The power take-off (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. The PTO solenoid will engage if all of the engage conditions are met. If any of the PTO engage conditions for the specific operating mode are no longer valid, the engine controller will exit out of PTO mode.
The PTO system is integrated with the cruise control and engine control system. The PTO system utilizes the following GM parts:
| • | The PTO enable switch |
| • | The cruise control multi-function switch |
| • | The PTO relay |
| • | The powertrain control module (PCM) with 8.1L gasoline only |
| • | The engine control module (ECM) with 6.6L or 7.8L diesel only |
| • | The throttle actuator control (TAC) module with 8.1L gasoline only |
| • | The TAC motor with 8.1L gasoline only |
The following parts are upfitter supplied and installed:
| • | The load engage switch |
| • | The PTO solenoid |
Voltage is supplied with the ignition switch in RUN and START through the engine 1 fuse to the secondary side of the PTO relay and the PTO enable switch. When the PTO switch is turned ON, voltage will be present through the primary side of the PTO relay and at the PTO ON switch signal circuit at the PCM. If the PCM sees all of the PTO engage conditions it will provide a ground for the relay. Current will flow through the coil in the PTO relay and close the normally open contacts. This will allow current to flow to the load engage switch. When the operator closes the load engage switch current will flow to the PTO solenoid. If the PCM sees all of the PTO engage conditions the PCM will send a class 2 signal to the TAC module. The TAC module will use the TAC motor to increase the engine RPM to the standby speed. In the event that any of the PTO requirements are no longer met then the PCM will return the engine RPM to idle and turn the PTO solenoid OFF.
For diesel applications, engine speed is controlled by the ECM and the fuel injection control module (FICM).
Modes of Operation
PTO -- Preset Mode-Factory Default Setting
The PTO preset mode is normally used for stationary applications. The PTO solenoid 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 that varies depending on the engine and transmission combination. This mode also provides 2 default PTO control speeds. Please note that the standby speed is not to be used as a PTO control speed. All vehicles that have the PTO option should be equipped with cruise control.
Programmable Parameters | Factory Setting (RPM) | Minimum Value (RPM) | Maximum Value (RPM) |
|---|---|---|---|
PTO Max Engine Speed | 2200 | L18-2100 LB7/LG4-950 | L18-4200 LB7/LG4-3100 |
PTO Standby RPM | F2 Auto/F2 Manual/F3 L18-800/850/1200 LB7-850/850 | Normal engine idle will override if higher than requested PTO base idle | 1300 |
PTO Set Speed | All/L18 Fam 3 1250/1400 | PTO Set speed cannot be set below PTO Standby speed | L18/LB7,LC8,LG4 4200/3100 |
PTO Resume Speed | 1700 | PTO Set Speed | L18/LB7,LC8,LG4 4200/3100 |
PTO Preset Mode Function Table
The following chart shows how to toggle between the three PTO speeds through the SET and RESUME switches on the cruise control multifunction switch.
Present PTO State | Pressing | Resulting PTO State |
|---|---|---|
PTO Standby Speed | SET | PTO Set Speed |
PTO Standby Speed | RESUME | PTO Resume Speed |
PTO Set Speed | SET | PTO Standby Speed |
PTO Set Speed | RESUME | PTO Resume Speed |
PTO Resume Speed | SET | PTO Set Speed |
PTO Resume Speed | RESUME | PTO Standby Speed |
PTO -- Variable Mode
This 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 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. Variable engine RPM may be obtained by holding either the SET or RESUME switch.
