GMC Sierra Classic 2007 Owner's Manual page 459

Variable Mode: PTO speed control mode can be
used for both stationary and mobile applications.
This mode allows PTO to be controlled in a fashion
similar to how cruise control sets vehicle speed.
The operator can set to an rpm (between 800 and
2,200) with the cruise control feature or with the
SET position on the PTO switch. The operator
is then able to increase rpm in increments of
100, or "tap up" with the resume position on the
cruise control. The operator can also decrease rpm
in increments of 100 with the SET switch on the
PTO or the cruise control.
Vehicles not equipped with cruise control will not
have the "tap up" capability.
All engine speed values listed above are factory
preset values. With the exception of the TCC
lockup speed (1,100 rpm) and the "tap" increment
(100 rpm), all of these values are programmable
and can be adjusted by your dealer or body
upfitter. If the PTO software settings do not match
the settings shown here, then they may have
already been altered in order to satisfy the
requirements of the installed PTO system and
body equipment.
When the PTO switch is turned on, the engine is
limited to 2,200 rpms. If there are no accessories
installed on the vehicle, make sure the PTO switch
is turned off.
Step-Bumper Pad
Your vehicle has a rear step bumper with a rear
step pad at the center of the bumper.
If your step-bumper has only one cutout circle,
you will have to cut out the circle, then remove it to
install the trailer ball.
If you will be using the
bumper to tow a trailer,
and your step-bumper
has three cutout circles,
you must push out the
center cutout circle to
install the trailer ball.
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