GMC 1996 Sierra Owner's Manual page 211

Turn Signals When Towing a Trailer
The green arrows on your instrument panel will flash
whenever you signal a turn or lane change. Properly
hooked up, the trailer lamps will also flash, telling other
drivers you're about to turn, change lanes or stop.
When towing a trailer, the green arrows on your
instrument panel will flash for turns even if the bulbs on
the trailer are burned out. Thus, you may think drivers
behind you are seeing your signal when they are not. It's
important to check occasionally to be sure the trailer
bulbs are still working.
Driving On Grades
Reduce speed and shift to
a
lower gear b e ~ m you start
down a long or steep downgrade. I f you don't shift
down, you might have to use your brakes so much that
they would get hot and no longer work well.
On a long uphill grade, shift down and reduce your
speed to around 45 mph (70 km/h) to reduce the
possibility of engine and transmission overheating.
If you have an automatic transmission, you should use
THIRD (3) (or, as you need to, a lower gear) when
towing a trailer. Operating your vehicle in THIRD (3)
when towing a trailer will minimize heat build-up and
extend the life of your transmission
If you have a manual transmission and you are towing a
trailer, it's better not to use FIFTH (5) gear. Just drive in
FOURTH (4) gear (or, as you need to, a lower gear).
When towing at high altitude on steep uphill grades,
consider the following: Engine coolant will boil at a
lower temperature than at normal altitudes. If you turn
your engine off' immediately after towing at high altitude
on steep uphill grades, your vehicle may show signs
similar to engine overheating. To avoid this, let the
engine run while parked (preferably on level ground)
with the automatic transmission in PARK (P) (or the
manual transmission out
of
gear and the parking brake
applied) for a few minutes before turning the engine off.
If you do get the overheat warning, see "Engine
Overheating'' i n the Index.