GMC 1994 Sonoma Owner's Manual page 76

To shift from 2H1, or 4HI to 4LO : The vehicle must be stopped or
moving less than 3 mph (4.8 km/h) with the transmission in "N" (Neutral)
or the clutch pedal depressed. The preferred method for shifting into 4LO is
to have your vehicle slowly moving 1 to 2 mph (1.6 to 3.2 k d h ) . Press and
release the 4LO switch. You must wait for the amber 4LO indicator light to
stop flashing and go solid amber before shifting your transmission into gear
or releasing the clutch pedal.
If the 4LO switch is pressed when your vehicle is in gear and/or moving,
the amber 4LO indicator light will flash for 30 seconds and not complete
the shift unless your vehicle is below 3 mph (4.8 k d h ) and the transmission
is in "N" (Neutral) or the clutch pedal depressed.
On automatic transmission equipped vehicles: If your transfer case does not
shift into 4L0, your transmission indicator switch may require adjustment.
With your transmission in "N" (Neutral), press and release the 4LO switch.
While the amber 4LO indicator light is flashing, shift your transmission into
P (Park).
Wait until the 4LO indicator light goes solid amber before shifting your
transmission into gear. This will get you into 4L0, but you should take your
vehicle in for service so normal operation can be restored.
To shift from 4LO to 4HI: Your vehicle must be stopped or moving less
than 3 mph (4.8 km/h) with the transmission in "N" (Neutral) and the clutch
pedal depressed. The preferred method for shifting out of 4LO is to have
your vehicle slowly moving 1 to 2 mph (1.6 to 3.2 km/h). Press and release
the 4HI switch. You must wait for the 4HI indicator light to stop flashing
and go solid amber before shifting your transmission into gear or releasing
the clutch pedal.
If the 4HI switch is pressed when your vehicle is in gear and/or moving, the
4HI indicator light will flash for 30 seconds but not complete the shift
unless the vehicle is below 3 mph (4.8 km/h) and the transmission is in
"N" (Neutral) or the clutch pedal depressed.
On automatic transmission equipped vehicles: If your transfer case does not
shift into 4H1, your transmission indicator switch may require adjustment.
With your transmission in "N" (Neutral), press and release the 4HI switch.
While the 4HI indicator light is flashing, shift your transmission into
P
(Park). Wait until the 4HI indicator light goes solid amber before shifting
your transmission into gear. This will get you into 4H1, but you should take
your vehicle in for service so normal operation can be restored.
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