Programmable Parameters | Factory Setting (RPM) | Minimum Value (RPM) | Maximum Value (RPM) |
|---|---|---|---|
PTO Max Engine Speed | 2200 | L18-2100 LB7/LG4-950 | L18-4200 LB7/LG4-3100 |
PTO Standby RPM | F2 Auto/F2 Manual/F3 L18-800/850/1200 LB7-850/850 | Normal engine idle will override if higher than requested PTO base idle | 1300 |
PTO -- Variable Mode Function Table
Switch | Set | Resume (w/Cruise Only) |
|---|---|---|
Pressed Once | Engine speed will decrease by 100 RPM increments and will maintain new engine speed value. Engine speed can be decreased down to PTO Standby RPM speed value, all further SET inputs will be ignored | Engine speed will increase by 100 RPM increments and will maintain new engine speed value. Engine speed can be increased up to the PTO max engine speed value, all further RESUME inputs will be ignored |
Held | Decelerates engine speed down to the PTO standby speed value | Increases engine speed up to the PTO max engine speed value |
Reprogramming the PCM/ECM for fast idle/PTO using the Scan Tool
The scan tool must be used to enable the Fast Idle and PTO options and adjust the factory preset parameters to the desired settings. The scan tool menu driven device is a hand held scan tool that plugs into the datalink connector located underneath the dash near the driver's seat. Once the scan tool is connected, the following chart will guide the user.
Step | Current Menu | Select |
|---|---|---|
1 | Main | F0: Diagnostics |
2 | Vehicle Identification | Year of Vehicle |
3 | Vehicle Identification | Medium Duty Truck |
4 | System Selection | F0: Powertrain |
5 | Vehicle Identification | L65, LB7, LG4, L18 |
6 | Vehicle Identification | Automatic or Manual |
7 | Powertrain | F2: Special Functions |
8 | Special Functions | F3: PTO Options |
9 | Current PTO Settings | Change Options |
10 | PTO Options | -- |
To Adjust the Mode of Operation:
The PTO options menu will appear with the following selections:
| • | F0: Fast Idle |
| • | Preset PTO Speed Control |
| • | Variable PTO Speed Control |
Once the mode of operation is selected the Current PTO Settings menu will appear with the adjustable parameters for the current mode.
This screen allows the user to increase or decrease engine RPM values using the INCREASE and DECREASE buttons. If the Engine Fault Shutdown feature is selected, the YES or NO Buttons will determine if this feature is enabled.
Important: Please be aware that UF3 and PTO high idle settings must be reprogrammed in the event that the PCM is replaced. It may be advisable to provide the PCM settings information to the customer in case the PCM requires servicing during some point in the life of the vehicle.
After all the settings are adjusted, the user will press the REPROGRAM button and the Current PTO Settings menu will appear with the changes. The ignition must be turned OFF for 10 seconds to ensure that the program values are stored in the PCM.
PTO Components
PTO Switch
The PTO switch is mounted in the left side of the instrument panel. The PTO switch has 2 positions, ON and OFF. When the PTO switch is in the ON position, voltage will be present on the PTO On switch signal circuit at the relay and the PCM. The PTO indicator is an LED and is integral with the switch The indicator 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 must be used in conjunction with the PTO switch. Illumination of the switch is provided through the dimming circuit.
Cruise Control Multi-Function Switch
Engine speed control of the PTO system must controlled by the function control switches located on the multifunction turn signal lever. The cruise control function control switches includes the ON/OFF, SET/COAST, and RESUME/ACCEL. The switch assembly provides driver control of the PTO as well as the cruise control system. The SET/COAST and RESUME/ACCEL switches may be used to increase and decrease engine RPM. See the chart above for recommended speeds.
PTO Relay
The PTO relay is located on the side of the underhood fuse block. Voltage is supplied with the ignition switch in RUN and START through the Engine 1 fuse to the secondary side of the PTO relay. The PTO switch controls the voltage to the primary side of the PTO relay while the PCM controls the ground.
Load Engage Switch
The load engage switch is an upfitter supplied and installed part. The switch is remote. Voltage is supplied by the PTO relay to the normally open contacts of the switch. When the switch is closed current will flow to the PTO solenoid.
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 PCM is the control center of the PTO system. Applications with a 6.6L or 7.8L diesel use an ECM. The PCM monitors numerous engine and vehicle functions. To engage the PTO, the PCM must see the following conditions:
| • | The engine must be running |
| • | The transmission must be in PARK or NEUTRAL |
| • | The park brake must be set for manual transmission only |
| • | The vehicle speed must be zero in Preset Mode only |
| • | The 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.
