GMC C series 2002 Owner's Manual page 167

At this point, the engine speed can be adjusted by doing
either of the following:
D
Press in the PTO DOWN (SET) button until you
reach the desired speed; then release the button. To
decrease the speed in small amounts, briefly press
this button. Each time you do this the engine speed
will decrease by 100 rpm. The engine speed can be
reduced to the PTO standby speed.
4-34
D
Move the PTO switch from ON all the way to PTO
UP (R/A). Hold the switch there until you reach the
desired speed; then release the switch. To increase
the speed in small amounts, briefly move the switch
to PTO UP (R/A). Each time you do this, the idle
speed will increase by 100 rpm. The engine speed
can be increased to the PTO maximum engine speed,
a default setting of 2,200 rpm.
The engine speed will return to a normal idle if the brake
or clutch pedal is pressed. PTO UP (R/A) can be used to
return the engine speed to the prior PTO operating speed.
PTO DOWN (SET) can be used to set a new PTO
operating speed.
For more information regarding the PTO settings, see
your GM dealer for assistance.