1999 LAND ROVER DISCOVERY Workshop Manual

AUTOMATIC GEARBOX - ZF4HP22 - 24
44-12 DESCRIPTION AND OPERATION
Reverse gear
Clutches (5), (8) and (11) are engaged. The front planet gear carrier of gear set (9) is locked. Gear set (10) also rotates
as a solid block.
Gear position switch
The gear position switch outputs signals that are related to the position of the selector lever assembly. The switch is
installed on the selector shaft on the left side of the gearbox. Slotted mounting holes allow the switch to be turned
relative to the shaft for adjustment. A fly lead connects the switch to the vehicle wiring.
Movement of the selector lever assembly turns the selector shaft, which operates six pairs of contacts in the switch.
The pairs of contacts are identified as the W, X, Y, Z, Park/Neutral and Reverse contacts. When closed:
lThe W, X, Y and Z contacts output a 12V ignition supply from the BCU.
lThe Park/Neutral contacts output an earth.
lThe Reverse contacts output a 12V ignition supply from the passenger compartment fuse box.
The outputs of the W, X, Y and Z contacts are monitored by the EAT ECU and the BCU to determine the position of
the selector lever assembly.
Gear position switch W, X, Y, Z outputs
The Park/Neutral contacts output to the BCU and, on diesel models, the ECM. The Reverse contacts output to the
BCU, the reversing lamps, the SLABS ECU and, where fitted, the ACE ECU and the electrochromic interior mirror.
Switch
contactsOutput
PRND 3 2 1
W12V - 12V 12V - - -
X- 12V 12V - 12V - -
Y- - 12V 12V 12V - 12V
Z- - - 12V 12V 12V -

AUTOMATIC GEARBOX - ZF4HP22 - 24
DESCRIPTION AND OPERATION 44-13
Oil cooler
1Inlet connection
2Fixing bracket
3Outlet connection4Fixing bracket
5Temperature sensor
Transmission fluid from the gearbox is circulated through a cooler attached to the front of the radiator. Quick release
connectors on the transmission fluid lines attach to connections on each end tank of the cooler. A temperature sensor
on the RH end tank provides the instrument pack with an input of transmission fluid temperature. If the temperature
exceeds a preset limit, the instrument pack illuminates the transmission temperature warning lamp. The warning lamp
remains illuminated until the temperature of the fluid returns within limits.
EAT ECU
The EAT ECU operates the solenoid valves in the gearbox to provide automatic control of gear shifts and torque
converter lock-up. The EAT ECU is attached to a protective bracket which is secured to the cabin floor below the LH
front seat. A 55 pin connector links the EAT ECU to the vehicle wiring.
Software in the EAT ECU monitors hard wired inputs and exchanges information with the ECM on a Controller Area
Network (CAN) bus to determine gear shift and torque converter lock-up requirements. Resultant control signals are
then output to the gearbox solenoid valves.

AUTOMATIC GEARBOX - ZF4HP22 - 24
44-14 DESCRIPTION AND OPERATION
EAT ECU connector
EAT ECU connector pin details
Pin No. Description Inputs/Outputs
1 to 4 Not used -
5 Pressure regulator solenoid valve (MV 4) Output
6 Power earth -
7 Not used -
8 Gear position switch, X contacts Input
9 Gear position switch, Z contacts Input
10 to 12 Not used -
13 Transmission high/low switch Input
14 Gearbox output shaft speed sensor, negative Input
15 Gearbox output shaft speed sensor cable screen -
16 CAN, high Input/Output
17 to 24 Not used -
25 Sport mode warning lamp Output
26 Battery power supply Input
27 Not used -
28 Electronics earth -
29 Not used -
30 Shift control solenoid valve (MV 1) Output
31 Diagnostics, K line Input/Output
32 Converter lock-up solenoid valve (MV 3) Output
33 Shift control solenoid valve (MV 2) Output
34 Not used -
35 Not used -
36 Gear position switch, W contacts Input
37 Gear position switch, Y contacts Input
38 to 41 Not used -
42 Gearbox output shaft speed sensor, positive Input
43 Not used -
44 CAN, low Input/Output
45 Mode switch Input
46 to 50 Not used -
51 Manual mode warning lamp Output
52 Not used -
53 Solenoid valves power supply Output
54 Ignition power supply Input
55 Not used -

