Dog Clutch 'A'

WARNING: This page is about a different car, the 2018 Land Rover Discovery Sport. However, it is still accessible from the selected car via links, so may be relevant.

ITEM	DESCRIPTION
1	Planetary gear set 1
2	Sensing piston
3	Dog 'A'
4	ATF pressure supply for dog clutch 'A' release
5	Input shaft
6	Piston
7	Planet carrier
8	ATF pressure supply for dog clutch 'A' engagement

Dog clutch 'A' is located at the end of the input shaft and is controlled by a double acting piston located within the input shaft.

A double acting piston is located internally in the input shaft and can move within the shaft when ATF pressure is applied to either side of the piston. The piston is connected to the dog 'A' by a pin which moves in a slot in the input shaft.

Dog 'A' is a sleeve with internal and external splines. Dog 'A' is permanently engaged with the input shaft via the internal splines. When the piston moves dog 'A' along the input shaft to the 'closed' position, dog 'A' engages with splines on the gearset 1 and 2 planet carrier, transferring drive from the input shaft to the gear set 2. When the dog 'A' is to be disengaged to the 'open' position, ATF pressure is applied to the opposite side of the piston and dog 'A' is moved along the input shaft and is disengaged from the planet carrier. Dog 'A' is in the 'closed' position in gears 1 through 7.

The dog 'A' has two states; open and closed. The piston cannot determine if the dog 'A' has travelled its full distance into or out of engagement with the ring gear carrier or has remained in an intermediate position. The piston is fitted with a sensing piston and the ATF pressure leakage through the sensing piston can be measured by the pressure sensor in the sensor unit.

The sensing piston is hollow and moves axially within the piston. Referring to the below illustration, if ATF pressure is applied to the right side of the piston, the piston and the sensing piston are pushed to the left. During the movement of the piston, a small amount of ATF pressure is passed through the sensing piston. This leakage pressure is measured at the left side of the piston by the pressure sensor. When the piston has moved fully to the left and reached its end position, the leakage through the sensing piston is blocked. The pressure drop on the left side of the piston is sensed and the TCM can determine the dog clutch 'A' is fully disengaged with the ring gear carrier.

If the pressure on the left side of the piston does not drop within a specified shift time, the TCM can determine that the dog 'A' has stopped in an intermediate position.

The dog clutch 'A' has four possible states of operation as follows:

ITEM	DESCRIPTION
A	Dog 'A' open - piston at end position
B	Dog 'A' closing - piston at intermediate position
C	Dog 'A' closed - piston at end position
D	Dog 'A' opening - piston at intermediate position
1	ATF Pressure applied - dog 'A' open
2	ATF pressure applied - dog 'A' closed
3	Leakage through sensing piston for pressure sensing.

Dog 'A' Open - ATF pressure is applied to the piston from the left side chamber and the dog clutch 'A' is open. The ATF pressure in the right chamber is almost zero because the sensing piston is pushed to its limit of movement and leakage through the sensing piston is prevented.
Dog 'A' Closing - ATF pressure is applied from the right side chamber which starts the piston moving to the left into engagement with the ring gear carrier. The piston is now in the intermediate position and leakage pressure is passed through the sensing piston into the left side chamber. Pressure in the left side chamber can be measured and will be approximately 2 bar (29 lbf in³ ).
Dog 'A' Closed - ATF pressure is applied to the piston from the right side chamber, the dog 'A' is closed and fully engaged with the ring gear carrier. The ATF pressure in the left chamber is almost zero because the sensing piston is pushed to its limit of movement and leakage through the sensing piston is prevented.
Dog 'A' Opening - ATF pressure is applied to the piston from the left side chamber which starts the piston moving to the right and disengaging from the ring gear carrier. The piston is now in the intermediate position and leakage pressure is passed through the sensing piston into the right side chamber. Pressure in the right side chamber can be measured and will be approximately 2 bar (29 lbf in³ ).