GMC 1999 Suburban, Owner'S Manual, Page: 299 / 424

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Automatic Transmission Fluid
If your vehicle has a diesel engine, see “Automatic
Transmission Fluid” in the Diesel Supplement.
When to Check and Change
A good time to check your automatic transmission fluid
level is when the engine oil is changed.
Change both the fluid and filter every 50,000 miles
(83 000 km) if the vehicle is mainly driven under one
or more of these conditions:
D In heavy city traffic where the outside temperature
regularly reaches 90
_F (32_C) or higher.
D In hilly or mountainous terrain.
D When doing frequent trailer towing.
D Uses such as found in taxi, police or delivery service.
If you do not use your vehicle under any of these conditions,
change the fluid and filter every 100,000 miles (166 000 km).
See “Scheduled Maintenance Services” in the Index.
How to Check
Because this operation can be a little difficult, you
may choose to have this done at the dealership
service department.
If you do it yourself, be sure to follow all the
instructions here, or you could get a false reading
on the dipstick.
NOTICE:
Too much or too little fluid can damage your
transmission. Too much can mean that some of
the fluid could come out and fall on hot engine
parts or exhaust system parts, starting a fire. Be
sure to get an accurate reading if you check your
transmission fluid.
Wait at least 30 minutes before checking the
transmission fluid level if you have been driving:
D When outside temperatures are above 90_F (32_C).
D At high speed for quite a while.
D In heavy traffic
--
especially in hot weather.
D While pulling a trailer.
To get the right reading, the fluid should be at normal
operating temperature, which is 180
_F to 200_F
(82
_C to 93_C).