Sierra Classic 1500 4WD V8-5.3L (2007)
The PTO Relay is located on the side of the underhood fuse block. The secondary side of the relay is supplied with voltage at all times through the Fog
Lamp fuse. The PTO switch controls the voltage to the primary side of the PTO relay while the PCM controls the ground.
PTO Solenoid
The solenoid is an upfitter supplied part and is replaceable on most PTOs. The PTO solenoid is used on clutch activated PTOs. The PTO relay supplies
the solenoid with voltage when the relay is energized.
PCM
The powertrain control module (PCM) is the control center of the PTO system. Applications with a 6.6L diesel use an ECM. The PCM monitors
numerous engine and vehicle functions. To engage the PTO, the PCM must see the following conditions:
^
PTO switch is ON
^
Engine must be running
^
Transmission must be in PARK or NEUTRAL
^
Park Brake must be set for manual transmission only
^
Vehicle speed must be zero in Preset Mode only
^
Brake or Clutch must not be depressed
The PCM constantly looks at the information from various sensors and other inputs, and controls the systems that affect vehicle performance. The PCM
also performs a diagnostic test on the Electronic Throttle Control system.
TAC Module
The TAC module is the control center for the electronic throttle system. The TAC module and the PCM communicate through a dedicated redundant
serial data circuit. The TAC module and the PCM monitor the commanded throttle position and compare the commanded position to the actual throttle
position. This is accomplished by monitoring the APP and the throttle position (TP) sensor. These 2 values must be within a calibrated value of each
other or a DTC may be set. The TAC module also monitors each individual circuit of the TP sensor and the APP to verify proper operation. The TAC
module commands the TAC motor to increase or decrease the engine RPM.
TAC Motor
The TAC motor is part of the throttle body and controls the positions of the throttle. The motor is bi-directional and is controlled by the TAC module.
The TAC motor increases or decreases the engine RPM.
