Page 301 of 376
Underhood Fuse Block
When a circuit goes out, the problem could be in either
the primary or secondary underhood fuse blocks.
These blocks use blade-type fuses.
Both underhood fuse blocks are located in the engine
compartment, on the passenger side of the vehicle.
To access the fuse blocks, gently squeeze both sides of
the cover to unlatch the tabs at the top. Then, unsnap
both attachments at the bottom and remove the cover.
Notice:Spilling liquid on any electrical components
on the vehicle may damage it. Always keep the
covers on any electrical component.
Be sure to replace fuses with fuses of the same rating.
Do not use fuses of higher amperage than those
indicated on the fuse block.
Fuse Usage
RR DEFOG Rear Defogger
ENG 1 Engine 1
ENG 3 Engine 3 (L18/LF6/LF8)Primary Underhood Fuse Block
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Fuse Usage
PCM-B Powertrain Control Module
TCM Transmissions (LF8)
ENG 4 Engine 4 (LMM/LF6/LF8)
ENG 2 Engine 2 (L18/LMM)
HTD FUEL Heated Fuel (LMM)
BLANK Not Used
BLANK Not Used
NOTE 3 Fan Relay (LMM), Emissions (L18)
A/C COMP Air Conditioning Compressor
ABS 1 Antilock Brake System 1
ABS 2 Antilock Brake System 2
ABS 3 Antilock Brake System 3
ENGINE Engine
E/A PUMP Electronic/Automatic Pump
HORN Horn
NOTE 2Fuel (L18/LMM), Electronic Control
Module (LF6)Fuse Usage
NOTE 4 Electronic Control Module (LF6)
STUD A Spare
STUD B Spare
Relay Usage
NOTE 1 LMM/L18 Fuel Pump Relay
IGN B RELAY Ignition Relay
STARTER
RELAYStarter Relay
HORN RELAY Horn Relay
IGN A RELAY Ignition Relay
PTO/ECU*
RELAYPower Take-Off/Engine Control Unit
(*Diesel 7.8L LF8)
REVERSE
RELAYReverse Relay
FAN RELAY Fan Relay (LMM)
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Fuse Usage
IGN 1 Ignition 1
IGN 4 Ignition 4
IGN 3 Ignition 3
BATT/HAZ Battery/Hazard Warning Flashers
HEADLAMP Headlamps
LIGHTING Interior/Exterior Lamps
HVAC Climate Control System
NOTEC4/C5 Electric Brake,
C6/C7/C8 Brake Lamps
Secondary Underhood
Fuse Block
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Page 341 of 376

At Least Twice a Year
Wiper Blade Check
Inspect wiper blades for wear, cracking, or
contamination. Clean the windshield and wiper blades, if
contaminated. Replace wiper blades that are worn or
damaged. SeeWindshield Wiper Blade Replacement on
page 5-71andWindshield and Wiper Blades on
page 5-91for more information.
Weatherstrip Lubrication
Silicone grease on weatherstrips will make them last
longer, seal better, and not stick or squeak. Apply
silicone grease with a clean cloth. During very cold,
damp weather more frequent application may be
required. Part C tells you what to use.
Body Lubrication Service
Lubricate all exposed surfaces such as door checks,
door lock bolts, lock strike plates, door hinge bushings,
latches, and dovetail bumper wedges. Where oil
holes are provided, a dripless oil can be used. The seat
adjusters, seat track, door weatherstrips, and rubber
cap bumpers should also be lubricated. Part C tells you
what to use. More frequent lubrication may be required
when exposed to a corrosive environment.
Parking Brake Check
Park on a fairly steep hill and hold the vehicle with the
parking brake only. This checks holding ability. See
Parking Brake (With Hydraulic Brakes) on page 2-38or
Parking Brake (With Air Brakes) on page 2-41.
Starter Switch Check
{CAUTION:
When you are doing this inspection, the vehicle
could move suddenly. If the vehicle moves, you or
others could be injured.
1. Before you start, be sure you have enough room
around the vehicle.
2. Firmly apply both the parking brake and the regular
brakes.
Do not use the accelerator pedal, and be ready to
turn off the engine immediately if it starts.
3. On automatic transmission vehicles, try to start the
engine in each gear. The starter should work only in
P (Park) or N (Neutral). If the starter works in any
other position, your vehicle needs service. On
manual transmission vehicles, the starter should
work only when the clutch pedal is all the way down.
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Page 343 of 376

Mirrors and Sun Visors Check
Check that friction joints hold mirrors and sun visors in
place.
Seat Adjuster Check
When adjusting a manual seat, be sure seat adjusters
latch by attempting to move the seat after latching.
Lamps Check
Check panel lighting, warning lights, indicator lights, and
interior lamps. On the outside, check: license plate lamps,
sidemarker lamps, reflectors or lights on outside mirrors,
headlamps, parking lamps, identification and clearance
lamps, taillamps, brake lamps, turn signals, backup
lamps, and hazard warning flashers. Have headlamp aim
checked at once if beams seem improperly aimed.
Glass, Mirrors, Lamps, and/or
Reflectors Condition Check
Look for broken, scratched, dirty, or damaged glass,
mirrors, lamps, or reflectors that could reduce the view
or visibility or cause injury. Replace, clean, or repair
promptly.
Door Latches Check
Check that doors close, latch, and lock tightly. Check for
broken, damaged, or missing parts that might prevent
tight latching.
Hood Latches Check
Check that the hood closes firmly. Check for broken,
damaged, loose, or missing parts that might prevent tight
latching. Make sure the secondary latch, if the vehicle
has one, keeps the hood from opening all the way when
the primary latch is released.
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Page 344 of 376

