GMC 1995 Jimmy Owner's Manual page 200

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" in the Index.
Parking on Hills
You really should not park your vehicle, with a trailer attached, on a hill. If
something goes wrong, your rig could start to move. People can be injured,
and both your vehicle and the trailer can be damaged.
But if you ever have to park your rig on a hill, here's how to do it:
1.
2.
3.
4.
5.
6.
Apply your regular brakes, but don't shift into PARK (P) yet, or into
gear for a manual transmission. Then turn your wheels into the curb if
facing downhill or into traffic if facing uphill.
Have someone place chocks under the trailer wheels.
When the wheel chocks are in place, release the regular brakes until the
chocks absorb the load.
Reapply the regular brakes. Then apply your parking brake, and then
shift into PARK
(P),
or REVERSE (R) for a manual transmission.
If you have a four-wheel-drive vehicle with a manual transfer case
shift lever, be sure the transfer case is in a drive gear
-
not in
NEUTRAL (N).
Release the parking brake.
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