1999 GMC ENVOY ECU

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4-50 Driving with a Trailer
CAUTION:
If you have a rear-most window open and you
pull a trailer with your vehicle, carbon monoxide
(CO) could come into your vehicle. You can't see
or smell CO. It can cause unconsciousness or
death. (See ªEngine Exhaustº in the Index.) To
maximize your safety when towing a trailer:

Have your exhaust system inspected for
leaks, and make necessary repairs before
starting on your trip.

Keep the rear
-most windows closed.

If exhaust does come into your vehicle
through a window in the rear or another
opening, drive with your front, main
heating or cooling system on and with the
fan on any speed. This will bring fresh,
outside air into your vehicle. Do not use
RECIRCULATION because it only
recirculates the air inside your vehicle.
(See ªComfort Controlsº in the Index.)
Towing a trailer requires a certain amount of experience.
Before setting out for the open road, you'll want to get
to know your rig. Acquaint yourself with the feel of
handling and braking with the added weight of the
trailer. And always keep in mind that the vehicle you are
driving is now a good deal longer and not nearly as
responsive as your vehicle is by itself.
Before you start, check the trailer hitch and platform
(and attachments), safety chains, electrical connector,
lamps, tires and mirror adjustment. If the trailer has
electric brakes, start your vehicle and trailer moving and
then apply the trailer brake controller by hand to be sure
the brakes are working. This lets you check your
electrical connection at the same time.
During your trip, check occasionally to be sure that the
load is secure, and that the lamps and any trailer brakes
are still working.
Following Distance
Stay at least twice as far behind the vehicle ahead as you
would when driving your vehicle without a trailer. This
can help you avoid situations that require heavy braking
and sudden turns.
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4-54 Maintenance When Trailer Towing
Your vehicle will need service more often when you're
pulling a trailer. See the Maintenance Schedule for more
on this. Things that are especially important in trailer
operation are automatic transmission fluid (don't
overfill), engine oil, axle lubricant, belt, cooling system
and brake system. Each of these is covered in this
manual, and the Index will help you find them quickly.
If you're trailering, it's a good idea to review these
sections before you start your trip.
Check periodically to see that all hitch nuts and bolts
are tight.
Trailer Wiring Harness
The heavy-duty trailer wiring is an eight-wire harness
assembly. The harness is stored under the vehicle, on the
driver's side corner frame rear crossmember. The
heavy
-duty trailer wiring has a 30-amp feed wire with
an in
-line fuse located by the junction block. See ªFuses
and Circuit Breakersº in the Index. The harness does not
have a connector and should be wired by a qualified
electrical technician. The technician can use the
following color code chart when connecting the wiring
harness to your trailer.

Dark Blue: Use for electric trailer brakes or
auxiliary wiring.
Red: Use for battery charging; it connects to the
starter solenoid.

Light Green: Back
-up lamps.

Brown: Taillamps and parking lamps.

Yellow: Left stoplamp and turn signal.

Dark Green: Right stoplamp and turn signal.

White (Heavy Gauge): Ground wire.

White (Light Gauge): Auxiliary stoplamp.
Securely attach the harness to the trailer, then tape or
strap it to your vehicle's frame rail. Be sure you leave it
loose enough so the wiring doesn't bend or break, but
not so loose that it drags on the ground. Store the
harness in its original place. Wrap the harness together
and tie it neatly so it won't be damaged.
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5-7
Towing Your Vehicle
CAUTION:
To help avoid serious personal injury to you
or others:

Never let passengers ride in a vehicle that is
being towed.

Never tow faster than safe or posted speeds.

Never tow with damaged parts not
fully secured.

Never get under your vehicle after it has
been lifted by the tow truck.

Always secure the vehicle on each side with
separate safety chains when towing it.

Use only the correct hooks.
NOTICE:
Use the proper towing equipment to avoid
damage to the bumper, fascia or fog lamp areas
of the vehicle.
With current trends in automotive styles and design, it is
essential that the correct towing equipment is used to
tow a vehicle. Your vehicle can be towed with wheel lift
or car carrier equipment. When towing with wheel lift
equipment, the transfer case must be in NEUTRAL and
the following tow limits must be used:

Towing from the front
-- 55 mph (88 km/h) or
legal speed limit and distance is unlimited

