Automatic Transmission/Transaxle: Description and Operation

PURPOSE AND OPERATION
Land Rover uses three versions of the ZF four speed automatic transmission. The mechanical section of all three transmissions is similar. All have lock-up torque converters and an overdrive fourth gear (0.73:1) for highway speeds. Lock up occurs between about 48 to 52 mph.

The three ZF transmissions use the same gear ratios but the controls are hydraulic or electronic depending on the application. The ZF4HP24 construction and operation is similar to the ZF4HP22 but is a stronger transmission with the ability to handle and transfer more torque. Unlike many automatics, ATF level is checked cold, in neutral with the engine idling.

FUNCTION
ZF automatic transmissions used in Land Rover products were specifically adapted for the various applications within which they would have to operate. A three element torque converter delivers power to a four speed epicyclic (planetary) transmission.

First and second speeds are reduction gears, and third speed is direct drive. Fourth is an overdrive gear for highway cruising and fuel economy. A clutch is used to lock up the torque converter and operates in third and fourth gear. This illustration identifies the main mechanical components of the automatic transmission.





ELECTRONIC FUNCTION
This electronic automatic transmission is similar in mechanical function to the ZF 4HP-22 that is currently in use, the major difference is that this transmission is electronically controlled. It receives signals from the various sensors i.e. road speed, engine speed and engine load via the BeCM and GEMS ECM and transmission control unit. It uses these signals to control gear selection, gear change speeds, and torque converter lock up via electrovalves located in the transmission valve body.

^ Advantages of the ZF Electronic Transmission
^ Gear shift points are precise and consistent
^ Speed transducer is more accurate than a mechanical governor
^ Transmission is more reliable (less sensitive to dirt etc.)
^ Smoother shifts - retards ignition between shifts (ECM, TCU communication).
^ Torque converter lock-up 3&4 gears

GEAR RATIOS
1st - 2.48:1
2nd - 1.48:1
3rd - 1.00:1
4th - 0.73:1
Reverse - 2.09:1