Land Rover Discovery: Power Transfer Unit (PTU) / Operation
POWER TRANSFER UNIT (PTU)
The Active Driveline Power Transfer Unit (PTU) input shaft is driven directly
from the transmission differential. When the hydraulically operated triple
cone synchroniser is in the disengaged position (B in the following
illustration), the crown wheel shaft and the crown wheel is disconnected
from the input shaft and no drive is transferred to the driveshaft or the Rear
Drive Unit (RDU).
When the triple cone synchroniser is engaged (A in the following
illustration), drive is transferred from the transmission differential, through
the synchroniser to the crown wheel shaft. The crown wheel rotates and
transmits the drive through 90 degrees to the pinion gear and the drive
flange to rotate the driveshaft.
Triple Cone Synchroniser Operation
![Triple Cone Synchroniser Operation](images/books/227/index533(3).jpg)
- Engaged
- Disengaged
- Input shaft
- Input shaft engagement ring
- Blocker ring
- Synchroniser Sleeve
- Synchroniser detent (3 off)
The synchroniser is operated by a hydraulic piston with pressure supplied
from RDU pump via the hydraulic supply lines connecting the PTU to the All
Wheel Drive (AWD) valve block. Solenoid operated spool valves in the AWD
valve block direct hydraulic pressure to the piston in the PTU casing to
engage or disengage the triple cone synchroniser. A spring loaded detent
locks the synchroniser selector fork in either position, removing the
requirement for hydraulic pressure to maintain to hold the synchroniser
piston in the required position.
The All Wheel Drive Control Module (AWDCM) controls the operation of
the active driveline system and the operation of the PTU synchroniser
hydraulic operation.
The synchroniser process comprises a number of steps from disengagement
to full engagement.
- When the transmission differential is rotating the input shaft, the
engagement ring integral with the input shaft is also rotating. With the
synchroniser disengaged no drive is transmitted to the crown wheel
shaft and the crown wheel drive gear, the pinion drive gear and the
drive flange remain stationary.
- When engagement is required, hydraulic pressure is applied to the
piston and the synchroniser selector fork begins to move the
synchroniser sleeve in the engagement direction. The selector fork
moves the synchroniser sleeve in an axial direction and the
synchroniser detents contact the blocker ring. This starts to align the
blocker ring teeth with the teeth on the synchroniser sleeve.
- As the synchroniser selector fork continues to move axially, the
friction
caused by the detents between the synchroniser sleeve and the
blocker ring reduces the speed of both components until they are
rotating at the same speed. The teeth of the synchroniser sleeve now
start to mesh with the teeth on the blocker ring.
- With the blocker ring and the synchroniser sleeve rotating at the same
speed, the sleeve fully engages with the blocker ring teeth. However,
there is still a speed difference between the input shaft engagement
ring and the synchroniser sleeve. This speed difference is reduced as
friction surfaces (cones) between the input shaft synchroniser sleeve
and the blocker ring start to mesh.
- The synchroniser selector fork now moves the synchroniser sleeve
axially into contact with the teeth of the input shaft engagement ring.
The tapered teeth of both components now start to mesh and the
synchroniser sleeve rotational speed is increased as it starts to engage
with the engagement ring.
- When the synchroniser sleeve is fully engaged with the input shaft
engagement ring, the rotational speed of the input shaft and the
crown wheel shaft are the same and drive is passed via the crown and
pinion drive gears to the drive flange.
- Disengagement is a reverse of the engagement process. When the
synchroniser selector fork moves the synchroniser sleeve fully out of
engagement with the engagement sleeve and the blocker ring, it
touches the casing. Friction between the casing and the synchroniser
sleeve brakes the crown wheel shaft and drive gear, the pinion drive
gear and the drive flange to prevent them from rotating through oil
drag between the components.
Synchroniser Piston Operation
![Synchroniser Piston Operation](images/books/227/index534(3).jpg)
- Disengaged
- Engaged
- Detent
- Synchroniser sleeve
- Synchroniser selector fork
- Piston
The synchroniser piston is located in a bore in the PTU casing. Hydraulic
pressure is supplied from the RDU pump via the AWD valve block.
Two hydraulic lines from the valve block supply hydraulic pressure to each
side of the synchroniser piston to engage or disengage the synchroniser.
Once the piston is in the engaged or disengaged position, hydraulic
pressure is not required to hold it in this position.
The piston has an orifice which allows the system to be self bleeding,
allowing any air trapped to pass from one side of the piston, via the orifice,
to the other side of the piston. The system allows a measured leakage
through the orifice which allows the low pressure side of the piston to act as
a return line.
READ NEXT:
The RDU is driven by the driveshaft which is connected to the PTU. As the
PTU synchroniser is engaged, the driveshaft begins to rotate the stationary
components within the RDU. Pressure is then applie
Active Driveline Hydraulic Circuit Diagram
Reservoir
Suction filter
Electric actuator
RDU pump
Pressure Control Valve (PCV) P1
PCV P2
PCV P3
PCV P4
PTU synchroniser piston
Bleed orifice
COMPONENT LOCATION - ALL WHEEL DRIVE SYSTEM -
VEHICLES WITH OUT ACTIVE DRIVE LINE
Power Transfer Unit (PTU)
Driveshaft
Active On-Demand coupling
Rear differential
POWER TRANSFER UNIT (PTU) -
SEE MORE:
SPECIAL TOOL(S)
REMOVAL
WARNINGS:
After carrying out repairs, the fuel system must be checked
visually
for leaks. Failure to follow this instruction may result in personal
injury.
This procedure involves fuel handling. Be prepared for fuel
spillage
at all times and always observe fuel ha
COMPONENT LOCATION - SHEET1 OF 2
NOTE:
Right Hand Drive (RHD) illustration shown, Left Hand Drive (LHD)
illustration similar
Rear right wheel speed sensor
Rear left wheel speed sensor
Front right wheel speed sensor
Front left wheel speed sensor
Integrated Anti-lock Brake System (ABS) control