Manual Transmission; Air Suspension (If Equipped) - GMC T-Series 1999 Owner's Manual

Under light load conditions, first shift the axle into the
high range with the vehicle at a standstill and then
operate the transmission normally.

Manual Transmission

The switch for the rear axle shift control is on the shift
lever. When the switch is up, the rear axle is in the high
range. Push it down to get the low range.
-
The two speed axle should be in low when you park the
vehicle. You'll need to engage the transmission, even
move the vehicle a little, to be sure the axle is in the low
range. Then apply the parking brake, shift into
NEUTRAL and turn off the engine.
Also, always start the vehicle in motion with the
-
two speed axle in the low range. When you upshift the
rear axle from low to high, move the switch up for the
high range, disengage the clutch, pause, engage the
clutch and then press down on the accelerator. When
you shift the axle down from high to low, you don't
have to clutch. Just move the switch down to low,
release the accelerator and then quickly apply it again.
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Air Suspension (If Equipped)

Your vehicle may be equipped with the Hendrickson
-
HAS Series single axle air suspension which is designed
- -
for single axle on highway use.
With this air suspension system, the ride height is set
during vehicle assembly so that the vehicle has a
constant frame height. The system will then increase or
decrease air pressure in the air springs to maintain
this height.
Air Suspension Dump Control (If Equipped)
If your vehicle is equipped with the Hendrickson HAS
-
Series single axle air suspension, you may have this
control. The air suspension dump control allows the
operator to lower the deck height approximately
4.5 inches (11.5 cm) from the normal frame height.
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