Power Take-Off: Description and Operation
Power Take-Off (PTO) Description and Operation
The PTO is an Upfitter integrated system that allows the user to create an auxiliary power source that will drive either a hydraulic pump or a generator. When the operator requests PTO the PTO switch will be turned ON and the engine RPM will go to a default standby speed. Turning the PTO switch to OFF or pressing the brake pedal will turn the PTO off.
The PTO system is integrated with the cruise control system and the engine controls. The PTO system utilizes the following original equipment (OE) parts:
* PTO Switch
* Turn Signal/Multi-Function Switch
* PCM
* TAC Module
The PTO system utilizes the following upfitter installed parts:
* PTO Solenoid
* PTO Relay
* Remote Tap Up and Tap Down Switches
* Remote Emergency Shutdown Switch
When the ignition switch is in RUN or START, voltage will be present on the Ignition 1 Voltage circuit at the PTO switch. When the PTO switch is turned ON, the contacts will close and voltage will be present on the Ground circuit and the PTO On Switch-Signal circuit at the upfitter connector. When the PTO switch is turned ON the indicator will illuminate. The PTO Switch Output Enable circuit also supplies the PCM with a PTO Supply-Voltage requesting the PCM to engage the PTO. If all of the engage conditions are met, the PCM will ground the PTO Control circuit. Current will now flow through the coil in the relay and the normally open contacts will close. Solenoid engagement should only occur when the PTO relay contacts are closed. The PTO solenoid has a dedicated ground wire.
In the event that any of the PTO requirements are no longer met then the PCM or TAC Module will return the engine RPM to idle and turn the PTO Solenoid off.
Modes of Operation
PTO - Preset Mode Factory Default Setting
The PTO preset mode is used for stationary applications. PTO will remain engaged while mobile in the preset mode however PTO set and resume speed controls will be inoperative. The engine RPM will also return to normal. This mode provides a default standby speed of 800 or 850 RPM depending on the engine and transmission combination. This mode also provides 2 default PTO control speeds: PTO set speed and PTO resume speed. Please note that the standby speed is not to be used as a PTO control speed. Vehicles not equipped with cruise control will not have the resume speed capability.
PTO Preset Mode Function Chart
The following chart shows how to toggle between the 2 PTO speeds through the SET and RES switches.
PTO - Variable Mode
The PTO speed control mode is used for both stationary and mobile applications. This mode allows PTO to be controlled in a fashion similar to cruise control. The operator is able to hold an engine RPM with the SET or RESUME switch on the cruise stock. The operator can "tap up" to increase engine speed in 100 RPM increments with the RESUME switch. The operator may also "tap down" to decrease engine speed with the SET switch. Vehicles not equipped with cruise control will not have the "tap up" capability.
PTO Components
PTO Switch
The PTO Switch is mounted on the upper left side of the Instrument Panel. The PTO Switch has two positions: ON and OFF. The PTO indicator light is integral with the switch and will illuminate when the switch is turned ON. The presence of the indicator being on does not mean that the PTO Solenoid is engaged. The Cruise Control Multi-Function Switch may be used in conjunction with the PTO Switch. If the vehicle does not have cruise control there will not be any tap up or tap down capability.
Cruise Control Multi-Function Switch
The operation of the PTO system may be through the function control switches located on the multifunction turn signal lever. The cruise control function control switches includes the ON/OFF, SET/COAST, and R/A. The switch assembly provides driver control of the PTO as well as the cruise control system. The SET/COAST and R/A switches may be used to increase and decrease engine RPM. See the chart above for recommended speeds.
PTO Relay
The PTO Relay is an upfitter supplied and installed part. The PTO Switch controls the voltage to the coil of the PTO Relay while the PCM controls the ground.
PTO Solenoid
The PTO Solenoid is used on clutch activated PTOs. The PTO Relay supplies the solenoid with voltage when the relay is energized. The solenoid is an Upfitter supplied part and is replaceable on most PTOs.
PCM
The powertrain control module (PCM) is the control center of the PTO system. The PCM monitors numerous engine and vehicle functions. To engage the PTO, the PCM must see the following conditions:
* 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
* 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 Cruise 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 position of the throttle. The motor is bi-directional and is controlled by the TAC Module. The TAC motor increases or decreases the engine RPM. The motor is not serviceable.