AUTOMATIC GEARBOX - ZF4HP22 - 24
DESCRIPTION AND OPERATION 44-15
CAN communications between EAT ECU and ECM
Inputs from ECM Outputs to ECM
CAN version identifier Calculated gear
Emissions (OBD II) control Diagnostic information
Engine air intake temperature Emission (OBD II) fault status
Engine speed Engine torque reduction request
Engine speed fault flag Gear position switch information
Engine temperature Output shaft speed
Engine torque Mode information
Engine torque fault flag Shift information
Friction torque Torque converter lock-up status
Maximum engine torque
Reduced engine torque
Road speed
Status of engine torque reduction
Throttle position

AUTOMATIC GEARBOX - ZF4HP22 - 24
44-16 DESCRIPTION AND OPERATION
Warning lamps
Warning lamps and selector lever position indication
1Transmission temperature warning lamp (red)
2Gear selector lever position indication3Sport mode warning lamp (green)
4Manual mode warning lamp (green)
Illumination for each warning lamp is provided by a non serviceable LED.

AUTOMATIC GEARBOX - ZF4HP22 - 24
DESCRIPTION AND OPERATION 44-17
Operation
Refer to illustration.

+ AUTOMATIC GEARBOX - ZF4HP22 - 24, DESCRIPTION AND OPERATION, Control schematic.
When the ignition is switched on, a bulb check is performed on the transmission temperature warning lamp and the
mode warning lamps by the instrument pack and the EAT ECU respectively. The warning lamps are illuminated for
approximately 3 seconds and then extinguished.
The gear position switch outputs are monitored by the BCU and the EAT ECU. The BCU outputs gear position signals
to illuminate the position indicators each side of the gear selector lever and on the odometer LCD in the instrument
pack.
In D, 3, 2, and 1, the EAT ECU outputs control signals to the gearbox to select the required gear.
In D, all forward gears are available for selection by the EAT ECU. In 3, 2 and 1, a corresponding limit is imposed on
the highest gear available for selection. When R is selected, reverse gear only engages if the vehicle is stationary or
moving at 5 mph (8 km/h) or less. When R is deselected, reverse gear only disengages if the vehicle is moving at 4
mph (6 km/h) or less.
Selector lever interlock (where fitted)
The interlock solenoid on the selector lever is de-energised unless the foot brake is applied while the ignition is on.
While de-energised, the interlock solenoid allows the selector lever to move through the range unless P is selected.
On entering the P position, the interlock solenoid engages a latch which locks the selector lever. When the ignition is
on and the foot brake is applied, the BCU energises the interlock solenoid, which disengages the latch and allows the
selector lever to be moved out of P.
Economy, sport and manual modes
During the power-up procedure after the ignition is switched on, the EAT ECU defaults to an economy mode. Pressing
the mode switch causes the EAT ECU to change between the economy mode and the sport or the manual mode,
depending on the range selected on the transfer box:
lIf the transfer box is in high range, the EAT ECU changes to the sport mode and illuminates the sport mode
warning lamp in the instrument pack. In the sport mode the gearbox is more responsive to accelerator pedal
movement. Downshifts occur earlier and upshifts occur later.
lIf the transfer box is in low range, the EAT ECU changes to the manual mode and illuminates the manual mode
warning lamp in the instrument pack. Kickdown is disabled and the EAT ECU maintains the gearbox in the gear
selected on the selector lever (D = 4th gear) to give improved off road performance. Downshifts occur only to
prevent the engine stalling. From a standing start, the vehicle pulls away in 1st gear and, if a higher gear is
selected, upshifts almost immediately to the selected gear (shifts of more than one gear can occur).
After a second press of the mode switch the EAT ECU reverts to the economy mode, for the range selected on the
transfer box, and extinguishes the related mode warning lamp in the instrument pack.
Shift control
To provide the different driving characteristics for each mode of operation, the EAT ECU incorporates different shift
maps of throttle position/engine speed. Base shift points are derived from the appropriate shift map. When a shift is
required, the EAT ECU sends a request to the ECM for a reduction in engine torque, in order to produce a smoother
shift. The percentage of torque reduction requested varies according to the operating conditions at the time of the
request. When the EAT ECU receives confirmation of the torque reduction from the ECM, it then signals the shift
solenoid valves in the gearbox to produce the shift. To further improve shift quality, the EAT ECU also signals the
pressure regulating solenoid valve to modulate the hydraulic pressure and so control the rate of engagement and
disengagement of the brake clutches.
With time, the components in a gearbox wear and the duration of the gear shifts tends to increase, which has an
adverse effect on the brake clutches. To counteract this, the EAT ECU applies a pressure adaptation to each shift.
To calculate the adaptations, the EAT ECU monitors the pressure modulation used, and time taken, for each shift. If
a subsequent shift of the same type, in terms of throttle position and engine speed, has a longer duration, the EAT
ECU stores an adaptation for that type of shift in a volatile memory. The adaptation is then included in future pressure
calculations for that type of shift, to restore shift duration to the nominal.

