PTO Engagement Using The PTODE Message

Introduction

With automatic and automated manual transmissions, engaging the Power Take Off is not as simple as energizing the PTO solenoid. There is a request/consent process to ensure the transmission is ready to support PTO operation. The PTO control system must first request PTO engagement from the transmission and then wait for the transmission to consent to engagement. This is done to protect the transmission and PTO. Not following the proper procedure can result in damage to the PTO clutch or transmission. To understand why this is so important, let's first take a look at what is happening in the transmission and PTO during the engagement process.

The PTO Shift Process

Automatic Transmissions

Most PTOs installed on automatic transmissions are hotshift PTOs. That means they can be engaged and disengaged while the PTO input gear is spinning. This is critical because the PTO input gear can be connected directly to the engine input to the transmission, which means the PTO input is spinning if the engine is running. A friction clutch inside the PTO engages and disengages the PTO output shaft using hydraulic pressure from the transmission main pressure line. The PTO solenoid is the hydraulic valve that directs pressurized transmission fluid in and out of the PTO clutch actuator to engage/disengage the PTO output shaft.

When the automatic transmission controller receives a PTO engagement request, the valves inside the transmission are reconfigured to raise the transmission's main line pressure above 120 psi. If this does not happen, there may not be enough pressure available to the PTO clutch to avoid slipping. Operating the PTO without enough pressure to the clutch could cause overheating, damage the clutch, and potentially damage the transmission.

If the PTO input gear is driven by the torque converter instead of the engine input, the torque converter clutch needs to lock the torque converter input (engine) to the output that is driving the PTO gear. If this doesn't happen, the PTO can stall the torque converter when a load is applied to the PTO output shaft. A torque converter stall means the engine is spinning the input to the torque converter, but the output of the torque converter is not spinning because the load being applied by the PTO cannot be sustained by the fluid coupling through the torque converter. This condition is similar to holding your foot on the brake and revving the engine with the transmission in gear (commonly known as a brake stand). Operating this way for too long can overheat the transmission.

Automated Manual Transmissions

PTOs installed on automated manual transmissions can use friction clutches like automatic transmission PTOs, only they need to use the chassis air supply for shifting because there is no hydraulic system in the manual transmission that can provide pressurized fluid. Manual transmissions can also use PTOs with positive engagement clutches that cannot be engaged and disengaged while the PTO input gear is spinning. This is because with manual transmissions, the PTO input gear meshes with the transmission's countershaft. Disengaging the transmission clutch will stop the countershaft and allow the teeth of a positive engagement clutch to slide into each other for engagement.

When the automated manual transmission receives a request for PTO engagement, the clutch is momentarily disengaged to stop the countershaft and PTO input gear from spinning. When the countershaft stops, the engagement consent is sent to the PTO controller so the PTO clutch can be engaged. During this phase, the transmission clutch may partially engage for a moment to gently spin the PTO input gear or 'bump' the input gear so the teeth of a positive engagement clutch have a chance to completely engage.

The final step to manual transmission PTO engagement is to confirm the PTO clutch has been successfully engaged so that the transmission clutch can be released and transfer full power to the PTO input gear. This means the PTO feedback switch is critical to PTO operation, since the PTO may not get full power until the transmission can confirm it is engaged.

Implementing J1939 PTODE

There are 3 SPNs required to fully support J1939 PTODE. They are all part of PGN 0xFDA4. We will only consider controlling a single PTO using PTO 1, but the other PTO positions operate the same way.

The PTODE process begins when the PTO controller requires PTO engagement and has already determined conditions for PTO operation are allowable for the application. The transmission does not implement neutral, park brake, vehicle speed, or engine speed* interlocks, so you need to make sure your application checks any applicable interlocks before requesting PTO engagement. When the PTO controller is ready to engage the PTO, it sends PGN 0xFDA4 with SPN 3452 set to 0b01. When the transmission receives this message, it will prepare for PTO engagement. When transmission's conditions are met for PTO engagement, the transmission sends PGN 0xFDA4 with SPN 3456 set to 0b01. When the PTO controller receives this message, it is now allowed to send power to the PTO solenoid, which should engage the PTO. The PTO controller will now monitor the PTO engagement feedback circuit to confirm the PTO is indeed engaged. When the PTO controller sees a change of state on the PTO feedback circuit, it sends another PGN 0xFDA4 message with SPN 3460 set to 0b01. When the transmission receives this message, it knows the PTO is engaged and makes any further adjustments to support PTO loads. Not all transmissions require the PTO feedback signal, but it is good practice to include it in your implementation to ensure compatibility with all transmissions.

Here is a table summarizing the stages of PTODE resulting in a successful PTO engagement. Keep in mind that these messages are sent every 100 ms and the messages in the table only represent the changes in data that occur at each stage. In the example below, the transmission is using source address 0x03 and the PTO controller is using source address 0x07.

When PTO operation is no longer desired, the PTO controller should stop requesting PTO engagement consent from the transmission. With hotshift PTOs, the PTO controller does not need to wait for the transmission to disengage the PTO. With positive engagement PTOs installed on automated manual transmissions, refer to the transmission manufacturer's documentation for the correct disengagement procedure.