Fluid Leaks Check
Check for fuel, coolant, oil, or other fluid leaks by
looking at the surface beneath the vehicle after it has
been parked for awhile.
Underbody Inspection
Corrosive materials used for ice, snow removal, and dust
control can collect on the underbody. If these materials
are not removed, accelerated corrosion (rust) can occur
on underbody parts such as fuel lines, frame, floor pan,
and exhaust system. At least every spring, flush these
materials from the underbody with plain water. Take care
to clean well any areas where mud and other debris can
collect. Sediment packed in closed areas of the frame
should be loosened before being flushed.
Engine Cover Check
Check that the cab’s engine cover and seal, if the
vehicle has one, are not torn or damaged. Be sure that
the cover is clamped down firmly to the floor.
At Least Once a Year
Key Lock Cylinders Service
Lubricate the key lock cylinders with the lubricant
specified in Part C.
Underbody Flushing Service
At least every spring, use plain water to flush any
corrosive materials from the underbody. Take care to
clean thoroughly any areas where mud and other debris
can collect.
Tractor Protection (Breakaway) Valve
Check
On air brake models, remove, disassemble, clean, and
inspect the tractor protection (breakaway) valve.
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Usage Fluid/Lubricant
Key Lock
CylindersMulti-Purpose Lubricant, Superlube
(GM Part No. U.S. 12346241,
in Canada 10953474).
Chassis
LubricationChassis Lubricant
(GM Part No. U.S. 12377985,
in Canada 88901242) or lubricant
meeting requirements of NLGI #2,
Category LB or GC-LB.
Front Wheel
Bearings
(Except Oil
Filled Hubs)Wheel bearing lubricant meeting
requirements of NLGI #2,
Category GC or GC-LB
(GM Part No. U.S. 1051344,
in Canada 993037).
Front Wheel
Bearings with
Oil Filled HubsSAE 75W-90 Synthetic Axle
Lubricant for Medium Duty Trucks
(GM Part No. U.S. 89021675,
in Canada 10953512).
Transfer Case
(Four-Wheel
Drive)Manual Transmission Fluid
(GM Part No. U.S. 88861800,
in Canada 88861801).
Front Axle
(Four-Wheel
Drive)SAE 75W-90 Synthetic Axle
Lubricant for Medium Duty Trucks
(GM Part No. U.S. 89021675,
in Canada 10953512).Usage Fluid/Lubricant
Rear Axle and
Rear HubsSAE 75W-90 Synthetic Axle
Lubricant for Medium Duty Trucks
(GM Part No. U.S. 89021675,
in Canada 10953512).
Rear Axle Shift
Motor LubricantRefrigerant Oil
(GM Part No. U.S. 5416939,
in Canada 10953496).
Propshafts and
SplinesWheel bearing lubricant meeting
requirements of NLGI #2,
Category GC or GC-LB
(GM Part No. U.S. 1051344,
in Canada 993037).
Cab — Door
Hinges and
LatchesMulti-Purpose Lubricant, Superlube
(GM Part No. U.S. 12346241,
in Canada 10953474).
Weatherstrip
ConditioningWeatherstrip Lubricant
(GM Part No. U.S. 3634770,
in Canada 10953518) or
Dielectric Silicone Grease
(GM Part No. U.S. 12345579,
in Canada 992887).
Weatherstrip
SqueaksSynthetic Grease
with Teflon, Superlube
(GM Part No. U.S. 12371287,
in Canada 10953437).
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Page 361 of 376

ORDER TOLL FREE: 1-800-551-4123
Monday-Friday 8:00 AM - 6:00 PM
Eastern Time
For Credit Card Orders Only
(VISA-MasterCard-Discover), visit Helm, Inc.
on the World Wide Web at: helminc.com
Or you can write to:
Helm, Incorporated
P.O. Box 07130
Detroit, MI 48207
Prices are subject to change without notice and without
incurring obligation. Allow ample time for delivery.
Note to Canadian Customers: All listed prices are
quoted in U.S. funds. Canadian residents are to make
checks payable in U.S. funds.
Vehicle Data Recording and
Privacy
Vehicle Data Recording and Privacy
(Isuzu 7.8L L6 Engine)
Your GM vehicle has a number of sophisticated
computers that record information about the vehicle’s
performance and how it is driven. For example,
your vehicle uses computer modules to monitor and
control engine and transmission performance, to monitor
the conditions for airbag deployment and deploy
airbags in a crash and, if so equipped, to provide
antilock braking to help the driver control the vehicle.
These modules may store data to help your
dealer/retailer technician service your vehicle. Some
modules may also store data about how you operate the
vehicle, such as rate of fuel consumption or average
speed. These modules may also retain the owner’s
personal preferences, such as radio pre-sets, seat
positions, and temperature settings.
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