AUTOMATIC GEARBOX - ZF4HP22 - 24
44-18 DESCRIPTION AND OPERATION
Kickdown
The EAT ECU monitors the input of the throttle position sensor to determine when kickdown is required. When it
detects a kickdown situation, the EAT ECU immediately initiates a down shift provided the target gear will not cause
the engine speed limit to be exceeded.
Torque converter lock-up
The EAT ECU energises the lock-up solenoid valve to engage the lock-up clutch. Lock-up clutch operation is
dependent on throttle position, engine speed, operating mode and the range selected on the transfer box.
High range
Unique lock-up maps, similar to the shift maps, are incorporated in the economy and sport modes for all forward gears.
Engagement and disengagement of the lock-up clutch is dependent on throttle position and engine speed.
Low range
To enhance off road control, particularly when manoeuvring at low speeds, torque converter lock-up does not occur
when there is any degree of throttle opening. When the throttle is closed above a preset engine speed, the lock-up
clutch engages to provide maximum engine braking.
Increased load/reduced torque compensation
To aid performance and driveability in the high range economy mode, the EAT ECU has three adaptive shift and lock-
up maps. These maps delay upshifts and torque converter lock-up similar to the sport mode if the inputs from the
engine indicate:
lA sustained high load on the engine, such as occurs when the vehicle is ascending a steep gradient or towing a
trailer.
lA lower than normal engine torque, such as occurs at altitude or high ambient temperatures.
The EAT ECU monitors the engine inputs and selects the most appropriate adaptive map for the prevailing conditions.
Diagnostics
While the ignition is on, the EAT ECU diagnoses the system for faults. The extent of the diagnostic capability at any
particular time depends on the prevailing operating conditions, e.g. it is not possible to check torque converter lock-
up while the vehicle is stationary, or to check for a short circuit to earth if the circuit concerned is already at a low
potential.
If a fault is detected, the EAT ECU immediately stores a fault code and the values of three operating parameters
associated with the fault. Depending on the fault, there are four possible effects:
lThe fault has little effect on gearbox operation or vehicle emissions. The driver will probably not notice any
change and the warning lamps remain extinguished.
lThe fault has little effect on gearbox operation but may effect vehicle emissions. On NAS vehicles, if the fault is
detected on a second consecutive drive cycle, the MIL illuminates.
lAll gears are available but kickdown does not function. The sport and manual warning lamps flash. The MIL
remains extinguished.
lLimp home mode is selected and vehicle performance is greatly reduced. The sport and manual warning lamps
flash. In all markets, if the fault is detected on a second consecutive drive cycle, the MIL illuminates.
After the detection of a fault, the effects remain active for the remainder of the drive cycle. In subsequent drive cycles,
as soon as the EAT ECU diagnoses the fault is no longer present, it resumes normal control of the gearbox. The
conditions required to diagnose that the fault is no longer present depend on the fault. Some faults require the engine
to be started, others require only that the ignition is switched on.
After a fault has not recurred for forty warm-up cycles, the fault is deleted from the EAT ECU memory. Only five
different faults can be stored in the memory at any one time. If a further fault occurs, the fault with the lowest priority
will be replaced by the new fault.
Mechanical limp home
In the mechanical limp home mode, gear engagement is controlled by the manual valve. The gearbox is fixed in 4th
gear if the fault occurs while the vehicle is moving, or 3rd gear if the fault occurs while the vehicle is stationary. 3rd
gear is also engaged if a vehicle is brought to a stop and the selector lever is moved out of, and back into, D. Neutral
and reverse gear are also available.