Towing from the rear
-- 35 mph (55 km/h),
50 miles (80 km)
NOTICE:
If the towing limits must be exceeded, a towing
dolly must be used under the front wheels or
vehicle/powertrain damage will occur.
Consult your dealer or a professional towing service if
you need to have your vehicle towed. See ªRoadside
Assistanceº in the Index.
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5-28 Storing a Flat or Spare Tire and Tools
CAUTION:
Storing a jack, a tire or other equipment in the
passenger compartment of the vehicle could
cause injury. In a sudden stop or collision, loose
equipment could strike someone. Store all these
in the proper place.
NOTICE:
An aluminum wheel with a flat tire should always
be stored under the vehicle with the hoist.
However, storing it that way for an extended
period could damage the wheel. To avoid this,
have the wheel repaired as soon as possible.
Follow this diagram to store the
underbody
-mounted spare.
A. Retainer
B. Valve Stem
(Pointed Down)
C. Spare or Flat Tire
D. Spring
E. Wheel Wrench
F. Lower
G. Raise
H. Hoist Arm
1. Put the tire on the ground at the rear of the vehicle,
with the valve stem pointed down and to the rear.
2. Pull the retainer through the wheel.
3. Put the chisel end of the wheel wrench, on an angle,
through the hole in the rear bumper and into the hoist
shaft. Turn the wheel wrench clockwise until the tire
is raised against the underside of the vehicle.
You will hear two ªclicksº when the tire is secure,
but pull on the tire to make sure.
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5-29
Return the jack, wheel wrench and wheel blocks to the
proper location in your vehicle's rear area. Secure the
items and replace the jack cover.
A. Retainer
B. Rubber Band
C. Work Gloves
D. Ground Mat
E. Jack Storage CoverF. Wheel Blocks
G. Wheel Wrench
H. Jack
I. Jacking Instructions
CAUTION:
Make sure the tire and carrier are secure.
Driving with the tire or carrier unlatched could
injure pedestrians or damage the vehicle.
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6-13 Engine Oil Additives
Don't add anything to your oil. Your dealer is ready to
advise if you think something should be added.
When to Change Engine Oil
If any one of these is true for you, use the short trip/city
maintenance schedule:

Most trips are less than 5 to 10 miles (8 to 16 km).
This is particularly important when outside
temperatures are below freezing.

Most trips include extensive idling (such as frequent
driving in stop
-and-go traffic).

You operate your vehicle in dusty areas or
off
-road frequently.

You frequently tow a trailer or use a carrier on top of
your vehicle.

