Mack Truck PTO and Pumps Service Manual

Mack Truck PTO and Pumps Service Manual
Mack PTO & Pumps Service Manual PDF

PTO Speed Functions
For the purposes of this manual, PTO (Power Take–off) is a function to maintain engine speed regardless of engine load. Think of it as cruise control for the engine - only instead of maintaining a steady vehicle speed, it is maintaining engine speed. It is most often used to provide increased steady engine speed when operating auxiliary equipment such as hydraulic pumps or compressors. The term "PTO" has come to mean both the function of maintaining constant engine speed during use of auxiliary equipment, as well as a term for the auxiliary equipment itself.

Auxiliary equipment require power take-offs, either when the truck is stationary or when it is in motion. Various power take-off alternatives can be chosen, depending on the bodywork. The work is generally carried out by equipment which is powered by a hydraulic motor. The hydraulic motor, together with a pump and associated equipment, form the basis of the hydraulic system. The pump, which provides the hydraulic pressure and flow to the motor, is the heart of the hydraulic system.

All power take-offs covered by this chapter are available factory installed. Some variants can also be ordered. It is important to design an optimum hydraulic system, and to specify the correct pump size to provide sufficient oil flow and prevent overloading of the power take-off.

There are two types of PTO engine speed control used in Mack trucks:

  1. (EHT/PTO 0): This type of PTO is standard equipment; it is initiated and controlled using the cruise control "ON/OFF", "SET / Decel or Resume / Accel switches. Manufacturers refer to this function by different names - Mack calls it "Electronic Hand Throttle" (EHT), even though some of the parameters for programming this function will refer to it as "high idle". (Note that in the North American market, the term "high idle" usually refers to the maximum possible engine speed under no-load conditions).
  2. "Physical" or "Wired" PTO: This type of PTO is most commonly associated with the engagement of engine- or transmission "Wired" PTO driven accessories, where the user wishes to "remotely" activate the engine speed control function. It is usually accomplished by wiring the auxiliary equipment to the PTO function controller (VECU, BBM ECU, or Transmission ECU) so that engaging one will automatically activate the other.

Body Builder Module
PTO Control functionality, which is in the Transmission Control Unit (TCU) in AMT-D, has been removed in AMT-F. Therefore, this functionality needs to be provided in the vehicle through the VECU for single PTO and with Body Builder Module (BBM) when using Dual PTOs.

  • This is added to support dual PTOs with the AMT-F and AMT-FX transmissions.
  • MACK BBM is located on the doghouse BIW with a mounting plate, under the plastic doghouse cover not visible to the driver.

Electronic Hand Throttle PTO
The VECU reads the cruise control PTO switch status, and commands the engine into PTO mode when conditions are right. These conditions are program in the VECU and might include a maximum vehicle speed, park brake set or not, etc. The engine PTO 'set speed' is selected by either pressing the cruise control "RESUME" button, or by using the accelerator pedal to get the desired engine speed and pressing one of the cruise control "SET" buttons (either SET / Decel or Resume / Accel). Once the PTO function is active, the SET / Decel or Resume / Accel buttons can be used to adjust the PTO engine speed (within limits).

Physical or Remote/Wired PTO
This PTO function is activated by applying an external signal to a dedicated PTO input pin on the VECU. The set speed can be adjusted using the cruise control SET / Decel or Resume / Accel buttons, as above. The Mack VECU allows for up to 4 PTO speed controls plus the PTO (EHT) without the use of a BBM. Each PTO can be independently selected and controlled by separate speed settings as well as independent engagement and disengagement requirements.

A sub-function of the 'wired' PTOs in the VECU is the ability to activate PTO "outputs". Traditionally, the auxiliary device (PTO pump, bypass solenoid, etc.) is wired so as to be activated by the same switch which initiates the engine speed control function. This means that the hardware will be engaged whenever the switch is on, even if the engine speed is not being controlled. There also is no protection against an input switch that has been accidentally activated.

With the mDrive transmission the PTO "Output Control" function of the VECU and BBM ECU, it is possible to prevent engagement of the auxiliary equipment until certain conditions are met - park brake set, engine speed within a certain range, etc. This is accomplished by controlling the auxiliary equipment with the VECU, instead of directly from a switch. The ECUs still read the switch input, though, and will only allow the auxiliary device to engage when conditions are right.

Transmission-mounted PTO mDrive Power Take-off
The mDrive is an automated manual transmission. When you have the PTO engaged and put the transmission into gear by pressing the service brake and selecting D on the shift pad, the TECU commands the clutch to be released and engages the countershaft brake. This stops the countershaft, which in turn stops the rotation of the PTO when the driver releases the brake pedal and applies the throttle. The clutch will re-engage and the countershaft will start to rotate along the PTO.

When the driver stops the truck, the clutch will disengage if “R” is selected. The transmission will shift into reverse and when the driver releases the service brake and depresses the throttle, the clutch will re-engage and the PTO will start to spin again.

This is something any manual transmission would require, but the clutch would be depressed and released by the driver. The mDrive with transmission–mounted PTO may not be the solution for the Roll Off Chassis. The REPTO (MP only) unit is best suited for this application, where the truck is moved and uses the hydraulics at the same time. (This was possible with an Allison Transmission).

If the driver wants to operate the PTO while driving the truck, the transmission will only be able to operate in the starting gear that was selected. Depending on the vehicle options and starting gear selected, the maximum vehicle speed is usually around 6 mph.

Speed Ratios for mDrive Transmission Mounted PTOs

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