AUTOMATIC GEARBOX - ZF4HP22 - 24
DESCRIPTION AND OPERATION 44-19
Fault effects and warning indications
Fault code,
OBD II
(TestBook)Fault description Effect Warning lamp
illumination
MIL Sport/
Manual
P0705
(14, 23)* Gear position switch,
incorrect outputsMaintains current gear in low range, limp
home mode in high range. Shift pressure to
maximum, harsh gear shifts/engagement.On On
P0721
(21)* Downshift safety
monitor prevented
downshift which would
have caused engine
overspeedMaintains current gear in low range, limp
home mode in high range. Shift pressure to
maximum, harsh gear shifts/engagement.On On
P0722
(22)* Torque converter
slippingMaintains current gear in low range, limp
home mode in high range. Shift pressure to
maximum, harsh gear shifts/engagement.On On
P0731
(29)* Ratio monitoring,
implausible 1st gear
ratioNo apparent effect On On
P0732
(30)* Ratio monitoring,
implausible 2nd gear
ratioNo apparent effect On Off
P0733
(31)* Ratio monitoring,
implausible 3rd gear
ratioNo apparent effect On Off
P0734
(32)* Ratio monitoring,
implausible 4th gear
ratioNo apparent effect On Off
P0741
(5)* Torque converter
lock-up clutch faultMay affect driveability. On Off
P0743
(7, 25)* Torque converter
lock-up solenoid (MV
3), open or short circuitLimp home mode in low and high ranges. Shift
pressure to maximum, harsh gear shifts/
engagement.On On
P0748
(10, 28)* Pressure regulating
solenoid (MV 4), open
or short circuitLimp home mode in low and high ranges. Shift
pressure to maximum, harsh gear shifts/
engagement.On On
P0753
(8, 26)* Shift solenoid (MV 1),
open or short circuitLimp home mode in low and high ranges. Shift
pressure to maximum, harsh gear shifts/
engagement.On On
P0758
(9, 27)* Shift solenoid (MV 2),
open or short circuitLimp home mode in low and high ranges. Shift
pressure to maximum, harsh gear shifts/
engagement.On On
P1562
(24)* Battery supply below
9V while engine
runningMaintains current gear in low range, limp
home mode in range. Shift pressure to
maximum, harsh gear shifts/engagement.Off On
P1601
(4)* ECU, EEPROM
checksumLimp home mode in low and high ranges. Shift
pressure to maximum, harsh gear shifts/
engagement.On On
P1606
(3)* EEPROM No apparent effect On†Off
P1606
(6)* Watchdog No apparent effect On On
P1612
(2)* Solenoid valves
power supply relay,
sticking closed or open
circuitLimp home mode in low and high ranges. Shift
pressure to maximum, harsh gear shifts/
engagement.On On