The vehicle is used for delivery service, police, taxi
or other commercial application.
Driving under these conditions causes engine oil to
break down sooner. If any one of these is true for your
vehicle, then you need to change your oil and filter
every 3,000 miles (5 000 km) or 3 months
-- whichever
occurs first.If none of them is true, use the long trip/highway
maintenance schedule. Change the oil and filter every
7,500 miles (12 500 km) or 12 months
-- whichever
occurs first. Driving a vehicle with a fully warmed
engine under highway conditions causes engine oil to
break down slower.
Remote Oil Filter
The access door for the
remote oil filter is in the
steering linkage shield
assembly located under the
radiator support. Twist the
screw to unlock or lock the
door. If you open the door,
make sure it is securely
closed when you
are finished.
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9-2
Blizzard 4-38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Blizzard, Driving In 4-38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brake Fluid 6-26 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brake, Parking 2-25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brake System Warning Light 2-68 . . . . . . . . . . . . . . . . . . . . . .
Brakes
Adjustment 6
-30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Master Cylinder 6-26 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pedal Travel 6-30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Replacing System Parts 6-30 . . . . . . . . . . . . . . . . . . . . . . . .
Wear 6-29 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brake-Transmission Shift Interlock Check 7-41 . . . . . . . . . . .
Braking 4-6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Braking in Emergencies 4-9 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Break-In, New Vehicle 2-14 . . . . . . . . . . . . . . . . . . . . . . . . . .
Brightness Control 2-42 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BTSI Check 7-41 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bulb Replacement 6-31, 6-60 . . . . . . . . . . . . . . . . . . . . . . . . .
Canadian Roadside Assistance 8-7 . . . . . . . . . . . . . . . . . . . .
Capacities and Specifications 6-60 . . . . . . . . . . . . . . . . . . . . . .
Carbon Monoxide 2-9, 2-30, 4-39, 4-50 . . . . . . . . . . . . . . . .
Care of Your Cassette Tape Player 3-25 . . . . . . . . . . . . . . . . . .
Cargo Security Shade 2-52 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cargo Tie Downs 2-52 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cellular Telephone 2-57 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Center Rear Passenger Position 1-34 . . . . . . . . . . . . . . . . . . . .
Certification/Tire Label 4-41 . . . . . . . . . . . . . . . . . . . . . . . . . .
Chains
Safety 4
-49 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tire 6-45 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Changing a Flat Tire 5-19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Charging System Light 2-67 . . . . . . . . . . . . . . . . . . . . . . . . . . Check Gages Light 2
-75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Checking Your Restraint Systems 1-52 . . . . . . . . . . . . . . . . . .
Chemical Paint Spotting 6-52 . . . . . . . . . . . . . . . . . . . . . . . . . .
Child Restraints 1-38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Center Seat Position 1-46 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Different Types 1-38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Securing in a Rear Outside Seat Position 1-44 . . . . . . . . . . .
Securing in the Right Front Seat Position 1-46 . . . . . . . . . .
Top Strap 1-43 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Where Not to Put 1-46 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Where to Put 1-42 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cigarette Lighter 2-54 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Circuit Breakers and Fuses 6-55 . . . . . . . . . . . . . . . . . . . . . . .
Cleaner, Air 6-14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cleaning
Aluminum Wheels 6
-51 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exterior Lamps and Lenses 6-50 . . . . . . . . . . . . . . . . . . . . .
Fabric 6-46 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Glass 6-49 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inside of Your Vehicle 6-46 . . . . . . . . . . . . . . . . . . . . . . . . .
Instrument Panel 6-48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interior Plastic Components 6-48 . . . . . . . . . . . . . . . . . . . . .
Leather 6-48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Outside of Your Vehicle 6-50 . . . . . . . . . . . . . . . . . . . . . . . .
Stains 6-47 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tires 6-51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vinyl 6-47 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Weatherstrips 6-49 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wheels 6-51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Windshield, Backglass and Wiper Blades 6-49 . . . . . . . . . .
Wood Panels 6-48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Climate Control System 3-2 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock, Setting 3-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comfort Controls 3-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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9-6
Lights
Air Bag Readiness 1-22, 2-67 . . . . . . . . . . . . . . . . . . . . . . .
Anti-Lock Brake System Warning 2-69, 4-7 . . . . . . . . . . . .
Brake System Warning 2-68 . . . . . . . . . . . . . . . . . . . . . . . . .
Charging System 2-67 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Check Gages 2-75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gate Ajar 2-75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low Fuel 2-76 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Safety Belt Reminder 1-8, 2-66 . . . . . . . . . . . . . . . . . . . . . .
Security Warning 2-74 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Service 4WD 2-74 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Service Engine Soon 2-70 . . . . . . . . . . . . . . . . . . . . . . . . . .
Loading Your Vehicle 4-41 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Loading Your Vehicle for Off-Road Driving 4-15 . . . . . . . . . .
Locking Rear Axle 2-32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lockout Prevention 2-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lockout Switch 2-32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Locks 2-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Door 2-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ignition Transmission Check 7-42 . . . . . . . . . . . . . . . . . . . .
Key Lock Cylinder Service 7-40 . . . . . . . . . . . . . . . . . . . . .
Power Door 2-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rear Door Security 2-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Loss of Control 4-13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low Fuel Light 2-76 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lubricants and Fluids 7-45 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lubrication Service, Body 7-40 . . . . . . . . . . . . . . . . . . . . . . . .
Luggage Carrier 2-53 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lumbar Controls 1-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maintenance Record 7-47 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maintenance Schedule 7-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Long Trip/Highway 7-6, 7-27 . . . . . . . . . . . . . . . . . . . . . . . Owner Checks and Services 7
-39 . . . . . . . . . . . . . . . . . . . . .
Periodic Maintenance Inspections 7-43 . . . . . . . . . . . . . . . .
Recommended Fluids and Lubricants 7-45 . . . . . . . . . . . . .
Scheduled Maintenance Services 7-4 . . . . . . . . . . . . . . . . . .
Short Trip/City 7-5, 7-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maintenance, Underbody 6-52 . . . . . . . . . . . . . . . . . . . . . . . . .
Maintenance When Trailer Towing 4-54 . . . . . . . . . . . . . . . . .
Malfunction Indicator Lamp 2-70 . . . . . . . . . . . . . . . . . . . . . .
Memory Seat 1-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Methanol 6-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mirrors 2-44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Convex Outside 2-45 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrochromic Day/Night Inside Rearview 2-44 . . . . . . . .
Heated Outside Rearview 2-45 . . . . . . . . . . . . . . . . . . . . . . .
Illuminated Visor Vanity 2-55 . . . . . . . . . . . . . . . . . . . . . . .
Outside 2-45 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power Folding 2-46 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Model Reference iii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mountain Roads 4
-34 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multifunction Lever 2-33 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Net, Convenience 2-51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Neutral 2-19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
New Vehicle ªBreak-Inº 2-13 . . . . . . . . . . . . . . . . . . . . . . . . .
Night Vision 4-27 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Normal Maintenance Replacement Parts 6-61 . . . . . . . . . . . . .
Odometer 2-65 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Off-Road Driving with Your Four-Wheel-Drive
Vehicle 4-15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Off-Road Recovery 4-11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oil, Engine 6-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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