, gravel,
or other foreign objects from the valves and valve boots
as needed. Using light oil, lubricate brake pedal to
brake application valve linkage components. Check any
rubber boots for cracks, holes, or deterioration and
replace if necessary.
(38)= Estimated minimum. Actual mileage depends on
fuel and oil consumed.
(39)= If the vehicle has the Isuzu diesel engine and is
used primarily for long trip, highway service, change
the engine oil and filter every 18,000 miles (28 800 km),
or every 12 months, or every 750 hours of engine
operation, whichever occurs first.
(40)= Make sure any safety belt reminder light and
safety belt assemblies are working properly. Look
for any other loose or damaged safety belt system parts.
If you see anything that might keep a safety belt
system from doing its job, have it repaired. Have any
torn or frayed safety belts replaced.
6-25

A
Accessories and Modifications............................ 5-3
Accessory Power Outlets.................................3-19
Adding a Snow Plow or Similar Equipment.........4-22
Adding Equipment to Your Airbag-Equipped
Vehicle.......................................................1-58
Additives, Fuel................................................. 5-7
Add-On Electrical Equipment............................5-95
Air Cleaner/Filter, Engine.................................5-38
Air Conditioning..............................................3-20
Air Pressure Gage..........................................3-47
Air Suspension...............................................2-44
Air Suspension Seats........................................ 1-6
Airbag
Off Light.....................................................3-28
Readiness Light..........................................3-27
Airbag System................................................1-48
Adding Equipment to Your Airbag-Equipped
Vehicle...................................................1-58
Airbag Off Switch........................................1-55
How Does an Airbag Restrain?......................1-53
Servicing Your Airbag-Equipped Vehicle..........1-58
What Makes an Airbag Inflate?......................1-53
What Will You See After an Airbag
Inflates?..................................................1-54
When Should an Airbag Inflate?....................1-52
Where Are the Airbags?...............................1-50Antenna, Fixed Mast.......................................3-58
Antilock Brake System (ABS)............................. 4-5
Antilock Brake, System Warning Light................3-33
Antilock Brakes, Trailer System
Warning Light.............................................3-34
Appearance Care
Aluminum or Chrome-Plated Wheels..............5-92
Care of Safety Belts....................................5-89
Chemical Paint Spotting...............................5-93
Cleaning Exterior Lamps/Lenses....................5-90
Fabric/Carpet..............................................5-88
Finish Care.................................................5-90
Finish Damage............................................5-93
Instrument Panel, Vinyl, and Other
Plastic Surfaces.......................................5-89
Interior Cleaning..........................................5-87
Sheet Metal Damage...................................5-93
Tires..........................................................5-92
Underbody Maintenance...............................5-93
Washing Your Vehicle...................................5-90
Weatherstrips..............................................5-89
Windshield and Wiper Blades........................5-91
Ashtray.........................................................3-19
Audio System.................................................3-48
Chime Level Adjustment...............................3-58
Fixed Mast Antenna.....................................3-58
Radio Reception..........................................3-58
Setting the Clock.........................................3-48
Theft-Deterrent Feature................................3-57
1